DWQ-2008-020006

NINE MILE RESERVOIR

LIMNOLOGICAL ASSESSMENT OF WATER QUALITY

Utah Division of Water Quality

February 2008

Utah Department of Environmental Quality
Division of Water Quality
TMDL Section

Nine Mile Reservoir

Waterbody ID Nine Mile Reservoir
UT-L-16030004-001
Location Sanpete County, Central Utah
Pollutants of
Concern
Low dissolved oxygen
Excess total phosphorus
Impaired
Beneficial Uses
Class 3A: Protected for cold water species and their food chain.
Recommended
Action
Nine Mile Reservoir is recommended to be placed in Category 5B of the
State of Utah's 303(d) list and requested for removal from the current
listing of impaired waters.
Delisting
Rationale
• Recent data assessment indicates that the water body is supporting all of
its designated beneficial uses.
• There have been no observed dissolved oxygen exceedances (> 50% of
the water column maintained above 4 mg/L) since 1999.
• Average current total phosphorus values (1999–2006) are below the
threshold of 0.025 mg/L established by the State of Utah for reservoirs.
• Both monitoring data and simulation modeling indicate that the
reservoir is mesotrophic and is not at risk for eutrophication.
• Water quality in the reservoir shows a trend toward improvement as
measured by total phosphorus, chlorophyll a and Secchi depth.
• A comparison with nearby Palisades Lake, which is not listed as
impaired, indicates that water quality is generally better in Nine Mile
Reservoir with the important exception of pH exceedances.
• Exceedances of pH criteria are associated with naturally alkaline soils
underlying the reservoir and its watershed.
• Exceedances of temperature criteria are related to drought conditions,
exposure, and natural climatic factors in the San Pitch basin.
• No fish kills have been reported in Nine Mile Reservoir.
• Blue-green algal species are not prevalent in Nine Mile Reservoir.

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Table of Contents
TABLE OF CONTENTS .................................................................................................ii
LIST OF TABLES........................................................................................................... iv
LIST OF FIGURES......................................................................................................... vi
EXECUTIVE SUMMARY ...........................................................................................viii
ACKNOWLEDGMENTS............................................................................................... ix
PREPARERS AND CONTRIBUTORS ........................................................................ ix
1 INTRODUCTION........................................................................................................1
1.1 Summary of Impairment Listing for Nine Mile Reservoir................................1
1.2 The Total Maximum Daily Load (TMDL) Process ............................................1
1.3 Delisting Requirements Under the Clean Water Act.........................................1
2 CHARACTERIZATION OF WATERSHED...........................................................3
2.1 Physical and Biological Characteristics...............................................................3
2.1.1 Climate...........................................................................................................3
2.1.2 Hydrology......................................................................................................9
2.1.3 Geology and Soils........................................................................................12
2.1.4 Plants, Animals, and Fisheries.....................................................................14
2.1.5 Wildlife........................................................................................................16
2.2 Cultural Characteristics......................................................................................16
2.2.1 Land Use and Ownership.............................................................................17
2.2.2 Population....................................................................................................18
2.2.3 History and Economics................................................................................21
2.2.4 Recreational Uses of Nine Mile Reservoir..................................................22
2.2.5 Public Involvement......................................................................................22
3 WATER QUALITY CONCERNS AND STATUS.................................................24
3.1 Water quality Limited Water bodies.................................................................24
3.2 Beneficial Use Classifications for Nine Mile Reservoir....................................24
3.3 Applicable Water Quality Standards.................................................................24
3.4 Summary and Analysis of Existing Water Quality Data .................................28
3.4.1 Water Quality Data Coverage......................................................................28
3.4.2 Statistical Overview.....................................................................................30
3.5 Assessment of Beneficial Use Support................................................................40
3.5.1 Key Indicators of Support............................................................................40
3.5.2 Direct Exceedance of Numeric Criteria and/or Threshold Values ..............45
3.5.3 Trend Analysis.............................................................................................55
3.5.4 Reservoir Water Column Data Assessment.................................................58
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3.5.5 Comparison with Palisades Lake.................................................................65
3.5.6 Impairment Adjustments..............................................................................68
3.5.7 Support Status Summary..............................................................................72
4 WATERSHED AND RESERVOIR MODELING .................................................74
4.1 Watershed Model: Soil and Water Assessment Tool (SWAT)........................74
4.1.1 General Model Description..........................................................................74
4.1.2 Model Development for Nine Mile Reservoir Watershed...........................77
4.1.3 Simulation Period for the Nine Mile Reservoir Watershed.........................80
4.2 Reservoir Model: BATHTUB.............................................................................80
4.2.1 General Model Description..........................................................................80
4.2.2 Model Inputs for Nine Mile Reservoir.........................................................81
4.2.3 Reservoir Model Results..............................................................................83
4.2.4 Model Calibration and Validation ...............................................................84
4.3 Model Uncertainty and Variability....................................................................86
4.4 Conclusions...........................................................................................................86
5 SUMMARY OF EVIDENCE FOR DELISTING...................................................87
5.1 Water Quality Summary.....................................................................................87
5.1.1 Compliance with Water Quality Criteria.....................................................87
5.1.2 Trend Towards Improving Water Quality...................................................87
5.1.3 Comparison with Palisades Lake.................................................................87
5.2 Explanation of Observed Water Quality Exceedances ....................................88
5.2.1 Naturally Alkaline Soils...............................................................................88
5.2.2 Correlation Between Water Quality and Reservoir Level...........................88
5.2.3 Drought Associated Temperature Exceedance............................................88
5.3 Other Indicators of Trophic State......................................................................88
5.4 Modeled Water Quality.......................................................................................88
BIBLIOGRAPHY............................................................................................................89

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iv
List of Tables
Table 2.1 Manti, Utah Air Temperature Data Summary....................................................4
Table 2.2 Manti, Utah Precipitation Data Summary...........................................................6
Table 2.3 Gunnison, Utah Air Temperature Data Summary..............................................9
Table 2.4 Gunnison, Utah Precipitation Data Summary.....................................................9
Table 2.5 Soil Associations and Characteristics in the Nine Mile Reservoir
Watershed ..................................................................................................................13
Table 2.6 Land Ownership within the Nine Mile Watershed Study Area........................17
Table 2.7 Land Use within the Nine Mile Watershed Study Area ...................................18
Table 3.1 Water Quality Numeric Criteria and Pollution Indicator Values
Specific to Nine Mile Reservoir Found in Utah State Code RS 317-2-14................25
Table 3.1 Water Quality Numeric Criteria and Pollution Indicator Values
Specific to Nine Mile Reservoir found in Utah State Code RS 317-2-14,
continued....................................................................................................................26
Table 3.1 Water Quality Numeric Criteria and Pollution Indicator Values
Specific to Nine Mile Reservoir found in Utah State Code RS 317-2-14,
continued....................................................................................................................27
Table 3.2 Annual Flow Volumes and Ranking Relative to the 30-year Average for
the Sevier River at USGS gage #10217000...............................................................31
Table 3.3 Water Quality Monitoring Sites for the Nine Mile Reservoir Watershed........32
Table 3.4 Summary of Data Available for Nine Mile Reservoir and Watershed..............33
Table 3.5 Change in Total Phosphorus Mean Concentration (mg/L) Through
Exclusion of Outliers and Historic Data....................................................................38
Table 3.6 Change in Mean chlorophyll a Concentration (µg/L) Through Exclusion
of Outliers and Historic Data.....................................................................................38
Table 3.7 Nitrogen to Phosphorus Ratios in Nine Mile Reservoir....................................43
Table 3.8 Summary of Current Dissolved Oxygen Data (1999–2006) in Nine Mile
Reservoir Watershed..................................................................................................45
Table 3.9 Percent of Total Samples in Exceedance of Dissolved Oxygen Criteria
(>4 mg/L)...................................................................................................................45
Table 3.10 Nine Mile Exceedance – DO Less Than 4.0 mg/L (% of Water
Column).....................................................................................................................46
Table 3.11 Summary of Current (1999–2006) Dissolved Oxygen Saturation Data
Corrected for Depths >1 Meter in Nine Mile Reservoir............................................47
Table 3.12 Percent of Total Samples in Exceedance of Dissolved Oxygen
Saturation Criteria in Surface Samples (<110%).......................................................47
Table 3.13 Summary of Current (1999–2006) Surface Dissolved Oxygen
Saturation Data (<1 meter) in Nine Mile Reservoir...................................................47
Table 3.14 Summary of Current (1999–2006) Nitrate Data in Nine Mile Reservoir
Watershed. .................................................................................................................48
Table 3.15 Percent of Total Samples in Exceedance of Nitrate Criteria (<4 mg/L)
in Current Data (1999–2006).....................................................................................49
Table 3.16 Summary of Current (1999–2006) pH Data in Nine Mile Reservoir
Watershed ..................................................................................................................49
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Table 3.17 Percent of Total Samples in Exceedance of pH criteria (<9) .........................50
Table 3.18. Summary of Current Temperature Data (1999–2006) in Nine Mile
Reservoir watershed...................................................................................................52
Table 3.19 Summary of Current Total Dissolved Solids Data (1999 – 2006) in
Nine Mile Reservoir Watershed.................................................................................53
Table 3.20 Percent of Total Samples in Exceedance of Total Dissolved Solids
Criteria (<1200 mg/L)................................................................................................54
Table 3.21 Summary of Current Total Phosphorus Data (1999 – 2006) in Nine
Mile Reservoir Watershed.........................................................................................54
Table 3.22 Percent of Total Samples in Exceedance of Total Phosphorus
Indicator Threshold (<0.05 mg/L for streams and <0.025 mg/L for
reservoirs) ..................................................................................................................55
Table 3.23 Summary of Current (1999–2006) Chlorophyll a Data (µg/L) in Nine
Mile Reservoir. ..........................................................................................................56
Table 3.24 Depth-integrated Reservoir Monitoring Data.................................................58
Table 3.25 Nine Mile Viable Habitat................................................................................62
Table 3.26 Summary of Comparative Data Between Palisades Lake and Nine
Mile Reservoir ...........................................................................................................66
Table 3.27 TSI Values and Status Indicators.....................................................................69
Table 3.28 Summary of Current (1999–2006) TSI Data in Nine Mile Reservoir.............71
Table 4.1 Watershed Land Use and Area Breakdown Used for Load Calculations..........78
Table 4.2 Irrigation of Agricultural Lands of Six Mile and Nine Mile Watersheds..........80
Table 4.3 Empirical Models Selected for Reservoir BATHTUB Model of Nine
Mile Reservoir ...........................................................................................................81
Table 4.4 Calculation of Hypolimnetic Depth for Nine Mile Reservoir Average
Conditions..................................................................................................................82
Table 4.5 Summary of Phosphorus and Nitrogen Loads to Nine Mile Reservoir.............83

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List of Figures
Figure 2.1 Study area with the Nine Mile Reservoir watershed..........................................5
Figure 2.2 Annual average air temperature and precipitation conditions at the
Manti, Utah meteorological site, Utah (WRCC 2006). ...............................................7
Figure 2.3 Annual average air temperature and precipitation conditions at the
Gunnison, Utah meteorological site, Utah (data from WRCC, 2006).........................8
Figure 2.4 Soils map of Nine Mile Reservoir watershed...................................................15
Figure 2.5 Land ownership and populated areas map for Nine Mile Reservoir
watershed...................................................................................................................19
Figure 2.6 Land use map for Nine Mile Reservoir watershed...........................................20
Figure 2.7 Sheep being moved from high-elevation summer pasture to winter
range in the desert, Sanpete County, circa 1930 (AITC 2005)..................................21
Figure 3.1 Annual discharge volumes for the Sevier River below the San Pitch
River near Gunnison, Utah from 1975 through 2005 (USGS Gage
#10217000)................................................................................................................29
Figure 3.2 Total phosphorus related to reservoir level indicating that the low
reservoir level may explain outliers...........................................................................39
Figure 3.3 Chlorophyll a related to reservoir level indicating that the low reservoir
level may explain outliers..........................................................................................39
Figure 3.4 pH values in Nine Mile Reservoir showing seasonal trend as observed
during 2005 routine monitoring.................................................................................50
Figure 3.5 pH of soils in Nine Mile Reservoir watershed.................................................51
Figure 3.6 Water temperatures observed during routine monitoring at the Mid-
reservoir Site in Nine Mile Reservoir........................................................................52
Figure 3.7 Water temperatures observed during routine monitoring near the Dam
Site in Nine Mile Reservoir.......................................................................................53
Figure 3.8 Chlorophyll-a trend data for Nine Mile Reservoir...........................................56
Figure 3.9 Secchi depth trend data for Nine Mile Reservoir.............................................57
Figure 3.10 Total phosphorus trend data for Nine Mile Reservoir....................................57
Figure 3.11 Spring (15 June 2005) depth profile plots for dissolved oxygen,
temperature, and pH observed in Nine Mile Reservoir.............................................58
Figure 3.12 Summer (28 July 2005) depth profile plots for dissolved oxygen,
temperature, and pH observed in Nine Mile Reservoir.............................................59
Figure 3.13 Summer (11 August 2005) depth profile plots for dissolved oxygen,
temperature and pH observed in Nine Mile Reservoir..............................................59
Figure 3.14 Fall (08 September 2005) depth profile plots for dissolved oxygen,
temperature and pH observed in Nine Mile Reservoir..............................................60
Figure 3.15 Relative percent of the water column at Mid-reservoir Site (upper
plot) and near the Dam Site (lower plot) experiencing one or more
exceedances of water quality criteria.........................................................................63
Figure 3.16 Relative percent of the water column at Mid-reservoir Site (upper
plot) and near the Dam Site (lower plot) exhibiting viable habitat conditions.
For the above plots, viable habitat condition was defined as that portion of
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the water column where no exceedances of water quality criteria (joint or
single) were observed. ...............................................................................................64
Figure 3.17 Dissolved oxygen profiles in Nine Mile Reservoir and Palisades Lake
for Mean and Minimum Dissolved Oxygen..............................................................67
Figure 3.18 Trophic state index trend data for Dam Site...................................................70
Figure 3.19 Trophic state index trend data for Mid-reservoir Site....................................70
Figure 4.1. SWAT model schematic of water routing and processes (Neitsch
2002)..........................................................................................................................76
Figure 4.2 Correlation between reservoir volume and reservoir depth at dam..................82
Figure 4.3 Model validation graphs for Nine Mile Reservoir BATHTUB model.............85

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viii
Executive Summary
Under section 303(d) of the Clean Water Act, Nine Mile Reservoir has been identified as
water quality limited due to low dissolved oxygen and excess phosphorus loading to the
reservoir from the surrounding watershed. The State of Utah has designated the beneficial
uses of the reservoir as secondary contact recreation (2B), cold water game fish and the
associated food chain (3A), and agricultural water supply (4). The cold water game fish
designated use (3A) was identified as not supported on the State of Utah’s 2006 303(d)
list of impaired waters. However, analysis of current water quality in Nine Mile
Reservoir and its tributaries indicates that the reservoir is meeting all designated
beneficial uses. Since anthropogenic activities are not impairing water quality, Nine Mile
Reservoir is recommended to be placed in Category 5B of the State of Utah's 303(d) list
and requested for removal from the current listing of impaired waters. The following
rationales are presented to support this recommendation.
• An assessment of recent data indicates that the water body is supporting all of its
designated beneficial uses; there have been no observed dissolved oxygen
exceedances since 1999.
• Current total phosphorus values (1999–2006) are well below the threshold of
0.025 mg/L established by the State of Utah for reservoirs when standard methods
are used to include nondetect values and exclude outliers from the dataset.
• Both monitoring data and simulation modeling indicate that the reservoir is
mesotrophic and is not at risk for eutrophication.
• Water quality in the reservoir shows a trend toward improvement as measured by
total phosphorus, chlorophyll a and Secchi depth.
• A comparison with nearby Palisades Lake, which is not listed as impaired,
indicates that water quality is generally better in Nine Mile Reservoir with the
important exception of pH exceedances.
• Exceedances of pH criteria are associated with naturally alkaline soils underlying
the reservoir and its watershed.
• Exceedances of temperature criteria are related to drought conditions, exposure,
and natural climatic factors in the San Pitch basin.
• No fish kills have been reported in Nine Mile Reservoir.
• Blue-green algal species are not prevalent in Nine Mile Reservoir.

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Acknowledgments

SWCA staff gratefully acknowledges the time and effort that have been dedicated by so
many individuals and organizations whose help and support have been indispensable.
Their continuing support is very much appreciated.
We would like to specifically acknowledge the efforts of the USFS Manti-LaSal National
Forest, Gunnison Irrigation Company, Sterling/Six Mile Irrigation Company, Utah
Division of Water Quality, Utah Division of Wildlife Resources, Utah Department of
Agriculture, Utah Department of Water Resources, Utah Department of Water Rights,
Utah Geological Survey, US Geological Survey, and the USDA Natural Resources
Conservation Service, and Sanpete County, for their contributions of important
background information and their assistance with data location and interpretation for this
project.
Preparers and Contributors
Preparers
• Doug Davidson, SWCA, Watershed Modeling
• Tonya Dombrowski, SWCA, Water Quality Analysis
• Erica Gaddis, SWCA, Water Quality Analysis, Reservoir Modeling, and
Impairment Assessment
• Eric McCulley, SWCA, Soils and Geology
• Brian Nicholson, SWCA, Watershed Characteristics and Hydrology
• Tyson Schriener, SWCA, GIS
• David Reinhart, SWCA, GIS
Contributors
• Rolan Beck
• Polly Johnson
Nine Mile Reservoir Delisting February 2008

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1 INTRODUCTION
1.1 SUMMARY OF IMPAIRMENT LISTING FOR NINE MILE RESERVOIR
Under section 303(d) of the Clean Water Act (CWA), Nine Mile Reservoir has been
identified as water quality limited due to low dissolved oxygen and excess phosphorus
loading to the reservoir from the surrounding watershed. The State of Utah has
designated the beneficial uses of the reservoir as secondary contact recreation (2B), cold
water game fish and their associated food chain (3A), and agricultural water supply (4).
The cold water game fish designated use (3A) was identified as non-supported on the
State of Utah 2006 303(d) list. Secondary contact recreation and agricultural water
supply designated uses were reported as being fully supported on this same list.
1.2 THE TOTAL MAXIMUM DAILY LOAD (TMDL) PROCESS
A Total Maximum Daily Load (TMDL) is the amount of an identified pollutant that a
specific stream, lake, river or other water body can 'accommodate' without violating state
water quality standards. TMDLs are watershed-based plans for restoring designated
beneficial uses in water quality limited water bodies. These plans must identify the causes
of designated beneficial use impairment, estimate reductions in pollutant loads necessary
to meet water quality standards, and restore impaired designated beneficial uses within a
specified time.
Briefly, the TMDL process involves evaluating the available data from 303(d) listed
water bodies to determine point and nonpoint source pollution loads and using the data to
set maximum allowable loads from each of these sources. Loads are the quantity of
pollution contributed to a stream by a single source (e.g., a wastewater treatment plant) or
by a group of sources (e.g., all developments or agricultural fields along a stream).
In this framework, a TMDL can be best described as a watershed or basin-wide budget
for pollutant loading to a watercourse. A TMDL, in actuality, is a planning document.
The "allowable budget" is first determined by scientific study of a stream to determine
the amount of pollutants that can be assimilated without causing the stream to exceed the
water quality standards set to protect the stream's designated beneficial uses (e.g., game
fish, domestic water supply, etc.). This amount of pollutant loading is known as the
loading capacity. It is established taking into account seasonal variations, natural and
background loading, and a margin of safety. Once the loading capacity is determined,
sources of the pollutants are considered. Both point and nonpoint sources must be
included. As part of this process, habitat function is assessed to provide a qualitative
summary of the current support status of game fish beneficial uses.
1.3 DELISTING REQUIREMENTS UNDER THE CLEAN WATER ACT
Analysis of current water quality in Nine Mile Reservoir and its tributaries indicate that
the reservoir is meeting all designated beneficial uses. Since man-made activities have
not caused any water quality impairment, Nine Mile Reservoir is recommended to be
placed in Category 5B of the State of Utah's 303d list and petitioned for removal from the
current 303(d) listing of impaired waters.
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According to EPA regulations, each state must demonstrate good cause for not including
waters on the list (40 CFR. Part 130.7(b)(6)(iv)) or removing them from the list. These
include:
• A water body listed due to error in assessment, a water body listed incorrectly in
place of another water body, or any other error not based on a water quality
assessment.
• The most recent data assessment indicates that the water body is supporting all of
its designated beneficial uses.
• A total maximum daily load analysis has been completed and approved by the
EPA.
• New modeling information indicates no TMDL is required in order to maintain
water quality standards.
• Data assessment methodologies have been modified.
Utah may also request EPA delisting of a water body when:
• The water body is meeting all applicable water quality standards or is expected to
meet these standards in a reasonable time frame (e.g., two years) as a result of
implementation of required pollutant controls.
• Upon re-examination, the original basis for listing is determined to be inaccurate.
Of the criteria listed above, the following were used in the impairment assessment of
Nine Mile Reservoir leading to a recommendation for delisting. Analysis of recent
dissolved oxygen and phosphorus data indicate full support of beneficial uses and
watershed and reservoir models indicating that no TMDL is required in order to maintain
dissolved oxygen water quality standards. The Nine Mile Reservoir watershed has been
meeting all listed beneficial uses set by the State of Utah. Based upon available data and
computer model simulations, the impairment listing for dissolved oxygen is not supported
and therefore the reservoir should be removed from the Utah 303 (d) list.
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2 CHARACTERIZATION OF WATERSHED
2.1 PHYSICAL AND BIOLOGICAL CHARACTERISTICS
The Nine Mile Reservoir study area is located in Sanpete County, in central Utah
between the towns of Sterling and Gunnison (Figure 2.1). The reservoir and its drainage
area form a portion of the San Pitch River basin. The reservoir shoreline is privately
owned by the Gunnison Irrigation Company with unrestricted public access. The
watershed is a mixture of public and private land with land uses that include range,
agriculture and rural development. The uplands are predominately administered by the
U.S. Forest Service while the valley floor is privately owned pasture and cropland.
The primary land uses in the watershed are agriculture (cattle and sheep grazing, hayland
and pasture) and rural residential housing. Sedimentation and nutrient loading are the
major potential sources of nonpoint source pollution. There are no point source pollution
sources to the reservoir (UDWQ 2007). Major inflows to the reservoir include Six Mile
Creek (diverted to the reservoir for a portion of the year) and three natural springs located
along the eastern shore of the reservoir. UDWR stocks Nine Mile Reservoir with rainbow
trout, tiger trout (a brown trout / brook trout hybrid). The reservoir also contains non-
native smallmouth bass and crayfish.
Nine Mile Reservoir is located at the western base of the Wasatch Plateau at an elevation
of 5,402 ft (1,646 m). The reservoir has a maximum volume of approximately 3,500 acre-
feet of water. Maximum reservoir depth is approximately 11 meters, and the reservoir
shoreline extends 14,169 feet (4,320 m). Slopes within this watershed are complex, and
range from 0 to 45 degrees with an average slope in the direct drainage of the reservoir of
10 degrees. The average slope in the Six Mile Creek watershed is 16.5 degrees with a
range from 0 to 77 degrees. The highest point in the watershed is located near the
headwaters of Six Mile Creek along Skyline Drive at an elevation of 10,700 feet. The
lowest point is the reservoir outlet at an elevation of approximately 5,360 feet.
The earth-fill dam that forms the reservoir was completed in 1900 and rises to a structural
height of 46 feet (Utah Division of Water Rights 2006a). The reservoir is used for storing
irrigation water and recreation. Anecdotal information indicates that Nine Mile Reservoir
has been filled to capacity (3,500 acre-feet) most years and is often drawn down to below
the staff gage which records a minimum volume of 240 acre-feet. There is no
conservation pool established for this reservoir. The physical structure of the reservoir is
such that the elevation of the outlet does not allow full evacuation of all water behind the
dam, thus, even in low water years, a minimum of approximately 100 acre-feet of water
will remain in the reservoir.
2.1.1 CLIMATE
Climate in the Nine Mile Reservoir drainage basin ranges from semiarid in Sanpete
Valley to subhumid in the surrounding uplands. The area is characterized by large
seasonal and daily temperature variations, especially during the summer. Most of the
precipitation in the San Pitch River drainage basin falls as snow in the mountains,
particularly along the Wasatch Plateau, from November to April. The months of June
through August are generally the driest, although brief, intense thunderstorms can
produce locally large precipitation totals.
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The Nine Mile Reservoir watershed is generally hot and dry in the summer and cold and
dry in the winter. Precipitation is bimodal (peaking in March and September or October)
with intense, short duration summer storms and milder, longer duration winter storms.
Much of the water is derived from snowmelt runoff from high elevations along the
Wasatch Plateau and headwaters of the inflowing tributaries. Snow pack accumulation
generally occurs from November to April at higher elevations. Lower areas of the valley
receive between 10 inches (25 cm) and 14 inches (35.6 cm) of precipitation annually,
while elevations above 8,000 feet (2,500 meters) average approximately 24 inches (60
cm) of precipitation annually. Average annual evaporation in the San Pitch River
drainage is 3.5 times greater than the average annual precipitation (Robinson 1971).
Climate data are not available directly for the reservoir. However, two long-term climate
sites maintained by the Western Regional Climate Center (WRCC) are available near the
watershed boundaries at Manti, Utah, and Gunnison, Utah.
The Manti, Utah WRCC site is located at an elevation of 5,530 feet (1,815 meters),
approximately 8 miles northeast of the reservoir and is assumed to be generally
representative of conditions at the reservoir site (reservoir elevation is 5,402 feet (1,646
meters). The site has been in operation from January 1928 to present, and data are
available through to December 2005 (WRCC 2006). Average and extreme minimum and
maximum temperatures recorded over the period of record for the Manti, Utah WRCC
site are displayed in Table 2.1 and Figure 2.2. Average total monthly precipitation for this
site is displayed in Table 2.2 and Figure 2.2.
As the Manti, Utah WRCC site is located to the northeast of the watershed and may not
be fully representative of conditions in southwestern areas, additional data were collected
from the WRCC site located near Gunnison, Utah.
Table 2.1 Manti, Utah Air Temperature Data Summary
Monthly Average
Max (°F) Min (°F) Average (°F)
Extreme
High (°F)
Extreme
Low (°F)
Annual 61.7 33.8 47.7 103 Jul 1960 -27 Jan 1937
Winter 39.2 16.9 28.1 69 Feb 1986 -27 Jan 1937
Spring 60.4 32.5 46.4 90 May 1967 -5 Mar 1964
Summer 83.6 51.3 67.4 103 Jul 1960 27 Jun 1902
Fall 63.6 34.4 49.0 97 Sep 1950 -18 Nov 1931
Winter = December, January, and February; Spring = March, April, and May; Summer = June, July, and August; Fall =
September, October, and November. (WRCC data, period of record = 1928 to 2006)
Nine Mile Reservoir Delisting February 2008
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Figure 2.1 Study area with the Nine Mile Reservoir watershed.
Ninemile
Reservoir
Palisade Lake
San Pitch River
4946150
Six Mile Creek
Highland Canal
Six Mile Creek
San Pitch River
4946160
5943540
5943250
5943240
5943230
4946190
4946360
5943260
Spring 3
Spring 2
Spring 1
Diversion
Reservoir Main
Reservoir Inflow
Reservoir Outflow
Sterling
Gunnison
0120.5 Miles
0241Kilometers
Area Enlarged
Imagery taken from National Agricultural Imagery
Program (NAIP) natural color aerial photography1-meter resolution, 2006
SWCA Water Quality Sample Location
STORET Location
Diversion
Town
Stream/Drainage
Ninemile Reservoir Direct Drainage
Study Area
River/Reservoir
Road
Tuesday, April 3, 2007 3:06:46 PMF:\11046\Maps\Report\nm_hydro.mxd
Nine Mile Reservoir Delisting February 2008
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Table 2.2 Manti, Utah Precipitation Data Summary
Average (inches) High (inches) Low (inches)
Annual 12.87 21.55 1983 7.08 1934
Winter 3.13 6.16 1980 0.89 1931
Spring 3.97 9.47 1995 1.34 1934
Summer 2.48 6.80 1936 0.31 1931
Fall 3.29 8.80 1982 0.56 1932
Winter = December, January, and February; Spring = March, April, and May; Summer = June, July, and August;
Fall = September, October, and November. (WRCC data, period of record = 1928 to 2006)

The Gunnison, Utah WRCC site is located at an elevation of 5,125 feet (1,682 meters),
approximately 6 miles to the west-southwest of the reservoir and is representative of both
the general reservoir location and the topography and elevation of much of the Nine Mile
watershed. The site was in operation from March 1956 to April 1990 (WRCC 2006).
Average and extreme minimum and maximum temperatures recorded over the period of
record for the Gunnison, Utah WRCC site are displayed in Table 2.3 and Figure 2.3.
Average total monthly precipitation for this site is displayed in Table 2.4 and Figure 2.3.
Observed temperatures at the Gunnison, Utah WRCC site are slightly warmer and
precipitation somewhat lower than those observed at the Manti, Utah site. The Gunnison,
Utah site experiences warmer maximum temperatures by about 3 to 5°F (15–16°C), and
cooler minimum temperatures by about 3°F (15°C) than the Manti, Utah site. Annual
average precipitation is approximately 30% lower at the Gunnison, Utah site than that
observed at the Manti, Utah site. The differences in temperature and precipitation
observed between these two locations provide a relative variance for the lower watershed
elevations.
Mean precipitation data for a larger general area near Nine Mile Reservoir are available
from 13 SNOTEL sites managed by the Utah Division of Water Resources. Data are
available from 1996 to 2006 and include a 10-year average.
No SNOTEL sites are located directly within the Nine Mile Reservoir watershed. The
closest SNOTEL station is Seeley Creek (Station ID: 11k09s) located near Twelve Mile
Creek approximately 3 miles from the south-central boundary of the watershed, about 8
linear miles east of the reservoir. The SNOTEL site elevation is approximately 9,910 feet
and is assumed to be characteristic of climate conditions in the higher elevations within
the watershed.
Station data indicate that in the past 10 years, the average annual precipitation is 24.7
inches (62.74 cm) with a minimum of 17.3 inches (43.94 cm) recorded in 2002 and
maximum of 31.8 inches (80.77 cm) falling in 1998. Precipitation falls throughout the
fall, winter, and spring with lower precipitation rates in the summer months (May, June,
July, and August). The area is subject to high intensity thunderstorms in the summer.
Mean monthly high temperatures at the SNOTEL station from 1997 to 2006 range from
40.6°F (-1°C) in the winter to 75.7°F (22°C) in the summer.
Nine Mile Reservoir Delisting February 2008
7
- Extreme Max. is the maximum of all daily maximum temperatures recorded for the day
of the year.
- Ave. Max. is the average of all daily maximum temperatures recorded for the day of the
year.
- Ave. Min. is the average of all daily minimum temperatures recorded for the day of the
year.
- Extreme Min. is the minimum of all daily minimum temperatures recorded for the day of
the year.
The Nine Mile Reservoir watershed has experienced drought for much of the last five
years, with extremely dry conditions occurring during the summer of 2002, when the
Palmer Drought Severity Index (PDSI) reached near-record severity based on the last 100
years of instrumental data (NCDC 2004). These dry conditions have resulted in low water
and low flow conditions for the reservoir watershed and adjacent areas.

Figure 2.2 Annual average air temperature and precipitation conditions at the
Manti, Utah meteorological site, Utah (WRCC 2006).
Nine Mile Reservoir Delisting February 2008
8
- Extreme Max. is the maximum of all daily maximum temperatures recorded
for the day of the year.
- Ave. Max. is the average of all daily maximum temperatures recorded for the
day of the year.
- Ave. Min. is the average of all daily minimum temperatures recorded for the
day of the year.
- Extreme Min. is the minimum of all daily minimum temperatures recorded for
the day of the year.

Figure 2.3 Annual average air temperature and precipitation conditions at the
Gunnison, Utah meteorological site, Utah (data from WRCC, 2006).
Nine Mile Reservoir Delisting February 2008
9

Table 2.3 Gunnison, Utah Air Temperature Data Summary
Monthly Average
Max (°F) Min (°F) Average (°F)
Extreme
High (°F)
Extreme
Low (°F)
Annual 65.3 31.1 48.2 104 Jul 1960 -28 Feb 1989
Winter 41.8 14.6 28.2 72 Feb 1972 -28 Feb 1989
Spring 63.6 30.2 46.9 94 May 1960 -7 Mar 1964
Summer 88.7 48.4 68.5 104 Jul 1960 22 Jun 1976
Fall 66.9 31.0 48.9 96 Sep 1977 -11 Nov 1977
Winter = December, January, and February; Spring = March, April, and May; Summer = June, July, and August;
Fall = September, October, and November. (WRCC data, period of record = 1956 to 1990)

Table 2.4 Gunnison, Utah Precipitation Data Summary
Average (inches) High (inches) Low (inches)
Annual 8.93 18.37 1983 5.07 1958
Winter 1.94 4.16 1978 0.36 1961
Spring 2.78 5.99 1983 1.16 1972
Summer 1.61 4.78 1984 0.32 1989
Fall 2.60 5.74 1982 0.49 1956
Winter = December, January, and February; Spring = March, April, and May; Summer = June, July, and August;
Fall = September, October, and November. (WRCC data, period of record = 1956 to 1990)

2.1.2 HYDROLOGY
2.1.2.1 Surface Water Hydrology
Nine Mile Reservoir is a man-made impoundment that does not lie directly on any natural
waterway. It is maintained through the artificial diversion of Six Mile Creek and the
inflow of three local springs.
The reservoir is located in the San Pitch River drainage basin, bordered on the west by
the San Pitch Mountains or the Gunnison Plateau with a peak elevation of 9,700 feet
(3,000 meters), and on the east by the Wasatch Plateau with peak elevation of
approximately 11,000 feet (3,350 meters). The San Pitch River flows north to south
through the valley, entering Gunnison Reservoir immediately to the northwest of Nine
Mile Reservoir near the narrow southern end of the Sanpete Valley. The San Pitch River
exits Gunnison Reservoir and flows into the Sevier River, which flows generally north
and west through the Gunnison Plateau, discharging eventually into a terminal basin in
western Utah.
The natural watershed that directly feeds the reservoir encompasses approximately 4
square miles and includes discharge from three perennial springs. In addition to the
perennial spring flow, some water is also diverted to the reservoir from Six Mile Creek.
The actual contributing watershed to the reservoir, therefore, includes an additional
28,000 acres specific to the drainage of Six Mile Creek. The diversion of water from Six
Mile Creek represents approximately 40% of the total inflow to the reservoir during years
Nine Mile Reservoir Delisting February 2008
10
when the maximum volume of water designated for Nine Mile Reservoir (1,500 acre-
feet) is diverted.
Six Mile Creek is located in the southern end of the Sanpete Valley and flows generally
westward approximately 12 miles from its headwaters in the Wasatch Plateau toward the
San Pitch River. The creek is diverted to flow into both Palisades Lake, a small reservoir
located northwest of Sterling, Utah, and into Nine Mile Reservoir. The Gunnison
Irrigation Company holds water rights for 1,500 acre-feet of Six Mile Creek water to
augment the reservoir volume, however only 1,000 acre-feet is typically diverted. This
water is diverted on the east side of Sterling, Utah and flows through an open ditch, pipe
and incised channel enroute to Nine Mile Reservoir. There is no U.S. Geological Survey
gage on Six Mile Creek or the diversion canal.
Diversion of water from Six Mile Creek to the reservoir generally takes place during
March, April, and May though during dry years will also include October, November,
and December. The maximum annual volume diverted from Six Mile Creek is 1,500
acre-feet (Rolan Beck, Gunnison Irrigation Company, personal communication with
Tonya Dombrowski, 28 June 2006). Little or no water is diverted from Six Mile Creek to
the reservoir during the winter months. Therefore, during the late summer and winter
seasons, reservoir inflow is dominated by spring flows rather than surface water from Six
Mile Creek. Springs fill the reservoir at the rate of approximately 200 acre-feet per month
(Rolan Beck, Gunnison Irrigation Company, personal communication with Erica Gaddis,
16 April 2007).
The Highland Canal functions as the primary outflow to the reservoir with most water
being diverted by pipeline to support irrigation downstream during summer months. The
reservoir is drawn down for irrigation from June 15 to August 15 at an average rate of 50
acre-feet per day (Rolan Beck, Gunnison Irrigation Company, personal communication
with Erica Gaddis, 16 April 2007). The reservoir is also designed so that emergency
overflow can be diverted directly to the San Pitch River via the Spaniard Canal; however,
this diversion has not been used in recent history.
2.1.2.2 Groundwater Hydrology
Groundwater within the Nine Mile Reservoir watershed can be divided into two major
categories: natural groundwater and irrigation recharge. Natural groundwater refers to
groundwater that is present due to geological and hydrological processes. Irrigation
recharge refers to waters which are not transpired by the vegetation and percolates into
the shallow aquifer as recharge.
The primary groundwater discharge areas within the Nine Mile Reservoir watershed are
composed of poorly sorted Quaternary valley-fill deposits (Weiss 1994). These deposits
are located within the Sanpete Valley and are adjacent to Nine Mile Reservoir.
Groundwater springs surface along the mountain front and are associated with faults and
valley fill. The areas east of the mountain front are composed of mostly bedrock and
mass-wasting deposits. Spring runoff and summer cloudburst thunderstorms contribute to
the groundwater recharge in mountain areas (Lowe et. al. 2002).
The principal valley-fill aquifer of Sanpete Valley is confined by thick, fine-grained
sediments in much of the valley, located within a series of interwoven layers of clay, silt,
sand, and gravel. Coarser materials generally occur along the mountain margins while
finer grained material occurs in the center of the valley. On the eastern edge of the valley,
near Nine Mile Reservoir, alluvial sands and gravels extend farther into the valley. In the
Nine Mile Reservoir Delisting February 2008
11
southern end of the valley, several distinct confining layers are present and result in
widely varying depth-to-water levels and head pressure in wells in close proximity.
Depths to groundwater ranges from 5 feet (1.5 meters) to 200 feet (60 meters) in the
vicinity of Sterling, Utah, becoming shallower with increasing proximity to the river
(Snyder and Lowe 1998).
The surrounding mountains and associated alluvial fans represent the primary recharge
areas for the valley-fill aquifer. Discharge areas occur predominantly along the north-
south centerline of the valley. Nine Mile Reservoir and its associated springs are located
in a primary recharge area of the valley floor in an area of unconsolidated valley fill.
Discharge areas do not occur in association with the reservoir, and are generally located
to the north of Gunnison Reservoir and the community of Manti, Utah (Snyder and Lowe
1998).
The quality of groundwater in the valley is generally high, with some areas experiencing
localized nitrate contamination. Most of the groundwater in the valley is classified as IA
(pristine water quality, defined as having less than 500 mg/L total dissolved solids) or II
(drinking water quality, defined as having between 500 and 3,000 mg/L total dissolved
solids) (Snyder and Lowe 1998).
High nitrate concentrations have been observed in groundwater at several locations in the
Sanpete Valley. Wells near Moroni, Utah, located at the northern end of the valley, have
shown nitrate concentrations above the state standard maximum contaminant level
(MCL) of 10 mg/L. A groundwater well near Manti, Utah (approximately 6 miles
northwest of the reservoir) showed nitrate concentrations of 4.5 mg/L (Snyder and Lowe
1998). The source of the nitrate contamination in these wells has not been determined.
Potential sources include septic tanks, fertilizers, feedlot drainage, and natural sources
such as potassium nitrates in the rock and caliche found in the Sanpete Valley.
The primary inflow source for Nine Mile Reservoir is the discharge from three perennial
springs located along the eastern edge of the reservoir. Measured flow data for the
springs are not available and information on spring water volumes delivered to the
reservoir is anecdotal. The three springs are estimated to deliver 1 to 3, 1 to 1.5, and 0.5
cubic feet per second, respectively (spring flows identified are for springs located from
south–north along the reservoir's eastern shoreline respectively). The combined inflow
from these three springs is estimated at approximately 3.0 to 3.5 cfs (Rolan Beck,
Gunnison Irrigation Company, personal communication with Tonya Dombrowski, 28
June 2006) and amounts to a total delivery volume of between 2,000 and 2,500 acre-feet
of water annually. Peacock Spring is the main spring inflow to the reservoir and is
estimated to contribute approximately 80% of the spring flow on an annual basis.
Spring flow does not appear to vary significantly from season to season, and shows little
variation in flow from year to year. Some incremental decrease in spring flow was noted
during the drought (discussed earlier) but it was not defined as a substantial decrease by
the irrigation company (Rolan Beck, Gunnison Irrigation Company, personal
communication with Tonya Dombrowski, 28 June 2006).
Topographic maps indicate the presence of additional springs within the watershed, but
none are reported to deliver water to the reservoir. The groundwater quality in the area
surrounding Nine Mile Reservoir, including the springs that act as the major source of
water for the reservoir, are classified as Class 1A (pristine water quality, defined as
having less than 500 mg/L total dissolved solids ) by the Utah Geologic Survey (Lowe et
al. 2002). Nitrate concentrations in the area are generally less than 3.0 mg/L although
Nine Mile Reservoir Delisting February 2008
12
high nitrate concentrations are found in groundwater in other areas of Sanpete Valley
(Lowe et al. 2002). The area that makes up the direct drainage to Nine Mile Reservoir is
primarily classified as valley fill representing historic landslides and alluvial deposits,
whereas bedrock makes up much of the Six Mile Creek drainage area. This area was not
included in the groundwater transport modeling studies conducted for Sanpete Valley in
2002 (Lowe et al. 2002).
2.1.3 GEOLOGY AND SOILS
2.1.3.1 Geology
The Nine Mile Reservoir watershed lies within the Basin and Range-Colorado Plateau
transition zone (Stokes 1986), which contains features characteristic of both the Basin
and Range and Colorado Plateau Physiographic provinces. Spieker (1946) described
these geologic features as follows:
The eastern margin of the plateau [Wasatch Plateau] is a sweeping stretch of barren
sandstone cliffs, a southward continuation of the Book Cliffs, surmounted by higher
tabular masses, in all of which the strata dip at low angles and are essentially parallel, in
the general habit of the Colorado Plateaus [sic]. On the western margin the strata plunge
to-ward Sanpete and Sevier Valleys in the great Wasatch monocline, at the base of which
the structure is complex and a variously deformed rock succession is broken by several
angular unconformities; the geologic features here are typical of the Great Basin, and
their eastern limit follows in a general way the western border of the plateau.
Nine Mile Reservoir is located along the complex structural area west of the Wasatch
Plateau. Local lithology is predominantly Tertiary and Mesozoic sedimentary rocks,
limestone and sandstone, and minor amounts of mudstone and clay stone. The regional
structure of these bedded rocks dips approximately 1 to 20 degrees to the west. The
layered nature of the rocks and presence of underlying clayey rock units have created
many large-scale landslide deposits during the late Pleistocene. These massive debris
flow deposits cover a large part of the Six Mile Creek watershed. Sanpete Valley-fill
deposits consist of unconsolidated alluvial and colluvial debris and grade into fans and
alluvial/colluvial deposits along valley margins (Weiss 1994). Deposits of Arapien Shale
are located adjacent to Nine Mile Reservoir and contain high amounts of gypsum and
halite.
There are two major erosional processes within the Nine Mile Reservoir subwatershed:
surface erosion and mass wasting. Surface erosion is the transport of soil particles from
the soil surface. Common causes are meteorological and occur with overland flow caused
by snowmelt, rain impact and runoff, and wind or freeze/thaw forces on steep slopes.
Mass wasting includes all forms of erosion in which large masses of soil are displaced.
Typical mass-wasting events may include small slumps and large debris flows, with
small earth flows and rock falls present locally. The steep slopes of the Wasatch Plateau
along with unstable lithology contribute to these mass-wasting events.
The watershed is ecologically transitional with the eastern half of the watershed within
the Wasatch Montane Zone and Semi Arid Foothills (Omernik and Gallant 1986),
characterized by the high mountains and foothills of the Wasatch Plateau. The western
half of the watershed is found within the Sagebrush Basins and Slope Ecoregion with
geology and soils typical of the valleys in the Great Basin.
Nine Mile Reservoir Delisting February 2008
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2.1.3.2 Soils
Soil data for the Nine Mile Reservoir watershed were collected from the USDA Soil
Conservation Service (USDA SCS) and the State Soil Geographic Database
(STATSGO). Detailed soil maps within the Soil Survey of Sanpete Valley (USDA 1981)
were also used as a reference. Soil locations and extents are detailed in Figure 2.4. Soils
in valley bottoms are mostly medium to fine textured with coarser soils towards valley
margins and in mountainous areas. The dominant soil types in the Nine Mile Reservoir
watershed are detailed in Table 2.5. Soils in the area range in texture from fine silty to
loamy skeletal and generally have medium erodibility ratings with k factors ranging from
0.24 to 0.43. The most common soil association in the Six Mile Creek watershed is the
Ute-Richen-Kildor-Embargo-Cluff-Castino association. This association is a well-
drained, pH neutral, extremely cobbly loam that is derived from sandstone parent
material occurring on alluvial fans, hillslopes, and mountainsides. The most common soil
type in the direct drainage area to Nine Mile Reservoir is the Sanpete-Rock outcrop-
Amtoft. This association is a shallow, well-drained, moderately to strongly alkaline
flaggy loam derived from colluvium and residuum of calcareous rocks found on hills and
mountain ridges.
Table 2.5 Soil Associations and Characteristics in the Nine Mile Reservoir
Watershed
Soil Name Soil
Texture
Soil
Erodibility
(K Factor)
Percent of Six
Mile Creek
Watershed
Percent of Nine Mile
Reservoir Direct
Drainage
Sanpete-Rock outcrop-
Amtoft
Loamy
skeletal .32 5% 39%
Sanpete-Lisade-Freedom-
Denmark-Arapien Fine loamy .37 3% 34%
Woodrow-Quaker-Linoyer-
Genola Fine silty .43 0% 9%
Slickspots-Skumpah-Ravola-
Mayfield 00% 2%
Lodar-Fontreen-Borvant Loamy
skeletal .32 8% 16%
Rogert family-Myton family-
Kamack-Castino family
Loamy
skeletal .32 12% 0%
Ute-Richens-Kildor-
Embargo-Cluff-Castino Fine .28 47% 0%
Rock outcrop-Mower-Lundy-
Lizzant-Hamtah-Agassiz
Loamy
skeletal .24 25% 0%
Sanpete-Rock outcrop-
Amtoft
Loamy
skeletal .32 5% 39%

Nine Mile Reservoir Delisting February 2008
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A more detailed soil analysis was conducted for the area immediately surrounding Nine
Mile Reservoir and the diversion from Six Mile Creek. This analysis used the updated
SSURGO dataset provided by the NRCS in order to assess the natural impact of alkaline
soils in the immediate vicinity. The Genola loam soil just south of Nine Mile Reservoir
has a particularly high pH value (10.1). In addition, other soils surrounding the reservoir
are also alkaline with pH values that range from 8 to 10. These soils include Arapien,
Woodrow, and Poganeab soils and it is reasonable to assume that they extend underneath
Nine Mile Reservoir and naturally contribute to the alkalinity of the reservoir water.
Approximately 70% of the land area in the direct drainage to Nine Mile Reservoir is
covered in soils that have a representative pH value greater than 8.0. (See Figure 3.5)
2.1.4 PLANTS, ANIMALS, AND FISHERIES
2.1.4.1 Upland Plant Communities
Plant community composition in the Nine Mile Reservoir watershed is determined by a
combination of climate and geologic factors including elevation, exposure, soil type and
depth, and moisture or precipitation. Elevations over 8,000 feet (2,500 meters) near the
headwaters of Six Mile Creek are forested with ponderosa pine (Pinus ponderosa),
spruce (Picea spp.), Douglas fir (Pseudotsuga menziesii), white fir (Abies concolor), and
patches of quaking aspen (Populus tremuloides) in the overstory. Understory species
include serviceberry (Amelanchier spp.), alder-leaf mountain-mahogany (Cercocarpus
montanus) and Gambel’s oak (Quercus gambelii).
The pinyon-juniper community is dominant at elevations from 6,000 to 8,000 feet
(1,828–2,500 meters). The overstory consists of pinyon pine (Pinus edulis), Utah juniper
(Juniperus osteosperma), and Gambel’s oak. Dominant understory species include big
sagebrush (Artemisia tridentata), rabbitbrush (Chrysothamnus spp.), squaw apple
(Peraphyllum ramosissimum) and serviceberry (Amelanchier spp.). cheatgrass (Bromus
tectorum), Indian ricegrass (Achnatherum hymenoides), winterfat (Krascheninnikovia
lanata), and wiregrass (Juncus spp.) (Welsh 2003).
The dominant plant species at elevations below 6,000 feet (1,828 meters) include low-
growing rabbitbrush, shadscale (Atriplex spp.), various sagebrushes (Artemisia spp.),
greasewood (Sarcobatus spp.) and a variety of grasses including saltgrass (Distichlis
spicata), wiregrass, Indian ricegrass, and others.
Two substantial wildfires have occurred recently within the upper Simile Creek drainage.
In 1992, 794 acres burned during a prescribed burn and in 2004 approximately 4,794
acres burned due to lightning strikes. The USFS allowed the fire to burn naturally except
where it threatened to impact Sterling, Utah. Approximately 555 acres were common to
both fires.

Nine Mile Reservoir Delisting February 2008

15

Figure 2.4 Soils map of Nine Mile Reservoir watershed
Ninemile Reservoir
Palisade Lake
San Pitch River
Six Mile Creek
Highland Canal
DiversionSterling
0120.5 Miles
0241Kilometers
Area Enlarged
Base map taken from Manti, UT 30 x 60 minuteUSGS topographic quadrangle.
Town
Diversion
Stream/Drainage
Ninemile Reservoir Direct Drainage
Study Area
River/Reservoir
Lodar-Fontreen-Borvant (s8161)
Rock outcrop-Mower-Lundy-Lizzant -Hamtah-Agassiz (s8168)
Rogert family-Myton family-Kamack- Castino family (s7993)
Sanpete-Lisade-Freedom-Denmark- Arapien (s8157)
Sanpete-Rock outcrop-Amtoft (s8162)
Slickspots-Skumpah-Ravola-Mayfield (s8165)
Ute-Richens-Kildor-Embargo-Cluff- Castino (s7994)
Woodrow-Quaker-Linoyer-Genola (s8156)
Monday, April 2, 2007 11:10:45 AMF:\11046\Maps\Report\nm_soils.mxd
Nine Mile Reservoir Delisting February 2008
16
2.1.4.2 Riparian Plant Community
Riparian areas constitute only a small portion of the overall watershed area, but are
ecologically important in terms of plant diversity, wildlife habitat, and erosion control
along waterways. Local riparian communities are characterized by yellow willow (Salix
lutea), whiplash willow (Salix exigua), wild rose (Rosa woodsii), wiregrass (Juncus spp.)
and Nebraska sedge (Carex nebrascensis). Other associated riparian plant species include
Douglas sedge (Carex douglasii), veronica (Veronica spp.), golden currant (Ribes
aureum), redtop (Agrostis gigantea), clover (Trifolium spp.), trefoil (Lotus spp.), and
narrowleaf cottonwood (Populus angustifolia) (USFS 2005 and Welsh 2003).
2.1.5 WILDLIFE
Wildlife found in the Nine Mile Reservoir watershed are indicative of pinyon-juniper
woodlands, mountain brush communities, willow/riparian, and rock habitats at mid-
elevations in Utah. Game species include mule deer, Rocky Mountain elk, and wild
turkey. Other mammals in the area include bobcat, coyote, red squirrel, and various
smaller rodents. Several bat species can be observed at night flying along the roadway
adjacent to Six Mile Creek. Other nocturnal wildlife species include great-horned owls,
striped skunks, and raccoons.
The most common birds found during the spring and summer months include western
scrub jay, mourning dove, American robin, yellow warbler, spotted towhee, chipping
sparrow, vesper sparrow, red-winged blackbird, western meadowlark, and less often,
yellow-breasted chat, and broad-tailed hummingbird. Turkey vultures and common
ravens are also seen frequently.
2.1.5.1 Fisheries
Nine Mile Reservoir supports stocked fish species that include rainbow trout, tiger trout,
and a brown trout/brook trout hybrid. Six Mile Creek supports a population of wild
cutthroat trout, while brook trout and rainbow trout have been stocked in some ponds and
reservoirs in the Six Mile Creek drainage (email communication, Utah Division of
Wildlife Resources, Central Region). No documented fish kills have occurred in the
reservoir (UDWQ 2007).
2.1.5.2 Special Designations
The Utah Division of Wildlife Resources (UDWR) has records of occurrence for
Bonneville cutthroat trout and Nine Mile pyrg (spring snail) within the project area. In
addition, in the vicinity, there are records of occurrence for bald eagle, Colorado River
cutthroat trout, and leatherside chub. All of the aforementioned species are included in
the Utah Sensitive Species List (Utah Division of Wildlife Resources 2006).
2.2 CULTURAL CHARACTERISTICS
Primary cultural influences in the Sanpete Valley have included the Fremont-Sevier
agriculturalists (present until around A.D. 1300), the San Pitch Tribe and Ute Tribe, and
the Mormon settlers. The latter came to the Manti area in the fall of 1849, choosing to
settle in the area because of the warm spring nearby, good agricultural land and soils, and
the nearby limestone quarries (Utah History Online 2006).
Nine Mile Reservoir Delisting February 2008
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Sterling, Utah, located on U-89 midway between Manti and Gunnison, is the closest
population center to Nine Mile Reservoir. The town, settled in 1873, is reported to have
been named for the "sterling" qualities of its people, and has been known by various
names including Pettyville, Pettytown, Leesburg, and Buncetown (Utah History Online
2006).
2.2.1 LAND USE AND OWNERSHIP
The watershed is predominantly forested, both public and private. The largest landowner
is the USFS which manages 69.8% of the watershed area as part of the Manti-LaSal
National Forest. Private land consists of 15.6%, and Utah State land consists of 14.6%.
The private land is used for agricultural purposes including crops and grazing.
The town of Sterling is located inside of the watershed. Residential areas comprise less
than one percent of the total watershed area. Historically, land use within the watershed
was primarily forestry and agriculture. Numerous poultry farms are located in the area
and livestock grazing is also a key activity.
Geographic information system (GIS) coverages, satellite imagery, aerial photographs,
and other cartographic resources were employed in the preparation of this document to
determine accurate land use (Figure 2.5, Table 2.7) and ownership (Figure 2.6, Table 2.6)
values for the Nine Mile Reservoir Watershed on a subwatershed basis.
Table 2.6 Land Ownership within the Nine Mile Watershed Study Area
Six Mile Creek
Watershed
Nine Mile Reservoir
Direct Drainage
Total Study
Area
Area
(acres)
Percentage
of Total
Land
Area
(acres)
Percentage
of Total
Land

Percentage of
Total Land
US forest service (USFS) 19,696.1 77.2% 0.0 0.0% 69.8%
Private 2,785.6 10.9%1,622.0 59.5% 15.6%
State wildlife
reserve/management 2,827.2 11.1% 764.3 28.0% 12.7%
State trust land 92.7 0.4% 221.4 8.1% 1.1%
Water 51.2 0.2% 118.8 4.4% 0.6%
State parks and recreation 48.2 0.2% 0.0 0.0% 0.2%
TOTAL 25,501.0 100.0%2,726.5 100.0% 100.0%

Nine Mile Reservoir Delisting February 2008
18

Table 2.7 Land Use within the Nine Mile Watershed Study Area
Six Mile Creek Watershed Nine Mile Reservoir
Direct Drainage
Total Study
Area

Area (acres)
Percentage
of Total
Land
Area
(acres)
Percentage
of Total
Land
Percentage of
Total Land
Evergreen forest 12,236.7 48.0% 540.9 19.8% 45.3%
Shrub/scrub 5,353.9 21.0% 1,442.1 52.9% 24.1%
Deciduous forest 3,717.8 14.6% 0.0 0.0% 13.2%
Mixed forest 1,584.1 6.2% 0.0 0.0% 5.6%
Developed uses 259.8 1.0% 118.5 4.3% 1.3%
Grassland/herbaceous 1,097.9 4.3% 14.4 0.5% 3.9%
Barren land
(rock/sand/clay) 778.6 3.1% 31.5 1.2% 2.9%
Pasture/hay 343.6 1.3% 318.0 11.7% 2.3%
Open water 82.7 0.3% 196.9 7.2% 1.0%
Cultivated crops 29.1 0.1% 64.1 2.4% 0.3%
Woody wetlands 16.7 0.1% 0.0 0.0% 0.1%
TOTAL 25,500.9 100.0%2,726.4 100.0% 100.0%

2.2.2 POPULATION
The town of Sterling is the only population center within the watershed boundary and is
located approximately 1.5 miles northeast of the Nine Mile Reservoir. Total population
figures for Sterling average approximately 251 individuals (US Census Bureau 2007). In
addition to the local resident population, tourism and recreational opportunities have
created some transient (county and non-county resident) visitor use on Nine Mile
Reservoir and within National Forest areas. Total population figures for the Nine Mile
Reservoir watershed area are estimated at approximately 300 individuals, the majority of
which (200) live in the town of Sterling and adjacent unincorporated areas.
Nearby communities include Mayfield, approximately 5 miles to the south (population
425); Manti, the county seat, approximately 6 miles to the northeast (population 3,185);
Gunnison, approximately 9 miles west-southwest (population 2,700); Centerfield,
approximately 10 miles to the southwest (population 1,051); Fayette, approximately 12
miles to the west-northwest (population 204); Ephraim, approximately 14 miles to the
northeast (population 4,977); and Redmond, approximately 18 miles to the southwest
(population 790). Populations listed are those projected for 2005 by the 2000 US Census
figures (US Census Bureau 2007)
Many of these communities experienced substantial growth during the 1990s. An average
of 40% increase in population occurred in those communities located within a 10-mile
radius of Sterling. Similar growth is projected to occur within the county over the next
decade, with an associated conversion of agricultural land use to rural residential land use
(City Data 2006).

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Figure 2.5 Land ownership and populated areas map for Nine Mile Reservoir watershed
Ninemile Reservoir
Palisade Lake
San Pitch River
Six Mile Creek
Highland Canal
DiversionSterling
0120.5 Miles
0241Kilometers
Area Enlarged
Base map taken from Manti, UT 30 x 60 minuteUSGS topographic quadrangle.
Town
Diversion
Ninemile Reservoir Direct Drainage
Study Area
Stream/Drainage
River/Reservoir
Landowner
BLM
DOD
USFS National Forest
Private
State
UDWR Wildlife Reserve/Management Area
USP Parks and Recreation
Water
Monday, April 2, 2007 11:23:02 AMF:\11046\Maps\Report\nm_ownerhip.mxd
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Figure 2.6 Land use map for Nine Mile Reservoir watershed
Ninemile Reservoir
Palisade Lake
San Pitch River
Six Mile Creek
Highland Canal
Sterling
Diversion
0120.5 Miles
0241Kilometers
Area Enlarged
Imagery taken from National Agricultural ImageryProgram (NAIP) natural color aerial photography1-meter resolution, 2006
Town
Diversion
Ninemile Reservoir Direct Drainage
Study Area
Stream/Drainage
River/Reservoir
Road
DESCRIPTN
Barren Land (Rock/Sand/Clay)
Cultivated Crops
Deciduous Forest
Developed, Low Intensity
Developed, Medium Intensity
Developed, Open Space
Evergreen Forest
Grassland/Herbaceous
Mixed Forest
Open Water
Pasture/Hay
Shrub/Scrub
Woody Wetlands
Monday, April 2, 2007 10:45:57 AMF:\11046\Maps\Report\nm_landuse.mxd
Nine Mile Reservoir Delisting February 2008
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2.2.3 HISTORY AND ECONOMICS
Since settlement, Sanpete County's economy has been based almost exclusively on
agriculture. Cattle dominated in the early years following settlement, but only a few large
dairy farms still operate in the region.
Sheep dominated the local economy from the 1880s through the 1920s, and the county
held a prominent role in world markets for a time. During the 1920s, Sanpete County's
sheep herds were the largest in Utah (RMRS 2005).
Snow College, a two-year college located in Ephraim, also plays an important role in the
local economy. (Populations listed are those projected for 2005 by the 2000 (US Census
Bureau 2007).

Figure 2.7 Sheep being moved from high-elevation summer pasture to winter range
in the desert, Sanpete County, circa 1930 (AITC 2005).
During this period, high animal densities and lack of understanding of appropriate land
management practices led to severe overgrazing in the high-elevation watersheds on the
Wasatch Plateau in central Utah. This condition resulted in catastrophic flooding and
mudflows through adjacent communities. Manti, Utah was inundated by damaging flood
waters, as were other local communities, and affected citizens petitioned the Federal
government to establish a forest reserve in 1902. The Manti National Forest was
established by the Transfer Act of 1905, and the Great Basin Station, a forerunner of the
Intermountain Forest and Range Experiment Station, was created in 1911 to study the
influence of rangeland vegetation on erosion and floods. Terracing along the hillsides on
the east side of Sanpete Valley to reduce runoff and encourage infiltration of snowmelt
and rainfall, is still visible within the watershed and adjacent areas. Practices developed
and studies conducted at the Great Basin Experiment Station have enhanced and updated
many of the interpretations and guidelines for the management of high-elevation
watersheds throughout the intermountain west (RMRS 2005).
Nine Mile Reservoir Delisting February 2008
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Revegetation following overgrazing has resulted in changes to the plant communities in
high-elevation watersheds, and current vegetation composition is significantly different
than the original condition. New plant communities have reached thresholds where the
vegetative composition appears to be climate driven. Many of the higher elevation areas
however, still remain in unsatisfactory condition and experience routine active erosion
events (RMRS 2005).
Sheep remain an important element in the state's and Sanpete County's agricultural
economy, but the high densities that occurred in the 1920s are no longer present.
Turkeys, originally grown only casually, became a necessary and major cooperative as a
result of the 1930s Great Depression which led to a severe decline in wool prices. Today,
poultry is the dominant agricultural product of the county, which ranks among the top ten
turkey-producing counties in the United States (Utah History Online 2006).
The current economy of the region is based primarily on agricultural industries, generally
centered on livestock and poultry farming, with increasing input from tourism and
recreation on National Forest lands and wilderness areas. Livestock is grazed on public
and private land within the watershed and adjacent to the reservoir.
2.2.4 RECREATIONAL USES OF NINE MILE RESERVOIR
2.2.4.1 Boating and Related Activities
Although the reservoir is privately owned, access to the reservoir is unlimited. Access to
the Nine Mile Reservoir is via US-89 south of Sterling at the U-137 junction. The major
recreation activity for the reservoir is fishing from boats. This activity is limited during
the later part of the summer season as the reservoir levels drop to low levels.
2.2.4.2 Fishing, Hunting, and Wildlife Observation
The primary recreational use of the reservoir is fishing, although usage has been reported
as light. The Division of Wildlife Resources stocks the reservoir annually with either
15,000 catchable rainbow trout or 3,000 catchable and 25,000 advanced fingerling
rainbow trout. The reservoir was drained in 1981 to raise the dam to a new elevation. The
reservoir was also chemically treated in 1959 and 1970 by the DWR to reduce rough fish
levels.
2.2.4.3 Camping
There are no camping facilities located near the reservoir. Camping facilities are located
in the watershed at the Palisade State Park, located east of Sterling. There is also a USFS
campground located in Manti Canyon.
2.2.5 PUBLIC INVOLVEMENT
Prior to beginning the TMDL planning process for Nine Mile Reservoir, the San Pitch
River Watershed Stewardship Committee was established to inform the San Pitch River
Watershed Water Quality Management Plan, completed in 2003 (UDWQ 2003).
During the initiation of the Subbasin Assessment process, a structured citizen
involvement program was established and included participation by members of the San
Pitch River Watershed Stewardship Committee as an advisory group to the TMDL
process. This program was established so that the community could provide direction and
Nine Mile Reservoir Delisting February 2008
23
leadership in developing and implementing this plan. The watershed advisory group
membership includes local representatives from all major sectors of the local community
as follows:
• Agricultural interests
• Gunnison Irrigation Company
• Sterling/Six Mile Irrigation Company
• Citizens at large
• City of Sterling
• Local development representative
• Environmental concerns
• Sporting or recreational interests
• Timber interests
• Sanpete County Commissioners
• USFS Manti-LaSal National Forest
• US Bureau of Land Management
• Utah Division of Wildlife Resources
Nine Mile Reservoir Delisting February 2008
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3 WATER QUALITY CONCERNS AND STATUS
3.1 WATER QUALITY LIMITED WATER BODIES
As stated in the opening sections of this document, the main purpose of the CWA is to
improve and protect water quality through restoration and maintenance of the physical,
chemical and biological integrity of the nation's waterways. Under section 303(d) of the
CWA, each state must submit a list to the EPA identifying waters throughout the state
that are not achieving state water quality standards in spite of the application of
technology-based controls in NPDES permits. The waters identified on the 303(d) list are
known as water quality limited. Nine Mile Reservoir was identified under section 303(d)
of the CWA in 2006, as water quality limited due to low dissolved oxygen and excess
phosphorus loading to the reservoir from the surrounding watershed.
3.2 BENEFICIAL USE CLASSIFICATIONS FOR NINE MILE RESERVOIR
The State of Utah has designated the beneficial uses of Nine Mile Reservoir to be
secondary contact recreation (classification 2B), cold water game fish and their
associated food chain (3A), and agricultural water supply (4). The cold water game fish
designated use (3A) was identified as non-supporting on the State of Utah 2006 303(d)
list. Secondary contact recreation and agricultural water supply designated uses were
reported as being fully supported on this same list.
Recreation classifications are for water bodies that are suitable or are intended to be made
suitable for primary and secondary contact recreation; this includes fishing for
consumption. Secondary contact recreation refers to uses where intimate human contact
and ingestion of water is expected to occur to a lesser degree such as fishing, boating and
wading.
Waters designated and supportive of cold water game fish and associated food chain use
are required to exhibit appropriate levels of dissolved oxygen, temperature, pH,
ammonia, and turbidity. Nine Mile Reservoir is not listed as impaired for temperature or
pH. The analysis presented in Section 3.5 of this assessment indicates that exceedances of
water quality criteria are associated with natural conditions.
Waters designated as agricultural water supply (including irrigation water and livestock
watering) are required to be suitable for the irrigation of crops or as drinking water for
livestock. Waters designated for agricultural water supply are required to meet general
surface water quality criteria for toxic materials. These waters are also required to meet
narrative criteria related to sediment and excessive nutrients.
3.3 APPLICABLE WATER QUALITY STANDARDS
Water quality standards under the CWA consist of three main components: designated
beneficial uses, water quality criteria that are established to protect designated beneficial
uses, and antidegradation policies and procedures.
Water quality criteria can be either numeric limits for individual pollutants and
conditions, or narrative descriptions of desired conditions. Table 3.1 summarizes the
applicable State of Utah water quality criteria and lists specific citations where the full
code language can be found.
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Table 3.1 Water Quality Numeric Criteria and Pollution Indicator Values Specific to Nine Mile Reservoir Found in Utah State Code RS
317-2-14
Parameter
and
Designated
Beneficial
Use
Criterion Utah State
Code Table Comments
Bacteria
2B <206 E coli organisms per 100 ml as a 30 day geometric mean;
AND maximum less than 940 E coli organisms per 100 ml.
Table 2.14.1
3A N/A
4 N/A
Dissolved Oxygen (DO)
2B N/A
3A No less than 6.5 mg/L (30-day average), 9.5 early life stages/5.0
all life stages (7-day average), 8.0 early life stages/4.0 all life
stages (1-day average).
Table 2.14.2 Footnote #2: These limits are not applicable to lower water levels
in deep impoundments.
4 N/A
Biological Oxygen Demand (BOD)
2B No greater than 5 mg/L Table 2.14.1
3A No greater than 5 mg/L Table 2.14.2
4 No greater than 5 mg/L Table 2.14.1
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Table 3.1 Water Quality Numeric Criteria and Pollution Indicator Values Specific to Nine Mile Reservoir found in Utah State Code RS
317-2-14, continued
Nutrients–Ammonia as N
2B N/A
3A mg/L as N (Acute) = 0.275/(1+10 E7.204-pH)) + (39.0/(1+10
EpH-7.204))
Table 2.14.2
4 N/A

Nutrients - Nitrate as N
2B No greater than 4 mg/L Table 2.14.1
3A No greater than 4 mg/L Table 2.14.2
4 N/A
Footnote #5: Investigations shall be conducted to develop more
information where these pollution indicator levels are exceeded.
Nutrients - Total Phosphate as P
2B No greater than 0.05 mg/L Table 2.14.1
3A No greater than 0.05 mg/L Table 2.14.2
4 N/A
Footnote #5: Investigations shall be conducted to develop more
information where these pollution indicator levels are exceeded.
Footnote #12: Total phosphorus as P (mg/L) limit for lakes and
reservoirs shall be 0.025 mg/L.
pH
2B No less than 6.5 AND no greater than 9.0 pH units Table 2.14.1
3A No less than 6.5 AND no greater than 9.0 pH units Table 2.14.2
4 No less than 6.5 AND no greater than 9.0 pH units Table 2.14.1
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Table 3.1 Water Quality Numeric Criteria and Pollution Indicator Values Specific to Nine Mile Reservoir found in Utah State Code RS
317-2-14, continued
Turbidity
2B No greater than 10 NTU increase Table 2.14.1
3A No greater than 10 NTU increase Table 2.14.2
4 N/A
Total Dissolved Gas
2B N/A
3A Not to exceed 110% of saturation. Table 2.14.2
4 N/A
Total Dissolved Solids
2B N/A
3A N/A
4 < 1,200 mg/L (irrigation), < 2,000 (stock watering) Table 2.14.1

Temperature
2B N/A
3A No greater than 20°C, No greater than 2°C change Table 2.14.2
4 N/A
Footnote #3: The temperature standard shall be at background
where it can be shown that natural or un-alterable conditions
prevent its attainment. In such cases rulemaking will be
undertaken to modify the standard accordingly.
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3.4 SUMMARY AND ANALYSIS OF EXISTING WATER QUALITY DATA
3.4.1 WATER QUALITY DATA COVERAGE
The available dataset covers a range of water years and a variety of physical, chemical and
biological water quality constituents. To better evaluate the existing dataset, available data were
divided into several subsets to allow identification of temporal, spatial, and constituent coverage
and completeness in both a general and a specific fashion. Identified water quality concerns in
the Nine Mile Reservoir system were used as the primary basis for data collection and
delineation. Therefore, while additional data exist (such as metal and pesticide concentration
information), they have not been included in this data summary.
3.4.1.1 Temporal Coverage
Data available for this TMDL process has been divided into the following three categories: 1982
to 1992 (historic), 1993 to 1998 (recent), and 1999 to present (current). Data collected prior to
1982 will be categorized as "legacy data" and its use will be restricted to trend analysis within
the study.
Monitoring data included in this data summary are available from the mid-1970s through mid-
2006, covering a wide range of water years and flow scenarios. Data to be used for this TMDL
process have been restricted to data from 1999 to 2006. As detailed in Table 3.4, some
monitoring locations have consistent data throughout this time period, while others were
collected intermittently or for a single year or event.
Data collected prior to 1982 were assumed not to be representative of current conditions in the
watersheds and were therefore excluded from the water quality assessment database. In addition,
these data may have inherent liabilities associated with outdated sampling or analysis methods
resulting in a condition where direct comparison cannot be made between old and current
measurements. Additionally, flow, diversion, and land use management within the watershed has
changed considerably in some cases since the early 1980s and transport and delivery
relationships derived from early data are not likely to be representative of current conditions.
It should be noted that much of the data from the early 1990s through 2004 were collected under
moderate to extreme drought conditions. Physical water quality characteristics such as
temperature and dissolved oxygen concentrations measured during these water years will be
representative of critical watershed conditions as drought generally exacerbates such conditions
within the watershed. The most current data have been used for assessment of criteria or
threshold exceedance, pollutant transport and processing, and pollutant loading analyses.
Current data were the primary source of information used to determine the support level of
designated beneficial uses, and will be employed to help define appropriate endpoints or
thresholds (if applicable) for the Nine Mile Reservoir system.
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Figure 3.1 Annual discharge volumes for the Sevier River below the San Pitch River near
Gunnison, Utah from 1975 through 2005 (USGS Gage #10217000).
3.4.1.2 Hydrological Coverage
Data were collected over a wide range of hydrological conditions. As gaged flows were not
available for the Nine Mile watershed, nearby drainages with continuous gage information were
used as a surrogate measure of relative flow volume and intensity. Annual total flow volumes
calculated from gaged flows in the Sevier River below the San Pitch River inflow near
Gunnison, Utah (USGS gage # 10217000) are displayed in Figure 3.1.
Based on the assumption that ungaged flows in the Nine Mile Reservoir watershed are similar in
trend volume and flow-to-gaged flows in nearby drainages (Sevier River below the San Pitch
River inflow near Gunnison, Utah) early water years (1982–1992) represent low–average to
above average water years (average flow at 10217000 gage was 145% of the 30-year average for
that time period). More recent water years (1993 through 1998) represent primarily low to
average flow conditions (average flow at 10217000 gage was 93% of the 30-year average for
that time period). Current water years (1999 through 2006) are generally well below average
with the exception of 2005 which had a moderately high-water year at the gage location (average
flow at 10217000 gage was 66% of the 30-year average for that time period). Water years 2002
through 2004 represent years with less than 40% of the 30-year average flow values at the gage
location.
Annual flow volumes and ranking relative to the 30-year average for USGS gage #10217000 are
displayed in Table 3.2.
Current water quality data collected in the Nine Mile Reservoir watershed are representative of a
wide range of flow values and describe both very low (2002 and 2004, 39% of the 30-year
average flow volume measured at the Sevier River below the San Pitch River inflow near
Gunnison, Utah) and average (1999 and 2005, 118% and 114% of the 30-year average flow
volume measured at the Sevier River below the San Pitch River inflow near Gunnison, Utah)
hydrologic conditions. Therefore, data collected between 1999 and 2006 are expected to be
representative of high flow and low flow (critical conditions) within the watershed.
USGS 10217000 SEVIER RIV BLW SAN PITCH RIV NR GUNNISON, UT
19
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(
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)
Nine Mile Reservoir Delisting February 2008

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Data collection is somewhat weighted toward the late spring to late fall months, with fewer
winter data points in most datasets.
Assuming that ungaged flows in the Nine Mile Reservoir watershed are similar in trend volume
and flow-to-gaged flows in nearby drainages (Sevier River below the San Pitch River inflow
near Gunnison, Utah), seasonal data collection are assumed to cover the range of high (spring
runoff in May and June) and low (July, August and September) seasonal flows over the course of
a year.
On both a water year and on a seasonal basis, available data collection times were compared with
representative precipitation indices for high, average, and low water years. Data collection
generally occurred over the critical range of flow and precipitation regimes, indicating (to the
extent possible) that data coverage is representative of an adequate variety of flow and
precipitation events.
3.4.1.3 Spatial Coverage
Surface water quality data are available for Nine Mile Reservoir and its associated watershed
(Table 3.3). Sites represent inflow, in-reservoir, and outflow conditions. While all sites do not
share the same level of data density, cumulatively, these monitoring sites represent good spatial
coverage of the reservoir system.
While the sites identified provide good inflow and in-reservoir information, the outflow data
available is from two limited sample suites collected in the summer of 2006 and may not be
representative of all flow and/or water quality conditions.
3.4.2 STATISTICAL OVERVIEW
Primary information sources for water quality data used in this analysis included the EPA
STORET website, Utah Division of Water Quality (UDWQ), Utah Department of Water
Resources (UDWR), Utah Geological Survey (UGS), Utah Department of Natural Resources
(UDNR), US Geological Survey (USGS), US Forest Service (USFS), US Bureau of Land
Management (BLM), Natural Resources Conservation Service (NRCS), Gunnison Irrigation
Company, Sanpete County, state and local soil and water conservation services, irrigation
districts, and their associated databases, and others.
Groundwater flow and volume information is general in nature and available almost exclusively
from USGS, UGS and county studies and reports. Climate information was obtained from
WRCC and SNOTEL sites.
The UDWQ, USGS, EPA, and others have been monitoring water quality at a number of sites in
the Nine Mile watershed since the mid 1970s. Locations for which water quality information is
available include in-reservoir monitoring sites and Six Mile Creek, as well as other sites such as
groundwater wells.
Five intensive water quality monitoring sites were identified as appropriate to the TMDL efforts,
one site on Six Mile Creek upstream of the reservoir, two in-reservoir sites (Mid-reservoir Site
and Dam Site), one location on the inflow to the reservoir, and the canal receiving the outflow
from the reservoir. In addition, SWCA collected data from springs flowing into the Nine Mile
Reservoir. A listing of all sites used in this analysis and a summary of the raw data available is
presented in Tables 3.3 and 3.4.
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Table 3.2 Annual Flow Volumes and Ranking Relative to the 30-
year Average for the Sevier River at USGS gage #10217000
Water Year Flow (cfs) Percent of 30-year Average Flow
1970 315 94%
1971 238 71%
1972 165 49%
1973 349 104%
1974 298 89%
1975 217 65%
1976 181 54%
1977 117 35%
1978 148 44%
1979 249 74%
1980 435 130%
1981 276 82%
1982 277 83%
1983 1,130 338%
1984 1,346 403%
1985 787 235%
1986 539 161%
1987 356 106%
1988 275 82%
1989 204 61%
1990 135 41%
1991 147 44%
1992 132 39%
1993 261 78%
1994 164 49%
1995 458 137%
1996 246 73%
1997 292 87%
1998 438 131%
1999 395 118%
2000 246 74%
2001 147 44%
2002 130 39%
2003 126 38%
2004 132 39%
2005 381 114%
30-year average 334 100%

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In total, over 5,000 data points were identified and assessed for Nine Mile Reservoir, covering
the time period from 1979 through 2006.
Early monitoring consisted primarily of field parameters, nutrients, oxygen demand, dissolved
ions and metals analyses, and groundwater studies. This work was followed in the 1990s with
pesticide analyses, more in-depth nutrient and organic carbon studies, bacterial analysis, and
some trophic status-related parameters.
Recent and current data contain a variety of field parameters, nutrient, sediment, dissolved ion,
and metals analyses (Table 3.4). Quantitative data were not available for stream channel stability,
riparian corridor health, or stream morphology for the reservoir or tributary sites. Biological data
are primarily speciation of algal populations.
Water quality constituents determined to be critical to the assessment of designated beneficial
use support status are represented in Table 3.4. These constituents include those related to low
dissolved oxygen, excess nutrients, eutrophication, and water quality criteria specific to
beneficial uses in Nine Mile reservoir as defined in Table 3.1.

Table 3.3 Water Quality Monitoring Sites for the Nine Mile Reservoir Watershed
Org
Name Station ID Station Name
UDEQ 5943240 NINE MILE RES AB DAM 01 (Dam Site)
SWCA None NINE MILE RES AT DAM (Dam Site)
SWCA None NINE MILE RES INFLOW
UDEQ 5943260 NINE MILE RES INFLOW
UDEQ 5943250 NINE MILE RES MID-RESERVOIR 02 (Mid-reservoir Site)
SWCA None NINE MILE RESERVOIR OUTFLOW
UDEQ 4946190 HIGHLAND CNL AT US89 BL NINE MILE RES
5943430 HIGHLAND CNL BL NINE MILE RES
UDEQ 4946360 SIX MILE CK AB CNFL / SAN PITCH R NW OF STERLING
SWCA None SIX MILE CREEK AT DIVERSION TO NINE MILE RES
SWCA None SPRING #1 DIRECT INFLOW TO NINE MILE RESERVOIR
SWCA None SPRING #2 DIRECT INFLOW TO NINE MILE RESERVOIR
SWCA None SPRING #3 DIRECT INFLOW TO NINE MILE RESERVOIR
In-reservoir monitoring by UDEQ includes both grab (instantaneous) samples and depth-integrated profile data for some
parameters. Sites with Station ID #s are intensive monitoring sites managed by UDEQ-DWQ.
Nine Mile Reservoir Delisting February 2008

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Table 3.4 Summary of Data Available for Nine Mile Reservoir and Watershed
Characteristic N Start Stop Max Min Mean Median
5943260 NINE MILE RES INFLOW
Alkalinity, Carbonate as CaCO3 1 6/12/90 6/12/90 272 272 272 272
Dissolved oxygen (DO) 22 6/12/90 8/18/06 9.9 5.4 7.22 7.84
Dissolved solids 17 6/12/90 8/18/06 740 224 605 668
Flow 25 5/27/92 1/26/06 8.0 0.0 1.96 1.5
Nitrogen, ammonia as N 18 6/12/90 11/17/05 0.09 Nondetect 0.07 0.02
Nitrogen, Kjeldahl 4 6/12/90 8/18/06 Nondetect Nondetect Nondetect Nondetect
Nitrogen, Nitrite (NO2) + Nitrate
(NO3) as N 18 5/27/92 8/18/06 6.730 Nondetect 4.37 6.25
pH 23 6/12/90 8/18/06 8.7 7.6 8.1 8.06
Phosphorus as P total 19 5/27/92 8/18/06 0.274 Nondetect 0.047 0.01
Phosphorus as P dissolved 17 6/12/90 8/18/06 0.03 Nondetect 0.01 0.005
Specific conductance 22 6/12/90 8/18/06 1234 394 897.25 1023
Temperature, water 22 6/12/90 8/18/06 26.4 12.4 17.3 17.0
Total suspended solids (TSS) 17 6/12/90 11/17/05 269 4 33.2 8
Turbidity 2 6/12/90 8/18/06 1627.4 1.3 814.35 814
Volatile solids 10 5/27/92 5/28/02 31.0 2.0 4.78 2.0
4946360 SIX MILE CK AB CNFL / SAN PITCH R NW OF STERLING
Alkalinity, Carbonate as CaCO3 33 5/6/97 5/6/97 338 205 244.9 244
Dissolved oxygen (DO) 34 9/2/76 8/18/06 11.6 6.8 9.04 9.1
Dissolved solids 36 9/2/76 8/18/06 890 218 352 290
Flow 32 7/26/77 6/20/02 100.0 0.0 14.2 4.0
Nitrogen, ammonia as N 30 10/13/76 6/20/02 0.10 Nondetect 0.03 0.10
Nitrogen, Kjeldahl 8 9/2/76 8/18/06 2.50 0.20 0.70 0.40
Nitrogen, Nitrite (NO2) + Nitrate
(NO3) as N 28 4/2/96 8/18/06 2.800 0.140 0.512 0.330
pH 73 9/2/76 8/18/06 9.1 8.0 8.5 8.5
Phosphorus as P total 32 9/2/76 8/18/06 0.301 Nondetect 0.068 0.049
Phosphorus as P dissolved 26 4/2/96 8/18/06 0.097 Nondetect 0.005 0.013
Specific conductance 64 9/2/76 8/18/06 1490 308 599 512
Temperature, water 36 9/2/76 8/18/06 23.0 1.4 10.8 10.2
Total suspended solids (TSS) 34 9/2/76 6/20/02 829 Nondetect 110.65 54
Turbidity 33 9/2/76 8/18/06 223.0 0.4 51.6 15.0
5943240 NINE MILE RES AB DAM 01
Alkalinity, Carbonate as CaCO3 23 5/21/81 9/8/05 373 221 288 290
Chlorophyll a, uncorrected for
pheophytin 21 6/12/90 9/8/05 26.1 Nondetect 4.27 1.8
Dissolved oxygen (DO) 138 6/12/90 8/18/06 15.6 0.3 8.9 9.7
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Table 3.4 Summary of Data Available for Nine Mile Reservoir and Watershed
Characteristic N Start Stop Max Min Mean Median
Dissolved solids 23 5/21/81 8/18/06 934 336 550 530
Nitrogen, ammonia as N 56 5/21/81 9/8/05 0.52 Nondetect 0.09 0.0625
Nitrogen, Kjeldahl 11 5/21/81 7/19/06 1.38 0.19 0.53 0.40
Nitrogen, Nitrite (NO2) + Nitrate
(NO3) as N 52 5/27/92
7/19/06
2.130 Nondetect 0.28 0.175
pH 155 6/12/90 8/18/06 10.1 7.8 9.0 9.0
Phosphorus as P total 56 5/21/81 7/19/06 0.586 Nondetect 0.036 0.01
Phosphorus as P dissolved 52 9/7/90 7/19/06 0.31 Nondetect 0.02 0.01
Secchi disk depth 22 6/12/90 9/8/05 8.3 0.2 2.9 2.5
Specific conductance 147 5/21/81 8/18/06 1422 422 824 801
Temperature, water 138 6/12/90 8/18/06 25.8 14.1 19.85 19.8
Total suspended solids (TSS) 24 5/21/81 9/8/05 54 Nondetect 8.13 3
Turbidity 23 5/21/81 8/18/06 24.0 0.5 4.56 2.9
Volatile solids 12 5/27/92 8/8/02 11.0 Nondetect 3.25 2.0
5943250 NINE MILE RES MID-RESERVOIR 02
Alkalinity, Carbonate as CaCO3 2 6/12/90 9/7/90 297 259 278 278
Chlorophyll a, uncorrected for
pheophytin 17 6/12/90 9/8/05 43.2 Nondetect 5.6 1.8
Dissolved oxygen (DO) 102 6/12/90 8/18/06 15.2 0.6 9.43 10.0
Dissolved solids 2 6/12/90 7/19/06 586 494 540 540
Nitrogen, ammonia as N 35 5/21/81 9/8/05 0.35 Nondetect 0.09 0.064
Nitrogen, Kjeldahl 6 5/21/81 7/19/06 1.26 0.15 0.58 0.55
Nitrogen, Nitrite (NO2) + Nitrate
(NO3) as N 31 5/27/92
7/19/06
1.180 Nondetect 0.29 0.155
pH 104 6/12/90 8/18/06 9.9 7.9 9.0 9.0
Phosphorus as P total 34 5/21/81 7/19/06 0.282 Nondetect 0.032 0.01
Phosphorus as P dissolved 32 9/7/90 7/19/06 0.021 Nondetect 0.008 0.0075
Secchi disk depth 18 6/12/90 9/8/05 6.2 0.3 3.0 3.1
Specific conductance 102 6/12/90 8/18/06 1372 56 796 797
Temperature, water 102 6/12/90 8/18/06 24.4 13.8 20.0 19.9
Total suspended solids (TSS) 16 6/12/90 9/8/05 32 Nondetect 5.28 2
Turbidity 2 6/12/90 8/18/06 5.0 2.0 3.5 3.5
Volatile solids 9 6/28/94 5/28/02 12.0 Nondetect 3.88 8.0
Springs Discharging
to the Reservoir
Dissolved oxygen (DO) 5 7/19/06 8/18/06 7.2 2.72 6.8
Dissolved solids 6 7/19/06 8/18/06 1300 321 711 630
Flow 5 7/19/06 8/18/06 0.024 0 0.005 --
Nitrogen, Kjeldahl Total 3 7/19/06 7/19/06 0.21 0.07 0.12 0.09
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Table 3.4 Summary of Data Available for Nine Mile Reservoir and Watershed
Characteristic N Start Stop Max Min Mean Median
Nitrogen, Nitrite (NO2) + Nitrate
(NO3) as N 3 7/19/06 7/19/06 6.700 0.025 2.26 --
pH 5 7/19/06 8/18/06 8.4 7.4 8.1 8.3
Phosphorus as P total 3 7/19/06 7/19/06 0.016 0.006 0.011 0.012
Phosphorus as P dissolved 3 7/19/06 7/19/06 0.014 0.005 0.009 0.007
Specific conductance 5 7/19/06 8/18/06 2180 672 1352 1126
Temperature, water 5 7/19/06 8/18/06 22.3 12.0 18.1 21.6
Turbidity 5 7/19/06 8/18/06 2636.0 484.5 624.1 1560.3

3.4.2.1 Analytical Methods
Data collected and assessed for the Nine Mile Reservoir TMDL consisted of samples evaluated
by four primary categories of analytical methodology: APHA, USEPA, Utah DWQ generic, and
Utah DWQ field methods.
3.4.2.1.1 APHA Methods.
These methods refer to the American Public Health Association (APHA 1992). APHA-approved
methods specific to the available database for Nine Mile Reservoir TMDL include analytical
procedures for measuring alkalinity, chemical oxygen demand, chloride, chlorophyll, dissolved
solids, fecal coliform bacteria, fecal streptococcus group bacteria, fixed solids, pH, total coliform
bacteria, total organic carbon, total suspended solids, volatile solids, and others not pertinent to
this TMDL effort.
3.4.2.1.2 USEPA Methods
These methods refer to methods approved by the US Environmental Protection Agency (EPA
1983). USEPA-approved methods specific to the available database for Nine Mile Reservoir
TMDL include analytical procedures for measuring ammonia, biochemical oxygen demand,
chloride, nitrate + nitrite, phosphorus, specific conductance, total suspended solids, turbidity,
volatile solids, and others not pertinent to this TMDL effort.
3.4.2.1.3 Utah DWQ Generic Methods (generic method and generic method 2)
These refer to the Utah Division of Water Quality (DWQ) methods entered in the STORET
database.
UTAH DWQ generic methods (generic method and generic method 2) specific to the available
database for Nine Mile Reservoir TMDL include measurements of alkalinity, ammonia,
biochemical oxygen demand, chemical oxygen demand, chloride, chlorophyll a, nitrate, nitrate +
nitrite, pH, orthophosphate, phosphorus, specific conductance, total Kjeldahl nitrogen, total
organic carbon turbidity, and others not pertinent to this TMDL effort.
Due to the fact that the data in this analysis category were collected, reviewed, and submitted to
the STORET database by DWQ, it was assumed that all sampling protocols and analytical
methods employed were carried out in a fashion approved by DWQ and contained and attained a
DWQ approved level of quality assurance and quality control.
3.4.2.1.4 Utah DWQ Field Measures
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These refer to the Utah Division of Water Quality (DWQ), Quality Assurance/Quality Control
Manual (1996). Utah DWQ field measures approved methods specific to the available database
for Nine Mile Reservoir TMDL include analytical procedures for measuring chlorine, dissolved
oxygen, flow, pH, salinity, Secchi depth, specific conductance, and temperature (air and water).
3.4.2.2 Quality Assurance and Quality Control
The data were assessed to ensure that all data points included in the TMDL process met an
appropriate level of quality. Basic statistical analyses were used to characterize the range and
quality of data. Statistical parameters assessed included the number of data points, determination
of mean, median, maximum and minimum values, assessment of variance, and an analysis of
seasonality. The completeness of the dataset was also evaluated in a spatial, temporal, and
parameter-specific fashion, and critical data gaps were identified. Further evaluation is discussed
in the following sections.
3.4.2.2.1 Treatment of Nondetects
Many of the data points (10% of total data points) collected in this dataset are concentration
values identified as “below detection limits”, “greater than quantitation limits”, or “too numerous
to count.” For the purpose of analyzing the data, standard methods were used to statistically
interpret these values. This was accomplished by assigning a numeric value of one-half of the
detection limit (in the case of concentrations identified as below detection limits) or a value that
represents 1.5 times the quantitation limit (in the case of concentrations identified as greater than
quantitation limits).
Detection limits were reported in the STORET database for most data points and provided a
specific nondetect value for most data. If data point specific detection limits were not provided,
detection limits were applied based on specific analytical methods. In some cases, UDWQ
monitoring data did not identify a specific analytical method; instead identifying the analytical
procedure as “generic method” or “generic method 2.” Arne Hultquist of the UDWQ Monitoring
Section, provided method numbers and detection limits for nondetect data for which no detection
limits were reported in the STORET database.
In the case of bacteriological data, where numerous dilutions are used to determine the total
counts, an upper quantitation limit cannot be identified directly from the method summary. In
cases where total concentrations were listed as being greater than the quantitation limits, or too
numerous to count, a value of 1.5 times the highest quantified concentration was substituted.
This provided a numeric value that allowed statistical analyses to be performed. Such a
substitution most likely represents an underestimation of the total bacteria count present.
However, as the quantitation limits for the analysis of total coliform and fecal coliform bacteria
are rarely lower than the state criteria for contact recreation, the recommended substitution is not
expected to result in a situation where risk to recreationists is unidentified (no false negatives).
Furthermore, it is not likely to result in a situation where bacterial loading is grossly
overestimated within the watershed.
3.4.2.2.2 Treatment of Errors
An initial assessment of the data was performed to identify transcription and other errors such as
inappropriate values (e.g., a pH value of 129), inaccurate sample information (e.g., units of mg/L
for specific conductivity data), and errors in physical information (e.g., incorrect county or
latitude information for a known sample site). A small number of such errors were identified and
corrective action was taken as outlined below.
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A number of sample sites included data points of zero (0). It was not immediately obvious what
these values represented. Possible interpretations include the following:
• Mis-entry of an analytical Nondetect
• An error in a spread sheet used to enter data to STORET, or an error within the STORET
database that did not allow display of appropriate decimal places and resulted in values of
less than one being displayed and recorded as 0
• Direct transcription errors
• A combination of the above and other unknown errors
Because of this uncertainty, zero values were removed from all datasets, with the exception of
measured or estimated flow, water, and air temperature measurements where a zero value is
possible. The total number of zero values removed from the Nine Mile Reservoir dataset was 31
(0.7% of the water quality dataset). Zero values occurred in this dataset for total suspended solids
(19 points) and volatile solids (12 points).
The available data for site # 5943250 NINE MILE RES MID-RESERVOIR were erroneously
identified as being located in Uintah County. Site numbers and latitude/longitude information
were checked and found to be accurate thus all listings of Uintah County were changed to
Sanpete County for this site.
3.4.2.2.3 Treatment of Outliers
To identify a final dataset that was representative of water quality conditions within the Nine
Mile Reservoir watershed, a threshold of plus or minus three standard deviations from the mean
was applied to the available datasets. This resulted in the removal of approximately 27 data
points from the Nine Mile Reservoir dataset (less than 1% of the dataset). This mechanism for
identifying non-representative data is a standard method used by DWQ in assessing water bodies
and has been found to be the most objective method for excluding potentially erroneous or
nonrepresentative data. Outliers excluded from the dataset include three total phosphorus values
in Nine Mile Reservoir and one total phosphorus value in Six Mile Creek. One chlorophyll a
value in Nine Mile Reservoir was also excluded.
3.4.2.2.4 Analytical Implications of Outlier Exclusion and Nondetect Treatment
The exclusion of outliers results in the mean total phosphorus concentration in the reservoir to be
reduced from above the 0.025 mg/L threshold to below the threshold (Table 3.5). Exclusion of
historic data further reduces mean concentrations. Exclusion of historic data also reduces the
mean total phosphorus concentration in Six Mile Creek from above the 0.05 mg/L threshold for
streams to well below the threshold. It is important to note that more than half of the total
phosphorus data points available for the in-reservoir monitoring locations, the reservoir inflow,
and Six Mile Creek, are reported as nondetect. As a result, the median value for all of these sites,
regardless of outlier exclusion, is calculated at half the detection limit averaging to 0.005 mg/L.
Similar reductions in chlorophyll a are observed when outliers and historic data are excluded
from mean calculations.
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Table 3.5 Change in Total Phosphorus Mean Concentration (mg/L) Through Exclusion
of Outliers and Historic Data
Entire
Dataset
Exclusion of
Outliers
Current Data Only
(1999–2006) Site
Mean Mean Mean Median
Nine Mile Reservoir inflow 0.040 0.029 0.033 0.005
Nine Mile Reservoir outflow 0.063 0.063 0.083 0.083
Nine Mile Reservoir at dam 0.032 0.023 0.009 0.005
Nine Mile Reservoir at mid-reservoir 0.029 0.022 0.023 0.005
SIX MILE CK AB CNFL / SAN PITCH R
NW OF STERLING 0.059 0.053 0.017 0.005
Springs discharging to the reservoir 0.011 0.011 0.011 ---

Table 3.6 Change in Mean chlorophyll a Concentration (µg/L) Through Exclusion of
Outliers and Historic Data
Entire Dataset Exclusion of
Outliers Current Data Only (1999– 2006) Site
Mean Mean Mean Median
Reservoir at dam 5.3 5.3 3.8 1
Mid-reservoir 6.1 4.0 2.5 0.7

Since the exclusion of outliers has such a critical impact on the support status assessment of Nine
Mile Reservoir, potential causes for the outliers in the current dataset were explored. Of the three
total phosphorus outliers in Nine Mile Reservoir, only one occurs during the current period of
1999 to 2006. This value of 0.586 was collected by UDEQ on August 9, 2000. The other two
data points were collected on September 9, 1992 at different depths. The chlorophyll a outlier
was collected by UDEQ on August 9, 1994. These outliers appear to be associated with low
water levels in the reservoir, measured as maximum depth at dam at the time data was collected
(Figures 3.2 and 3.3). Since no conservation pool has been established for Nine Mile Reservoir,
when the reservoir is drawn down its minimum level it no longer can support fish due to a lack
of water. When the reservoir still maintains a small pool of water, it can quickly become stagnant
and warm thereby reducing viable habitat for a cold water fishery and promoting in-reservoir
algal growth. Watershed level nutrient reductions would not have a substantial improvement on
this condition which is primarily a function of reservoir level management and water rights. For
this reason, exclusion of outlier values is justified regardless of whether outliers represent
erroneous data or result from low reservoir volume.

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0
1
2
3
4
5
6
7
8
9
10
01020304050
Chlorophyll a µg/L
Re
s
e
r
v
o
i
r

D
e
p
t
h

a
t

D
a
m

(
m
)
Chl a at Dam Chl a at Mid-reservoir
30.5 µg/L = 3 SDs from mean

Figure 3.2 Total phosphorus related to reservoir level indicating that the low reservoir level
may explain outliers.

Figure 3.3 Chlorophyll a related to reservoir level indicating that the low reservoir level
may explain outliers.

0
1
2
3
4
5
6
7
8
9
10
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Total Phosphorus (mg/L)
Re
s
e
r
v
o
i
r

D
e
p
t
h

a
t

D
a
m

(
m
)
Dam Site Mid-Reservoir Site
0.26 mg/L = 3 SDs from mean
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3.5 ASSESSMENT OF BENEFICIAL USE SUPPORT
3.5.1 KEY INDICATORS OF SUPPORT
3.5.1.1 Low Dissolved Oxygen
Dissolved oxygen (DO) is important to the health and viability of fish and other aquatic life.
Aquatic life depends on high concentrations of dissolved oxygen (from 6–8 mg/L or greater).
Low dissolved oxygen (concentrations below 5 mg/L) can result in stress, reduced resistance to
other environmental stressors, and even death at very low levels (less than 2 mg/L).
In addition to direct effects on aquatic life, low dissolved oxygen concentrations can lead to
changes in water and sediment chemistry that can influence the concentration and mobility of
nutrients and toxins—e.g., changes in phosphorus, ammonia, and mercury levels in the water
column. Low dissolved oxygen at the sediment-water interface can result in substantial release of
sorbed phosphorus in the water column, which in turn can lead to increased algal growth and
decreased dissolved oxygen concentrations. Anoxic conditions, combined with available organic
matter, can result in higher rates of methyl mercury production. Methyl mercury represents a
significantly greater threat for bioconcentration and accumulation than elemental or mineralized
mercury compounds. Finally, increased water column concentrations of ammonia can result from
the chemical changes caused by anoxic conditions. Elevated ammonia levels threaten the health
of aquatic life forms and, at extreme concentrations, can result in death.
Low dissolved oxygen often results from high nutrient, organic, or algal loading to a surface
water system. Nutrients promote algae growth, which in turn consumes oxygen from the water
column during periods when respiration is the dominant process (generally at night). In addition,
dying algae in lakes and reservoirs settle to the bottom of the water body and decompose; aerobic
decomposition of the dead algae and other detritus (nonliving organic material) depletes the
oxygen supply in the overlying water and sediment. In systems where suspended solids are
primarily organic in origin, low dissolved oxygen levels may be correlated with sediment inputs
as well.
Dissolved oxygen concentrations are also reduced by pollutants that require oxygen in oxidation
processes. Biochemical oxygen demand (BOD) is a measure of the dissolved oxygen required to
oxidize material (usually organic), whether the material is naturally occurring, the result of
increased natural material, or contained in municipal, agricultural, or industrial wastes. Some of
the delivered organic material is algae and some is detritus. Both of these organic matter
components produce a certain amount of BOD. A substantial organic load may be delivered to
the reservoir during high volume and high velocity spring flow events.
3.5.1.2 pH
A key indicator of acidity or alkalinity of a system is pH, as measured by the hydrogen ion
activity in the water. A pH value of 7.0 is neutral, with values from 0 to 7 indicating acidic water
and those from 7 to 14 indicating alkaline water. Extremely acidic or alkaline waters can be toxic
to aquatic life. Even at less extreme levels, acidic or alkaline conditions can cause chemical
shifts in a system; acidic conditions can release metallic compounds from sediments while
alkaline conditions can increase ammonia toxicity and release sorbed phosphorus.
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Both living and dead (decomposing) algae can have minor effects in the pH of the water due to
the release of various acid and base compounds during respiration and photosynthesis.
3.5.1.3 Temperature
Appropriate temperature is crucial to water quality and support of aquatic habitat. Temperature
determines whether or not a water body can support warm or cold water aquatic species. High
water temperatures can be harmful to fish at all life stages, especially when high temperatures
combine with other habitat limitations such as low dissolved oxygen or poor food supply. As a
stressor to adult fish, elevated temperatures can lower body weight, reduce oxygen exchange,
and diminish reproductive capacity. Extremely high temperatures can result in death if they
persist for an extended length of time. Juvenile fish are more sensitive to temperature variations
and duration than adult fish and tend to experience negative impacts at a lower threshold value
than the adults.
Acceptable temperature ranges vary for different species of fish; warm water species adapt better
to rising water temperatures than cold water fish. Protective criteria have been established to
serve the needs of important cold and warm water species of aquatic life. The temperature
criteria are usually built around a maximum allowable value that relates to critical life-stage
requirements.
3.5.1.4 Nutrients
General concerns associated with excessive nutrient concentrations relate to both direct and
indirect effects. Direct effects include nuisance algae and periphyton growth. Indirect effects
include low dissolved oxygen, increased methyl mercury production, elevated pH, cyanotoxins
from cyanobacteria (blue-green algae) production, trihalomethane production in drinking water
systems, and maintenance issues associated with domestic water supplies.
Nuisance aquatic growth, both algae (phytoplankton, or water column algae, and periphyton, or
attached algae) and rooted plants (macrophytes) can adversely affect both aquatic life and
recreational water uses. Algal blooms occur where nutrient concentrations (nitrogen and
phosphorus) are sufficient to encourage excessive growth. Levels necessary for growth may
occur at concentrations well below the identified water quality thresholds and criteria. Available
nutrient concentrations, flow rates, velocities, water temperatures, and sunlight penetration in the
water column are all factors that influence algae (and macrophyte) growth. When conditions are
appropriate and nutrient concentrations exceed the quantities needed to support algal growth,
excessive blooms may develop. Commonly, these blooms appear as extensive layers or algal
mats on the surface of the water.
Algal blooms often create objectionable odors in water used for recreation and can produce
intense coloration of both the water and shorelines. Water bodies demonstrating sufficient
nutrient concentrations to cause excessive algal growth are said to be eutrophic. Algae is not
always damaging to water quality, however. The extent of the effect is dependent on both the
type(s) of algae present and the size, extent, and timing of the bloom. In many systems, algae
provide a critical food source for many aquatic insects, which in turn serve as food for fish.
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Algae growth also has indirect effects on water quality. When algae die, they sink slowly
through the water column, eventually collecting on the bottom sediments. As the algae
decompose, the biochemical processes that occur remove oxygen from the surrounding water.
Because most of the decomposition occurs within the lower levels of the water column,
dissolved oxygen concentrations near the bottom of lakes and reservoirs can be substantially
depleted by a large algal bloom. Low dissolved oxygen in these areas can lead to decreased fish
habitat and even fish kills if there are not other areas of water with sufficient dissolved oxygen
available where the fish can take refuge.
Both nitrogen and phosphorus represent nutrients that can contribute to eutrophication. Either
nutrient may be the limiting factor for algal growth depending on algal species. In systems where
cyanobacteria (blue-green algae) are the dominant population, nitrogen is not a limiting agent
based on this ability to fix nitrogen. Therefore, these organisms can grow where low nitrogen
concentrations may inhibit the growth of other algal species (Sharpley et al. 1995 and 1984;
Tiessen 1995). These systems are therefore phosphorus limited. Freshwater systems are usually
phosphorus limited, however there is a large body of literature concerning the impact of the
nitrogen-to-phosphorus ratio (N:P) in freshwater systems. Typically N:P ratios less than 10
suggest a nitrogen limited system, whereas higher ratios suggest that nitrogen and phosphorus
are either co-limiting or that the system is phosphorus limited. However, the cut off for an N:P
ratio below which nitrogen is likely the limiting agent ranges from 7 to 15 (EPA 2000).
The nitrogen-to-phosphorus ratio in Nine Mile Reservoir averages 23.4 at the dam and 22.1 at
mid-reservoir. The ratios range from a low of 0.05 to a high of 142. However, these estimates are
based on a very narrow dataset because there are very few dates for which total phosphorus and
total nitrogen data are available. Many data points do not come from the recent or current dataset
(defined as 1992 and later). These N:P data suggest that Nine Mile Reservoir is most likely
phosphorus limited, although it has previously been classified as a nitrogen limited system by the
State of Utah (UDWQ 2007).
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Table 3.7 Nitrogen to Phosphorus Ratios in Nine Mile Reservoir
N:P Mid-
reservoir N:P Dam
5/21/1981 23.3 24.0
6/12/1990 30.7 22.4
9/7/1990 15.2 19.2
5/27/1992 32.6 5.3
9/9/1992 11.0 9.5
6/28/1994 3.0 4.0
8/9/1994 2.8 2.9
6/19/1996 15.0 8.3
9/3/1996 18.0 18.4
6/30/1998 45.2 24.9
9/10/1998 24.0 27.5
6/14/2000 10.0 10.0
8/9/2000 3.6 0.5
5/28/2002 44.8 46.0
8/8/2002 -- 11.8
6/9/2004 50.0 33.6
7/20/2004 10.0 10.0
8/10/2004 -- 13.0
9/14/2004 -- 142.0
6/15/2005 11.2 48.5
8/11/2005 47.0 22.3
9/8/2005 22.0 10.0
Overall Average 22.1 23.36
Overall Maximum 50 142
Overall Minimum 2.79 0.54
Current Average (1999–2006) 24.8 31.6

Excess nutrient loading causes water quality problems due to the direct effect of high phosphorus
concentrations on excess algal growth within the water column, combined with the direct effect
of the algal life cycle on dissolved oxygen and pH within aquatic systems. As total phosphorus
(TP) includes both dissolved and particulate-bound phosphorus, it represents the phosphorus that
is currently available for growth as well as that which has the potential to become available over
time.
Consideration of flow is important in the evaluation of nutrients and phytoplankton, periphyton,
and rooted macrophyte concentrations. In a riverine system, flow transports phytoplankton and
nutrients from upstream to downstream in an advective or dispersive transport mode. In other
words, the riverine system is a dynamic system in which nutrients are being continually cycled as
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the water moves downstream. The flow regimen is important in determining the result of this
combination of component concentrations. High flows can flush dissolved constituents like
nutrients downstream. High flows can also scour periphyton and rooted macrophytes, reducing
their concentrations considerably in-stream and concentrating them in the receiving water body.
Finally, when high flows scour sediments and sediment is moved downstream, sediment-bound
nutrient concentrations also increase as buried sediment is exposed.
High total phosphorus concentrations can lead to increases in the rate of algal growth and in
overall productivity, up to the saturation point. The increased algal biomass production and
transport increases biological oxygen demand and decreases dissolved oxygen levels. Reservoir
systems that experience low flow-through rates during the growing season can experience
conditions that are optimal to algae growth and decomposition.
A separate consideration is the difference between algae concentrations and the rate of algal
growth. Algal concentrations are determined by the availability of nutrients on a continuing
basis, the availability of adequate light, and the presence of flows (velocities) that will permit
continued growth without losses due to flushing (of phytoplankton), sloughing (of attached algae
or periphyton), or mechanical breakage and scouring (of rooted macrophytes). In quiescent
systems like Nine Mile Reservoir, algal concentrations during the summer season are dependent
on nutrient availability, and only if nutrient concentrations have been depleted by algal uptake
does the growth rate approach zero and phytoplankton begin to die.
In streams and rivers, the nutrients also cycle between the water, sediment, living organisms, and
detritus; this is called nutrient spiraling. Generally, high velocities occur often enough to scour
attached and rooted vegetation and to keep concentrations of aquatic vegetation low. Under low
velocities, however, attached and rooted vegetation may increase to noticeable levels. As long as
nutrients continue to be available and flows are inadequate to cause losses of algae mass, the
algae will continue to grow and may reach levels that cause algal mats on the bottom or at the
surface. This is often the case in shallow lakes or ponds or in pools found in intermittent streams.
However, the presence of algal mats or attached algae does not necessarily indicate an excess of
nutrients.
Many sources and conditions in the environment add nutrients to water bodies. Phosphorus can
be present as a constituent of certain rock types (e.g., siliceous igneous rock) and in the mineral
apatite. Nitrogen is a major component of the atmosphere and enters biological systems through
nitrogen fixation and rock weatherization.
The environment itself can also be a factor in the phosphorus levels occurring within a region,
since the climate, pH of natural waters, and the presence of other substances that may adsorb or
release phosphorus (Hedley et al. 1995) can all potentially affect phosphorus levels. In addition,
soil chemistry, redox potential, and nutrient ratios affect the cycling of nitrogen in natural
systems. There are also anthropogenic (man-made) nutrient sources. Applied fertilizers in
farming, landscaping and pasture management, manure treatment, the duration and density of
livestock grazing, the creation of artificial waterways and water levels through irrigation and
water management practices, as well as the presence of sewage and septic waste (treated and
untreated) in the surface, subsurface, and groundwater of a region often represent significant
contributions to the phosphorus concentration in an area. Natural sources of nutrients include the
indigenous wildlife and waterfowl that use the watershed. While these populations are relatively
stable throughout much of the year, substantial increases in some populations are observed with
spring and fall migration patterns.
Nitrogen occurs in the environment in a variety of sources and forms. It can be present as a
mineral constituent of certain rock types, as a result of the decomposition of plant and other
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organic material, in rainfall (as a component of agricultural or urban/suburban runoff), and as a
constituent in septic discharges.
It is likely that both physical and chemical processes impact the transport and availability of
phosphorus and nitrogen in the Nine Mile Reservoir watershed. Physical processes (wind and
water movement) dominate in the transport of phosphorus contained within or adsorbed into
sediment and particulates. Chemical processes (i.e., changes in water chemistry such as dissolved
oxygen, pH levels, or redox) dominate in the transport of dissolved phosphorus to the system and
in the transformation of phosphorus from one form or state (i.e., free or adsorbed) to another,
within both the transport pathway and the water column.
3.5.2 DIRECT EXCEEDANCE OF NUMERIC CRITERIA AND/OR THRESHOLD VALUES
3.5.2.1 Dissolved Oxygen
Current data collected in Nine Mile Reservoir (1999-2006) indicate that dissolved oxygen
concentrations are generally quite high with mean concentrations ranging from 8.8 to 9.9 mg/L.
Minimum dissolved oxygen concentrations range from 0.28 to 1.37 mg/L and are limited to the
sediment-water interface at the bottom of the reservoir.
As fish and most other aquatic life species are mobile and can relocate to areas of suitable habitat
in the event of a localized criteria exceedance, the State of Utah has defined the support status of
game fish populations relative to the percentage of the total water column experiencing
depressed dissolved oxygen concentrations. A water body's dissolved oxygen concentration is
defined to have a non-support status for cold water game fish when less than 25% of the water
column depth exhibits dissolved oxygen concentrations of 4.0 mg/L or greater. If 25 to 50% of
the water column depth exhibits dissolved oxygen concentrations of 4.0 mg/L or greater, the
water body is defined as having a partial-support status. Where greater than 50% of the water
column depth exhibits dissolved oxygen concentrations of 4.0 mg/L or greater, a full-support
status has been defined. These criteria were assessed on average for each month in the algal
growth season (May-October) for which water column data were available.

Table 3.9 Percent of Total Samples in Exceedance of Dissolved Oxygen Criteria (>4 mg/L)
Site Name 2000 2001 2002 2004 2005 2006 Mean
Inflow 0% 0% 0% 0% 0% 0%
Dam Site 20% 0% 8% 13% 26% 14%
Mid-reservoir Site 8% 0% 11% 5% 0% 5%

Table 3.8 Summary of Current Dissolved Oxygen Data (1999–2006) in Nine Mile
Reservoir Watershed.
Site Name N Mean Maximum Minimum Standard
Deviation
Inflow 17 7.53 9.16 5.85 1.09
Dam Site 135 8.84 15.58 0.28 3.66
Mid-reservoir Site 98 9.82 15.24 1.37 2.75
Nine Mile Reservoir Delisting February 2008

46
There have been no observed exceedances of the percent water column criteria for dissolved
oxygen established by the State of Utah. All dissolved oxygen water column exceedances have
affected less than 25% of the water column. At the dam, low dissolved oxygen (less than 4.0
mg/L) was experienced in 0% of the water column on average in May and September, 12% of
the water column in June, 9% of the water column in July, and 6% of the water column in
August (Table 3.10). The greatest incidence of dissolved oxygen exceedance observed at this site
involved 19% of the water column and occurred in August of 2006. On average, low dissolved
oxygen (less than 4.0 mg/L) was experienced in 0% of the water column at the Mid-reservoir
Site in May, 5% of the water column in June, 0% of the water column in July, 0% of the water
column in August and 0% of the water column in September (Table 3.10). The greatest incidence
of dissolved oxygen exceedance observed at this site involved 13% of the water column and
occurred in June of 2006.
Table 3.10 Nine Mile Exceedance – DO Less Than 4.0 mg/L (% of Water Column)
Site Name Month 2000 2001 2002 2003 2004 2005 2006 AVERAGE
May -- -- 0% -- -- -- -- 0%
June 14% -- -- -- 8% 13% 15% 12%
July -- -- -- -- 0% 18% 9%
Aug -- -- -- -- 0% 0% 19% 6%
Dam Site
Sept -- -- -- -- -- 0% -- 0%
May -- -- 0% -- -- -- -- 0%
June 0% -- -- -- 6% 0% 13% 5%
July -- -- -- -- 0% 0% -- 0%
Aug -- -- -- -- -- 0% 0% 0%
Mid-reservoir Site
Sep -- -- -- -- -- 0% -- 0%
3.5.2.2 Dissolved Oxygen Saturation
Dissolved oxygen saturation data are reported in the STORET database for both surface
measurements and water column measurements. The water quality criterion for dissolved gases
established by the State of Utah is 110% of saturation due to the stress supersaturated water can
cause for fish. Values greater than 110% are considered to be exceedances of state water quality
criteria. The measurements collected at depth and reported in the STORET database had not
been corrected for hydrostatic pressure. The amount of gas that can be dissolved in water is
influenced by atmospheric pressure, hydrostatic pressure, and water temperature. When oxygen
is produced below 1 to 4 meters of depth, more of it can remain dissolved because of hydrostatic
pressure leading to oxygen accumulation frequently exceeding several hundred percent
supersaturation relative to the pressure at the surface of the reservoir (Wetzel 2001).
In order to correct dissolved oxygen saturation for data collected below 1 meter of depth in Nine
Mile Reservoir, the following equation was applied (Wetzel 2001):
zPPz0967.00+=
Where Pz is the actual pressure at a given depth, P0 is the atmospheric pressure at the surface of
the reservoir, and z is the depth of the measurement in meters. Because atmospheric pressure
data was not available for each data collection date, a mean average atmospheric pressure of
0.866 atmospheres was assumed. Resulting calculated dissolved oxygen saturation values closely
matched data collected at the surface of the reservoir and reported in STORET. Measured
Nine Mile Reservoir Delisting February 2008

47
atmospheric pressure is commonly observed near this value for Manti, Utah close to Nine Mile
Reservoir. Correction of dissolved oxygen saturation data at depths greater than 1 meter greatly
affects the summary statistics for this parameter. Using the raw data reported by STORET, the
mean dissolved oxygen saturation value in the reservoir (at depths greater than 1 meter) ranged
from 111% at the dam site to 123% at the Mid-reservoir Site. The mean corrected dissolved
oxygen saturation data are significantly lower, ranging from 88% at the dam site to 98% at the
Mid-reservoir Site. These mean values are below the state water quality criteria established for
cold water fisheries of dissolved gases less than 110% of saturation.
Table 3.11 Summary of Current (1999–2006) Dissolved Oxygen Saturation Data
Corrected for Depths >1 Meter in Nine Mile Reservoir
Site Name N Mean Median Maximum Minimum Standard
Deviation
Dam Site 47 88% 89% 192% 2% 50%
Mid-reservoir Site 26 98% 106% 168% 10% 48%
There do remain exceedances of the dissolved oxygen saturation criteria in samples collected at
the reservoir surface (less than 1 meter of depth).
Mean surface dissolved oxygen saturation data range from 128% to 138% with maximum values
of 194% and 168%. Although such high dissolved oxygen saturation values during the day are
often used to indicate the presence of photosynthesizing algae, the data for the Nine Mile
Reservoir do not correlate well with chlorophyll-a measurements in the reservoir. High dissolved
oxygen saturation values do, however, correspond to exceptionally warm water temperatures.
One potential explanation for the high dissolved oxygen values observed at the surface of Nine
Mile Reservoir is diurnal temperature swings. As water becomes colder at night, it is able to
absorb more oxygen. As the water is warmed during the day, the saturation level of the water is
lowered. However, it may take several hours for oxygen to be released from the reservoir,
resulting in elevated dissolved oxygen saturation levels observed during daytime hours. These
diurnal trends can not be directly analyzed for Nine Mile Reservoir because no diurnal
temperature or dissolved oxygen data have been collected. A water temperature swing of 7°C,
Table 3.12 Percent of Total Samples in Exceedance of Dissolved Oxygen Saturation Criteria
in Surface Samples (<110%)
Site Name 2000 2001 2002 2004 2005 2006 Mean
Dam Site 67% 33% 67% 50% 50% 67% 55%
Mid-reservoir Site 100% 100% 100% 75% 50% 100% 80%
Table 3.13 Summary of Current (1999–2006) Surface Dissolved Oxygen Saturation Data
(<1 meter) in Nine Mile Reservoir
Site Name N Mean Media
n Maximum Minimum Standard Deviation
Dam Site
(corrected data) 22 128% 115% 194% 81% 34%
Mid-reservoir Site
(corrected data) 10 138% 146% 168% 104% 26%
Nine Mile Reservoir Delisting February 2008

48
which is a reasonable expectation of diurnal swings in Nine Mile Reservoir, relates to a 1 mg/L
change in dissolved oxygen at saturation. Assuming this 7 degree flux spans 17 to 25°C (the
observed high temperature in Nine Mile Reservoir), the 1 mg/L change in dissolved oxygen is
related to a flux in DO of 25%.
Although dissolved oxygen saturation exceedances are observed at the surface of Nine Mile
Reservoir, supersaturation does not extend throughout the water column, is unlikely to impose a
severe stress to fish, and does not cause an impairment of the cold water fishery beneficial use.
3.5.2.3 Nitrate
The water quality criterion for nitrate established by the State of Utah is 4 mg/L. Values greater
than 4 mg/L are considered to be in exceedance of state water quality criteria. Mean nitrate
concentrations in Nine Mile Reservoir are quite low, with a mean concentration of 0.3 mg/L at
both reservoir monitoring sites. Maximum concentrations range from 2.13 mg/L at the dam to
1.34 mg/L at the Mid-reservoir Site. These low concentrations are maintained throughout the
season despite high nitrate concentrations in the inflow to Nine Mile Reservoir. Mean
concentrations in the springs discharging to the reservoir and the reservoir inflow range from
2.26 mg/L to 2.63 mg/L. High concentrations of nitrate in the spring water discharging to Nine
Mile Reservoir indicate high concentrations of nitrate in local groundwater. Groundwater also
feeds the inflow to Nine Mile Reservoir especially at times when water is not being diverted
from Six Mile Creek. However, the reservoir itself does not show signs of elevated nitrate.
No exceedances of the nitrate criteria (no greater than 4.0 mg/L) were observed in either of the
in-reservoir datasets. Six Mile Creek diversion inflow data show no nitrate criteria exceedances.
Spring inflow data (although a relatively small dataset) show only one exceedance from the
northern-most spring with the lowest inflow volume. However, inflow data to the reservoir show
routine exceedances of the nitrate criteria of 4.0 mg/L.
Table 3.14 Summary of Current (1999–2006) Nitrate Data in Nine Mile Reservoir
Watershed.
Site Name N Mean Maximum Minimum Standard
Deviation
Inflow 14 2.63 6.52 0.03 2.92
Dam Site 35 0.34 2.13 0.03 0.39
Mid-reservoir Site 20 0.32 1.18 0.03 0.32
Six Mile Creek above
San Pitch confluence
15 0.95 2.80 0.29 0.68
Springs discharging to
the reservoir 3 2.26 6.70 0.03 3.84
Nine Mile Reservoir Delisting February 2008

49
3.5.2.4 pH
High pH values are observed routinely throughout Nine Mile Reservoir as well as the springs
and inflow discharging to the reservoir. Mean reservoir pH is above 9.0 at both sites with
maximum pH values over 10 at the Dam Site in the late summer months.
Table 3.16 Summary of Current (1999–2006) pH Data in Nine Mile Reservoir Watershed
Site Name N Mean Maximum Minimum Standard
Deviation
Inflow 18 8.25 8.73 7.79 0.29
Dam Site 149 9.01 10.07 7.47 0.56
Mid-reservoir Site 100 9.10 9.86 8.02 0.43
Six Mile Creek above San
Pitch confluence
37 8.41 8.97 7.27 0.30
Six Mile Creek at diversion 1 7.15 7.15 7.15
Springs discharging to
reservoir
5 8.11 8.44 7.40 0.41
Reservoir outflow 2 8.08 8.7 7.46 0.88
Exceedances of the water quality criteria for pH (no greater than 9.0 and no less than 6.5) occur
routinely in Nine Mile Reservoir. All observed exceedances were for high pH (alkaline)
conditions. The mid-reservoir dataset for Nine Mile Reservoir shows 60% exceedance of the pH
criteria. The dataset collected near the dam shows 54% exceedance, with maximum values
greater than 10 observed. In the following figures (Figure 3.4), the solid lines represent the upper
(9.0) and lower (6.5) limits of the pH range defined by the state water quality criteria.
Table 3.15 Percent of Total Samples in Exceedance of Nitrate Criteria (<4 mg/L) in
Current Data (1999–2006).
Site Name 2000 2001 2002 2004 2005 2006 Mean
Inflow 100% -- 100% 100% 0% 0% 36%
Dam Site 0% -- 0% 0% 0% 0% 0%
Mid-reservoir Site 0% -- 0% 0% 0% 0% 0%
Six Mile Creek above San Pitch
confluence -- 0% 0% -- -- 0% 0%
Outflow -- -- -- -- -- 0% 0%
Springs discharging to reservoir -- -- -- -- -- 33% 33%
Nine Mile Reservoir Delisting February 2008

50
While pH values observed in Six Mile Creek water quality monitoring were elevated (averaging
8.5), exceedances of the upper pH criteria of 9.0 did not occur in any of Six Mile Creek diversion
inflow data. Observed exceedances of the pH criteria occur more frequently in the later summer
months. The incidence and magnitude of exceedance increase with time as the summer
progressed, with the highest pH values recorded in the month of September (Figure 3.4). This
trend correlates well with the diversion of less alkaline water from Six Mile Creek from June 15
to August 15. During this period, the less alkaline diversion water dilutes the high pH water in
Nine Mile Reservoir and the incoming spring water. Once the diversion ends, the dilution
process ceases and pH values go back to their natural level of just over 9.
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Dam Site

Figure 3.4 pH values in Nine Mile Reservoir showing seasonal trend as observed during
2005 routine monitoring.
Due to the occurrence of high pH soils near and underlying portions of the reservoir, an attempt
was made to characterize the contribution of soil pH to the level of pH criteria exceedance
observed in the reservoir. Local and watershed soils are alkaline in nature, especially those soils
located near and under the southwest edge of the reservoir, which are highly alkaline in nature. A
soil analysis completed for the area surrounding Nine Mile Reservoir assessed that the alkaline
soils near the reservoir have a particularly high pH range of 8 to 10. (Figure 3.5) It is also
Table 3.17 Percent of Total Samples in Exceedance of pH criteria (<9)
Site Name 2000 2001 2002 2004 2005 2006 Mean
Inflow 0% -- 0% 0% 0% 0% 0%
Outflow -- -- -- -- -- 0% 0%
Dam Site 82% -- 0% 63% 52% 63% 54%
Mid-reservoir Site 92% -- 0% 83% 49% 76% 60%
Six Mile Creek above San Pitch
confluence -- 0% 0% -- -- 0% 0%
Six Mile Creek at diversion -- -- -- -- -- 0% 0%
Springs discharging to the
reservoir -- -- -- -- -- 0% 0%
Nine Mile Reservoir Delisting February 2008

51
assumed that the alkaline soils extend underneath the Nine Mile Reservoir and naturally
contribute to the alkalinity to the reservoir water. Soil leaching processes may therefore act as a
source of elevated pH levels in the reservoir. Thus, impairments associated with pH in Nine Mile
Reservoir are associated with naturally occurring conditions and can therefore not be corrected
with management practices.
Figure 3.5 pH of soils in Nine Mile Reservoir watershed.
3.5.2.5 Temperature
High temperature values are routinely observed in Nine Mile Reservoir. Mean temperature at
both reservoir sites is 20°C with maximum values greater than 24°C observed in the summer
months.
Exceedances of the water quality temperature criteria for cold water game fish (no greater than
20°C) occur routinely in both reservoir datasets. The Mid-reservoir Site dataset showed 53%
exceedance of the 1-day average criteria. The dataset collected at the Dam Site also showed 49%
exceedance. Exceedances of the coldwater temperature criteria occurred in approximately 10%
of the Six Mile Creek data (sampled above the confluence with the San Pitch River), and in 60%
of the spring inflow data.
Since the reservoir has little natural cover and the watershed is located in an area experiencing
warm, dry climate conditions, the State of Utah recently conducted an assessment of temperature
inputs to several local water bodies and determined that the primary source of temperature
loading was from solar radiation and heat transfer. Temperature increases in Nine Mile
Reservoir are influenced by natural heat exchange through high air temperatures and the effects
of direct solar radiation on the water surface, especially during the summer. In addition,
inflowing tributaries in hot climates can contribute to temperature increases particularly in the
Nine Mile Reservoir Delisting February 2008

52
Water temperature measured at mid-lake in Ninemile Reservoir
0
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summer (See Figure 2.3 for average daily air temperatures in the general area of the watershed.).
Exceedances of the cold water temperature criteria occurring in the Six Mile Creek diversion
inflow and spring inflow indicate that both inflow and in-reservoir heating processes are
responsible to some degree for the elevated water temperatures observed. Furthermore, native
vegetation in all but the highest elevations of most drainages is relatively low-growing and
sparse and provides little shade on major tributaries. These environmental factors play a major
role in raising water temperatures in Nine Mile Reservoir. Despite exceedances of the state
criteria, Nine Mile Reservoir was not determined to be impaired for temperature due to natural
heat loading. The assessment of temperature exceedance in this document is specific to the
determination of designated use support status for cold water game fish only.

Figure 3.6 Water temperatures observed during routine monitoring at the Mid-reservoir
Site in Nine Mile Reservoir.

Table 3.18. Summary of Current Temperature Data (1999–2006) in Nine Mile
Reservoir watershed
Site Name N Mean Maximum Minimum Standard
Deviation
Inflow 18 18.33 26.41 12.43 3.37
Dam Site 136 20.04 25.80 14.08 2.85
Mid-reservoir Site 100 20.22 24.36 13.80 2.61
Six Mile Creek above
San Pitch confluence
21 10.21 21.81 0.35 6.86
Six Mile Creek at
diversion
1 20.30 20.30 20.30 --
Springs discharging to
the reservoir
5 18.06 22.30 12.00 5.36
Reservoir outflow 2 20.9 21.4 20.4 0.71
Nine Mile Reservoir Delisting February 2008

53
Water temperature measured near Ninemile Dam
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Figure 3.7 Water temperatures observed during routine monitoring near the Dam Site in
Nine Mile Reservoir.
3.5.2.6 Total Dissolved Solids
Data collected between 1999 and 2006 indicate that total dissolved solids (TDS) concentrations
are moderate in Nine Mile Reservoir, with a mean concentration at the Dam Site of 480 mg/L.
The maximum observed TDS concentration in Nine Mile Reservoir is 854 mg/L, well below the
water quality criteria of 1,200 mg/L established by the State of Utah. (Table 3.19) High
concentrations of TDS are observed in the springs discharging to the reservoir, indicating high
dissolved solids in the groundwater. This is characteristic of waters in other parts of the San
Pitch River basin (UDWQ 2003).
Table 3.19 Summary of Current Total Dissolved Solids Data (1999 – 2006) in Nine Mile
Reservoir Watershed.
Site Name N Mean Maximum Minimum Standard
Deviation
Inflow 15 510 796 224 182
Outflow 2 358 490 225 187
Dam Site 15 480 854 336 136
Six Mile Creek above San
Pitch confluence
15 410 890 214 208
Six Mile Creek at
diversion
1 225 225 225 --
Springs discharging to the
reservoir
6 711 1300 321 378
The available data show no total dissolved solids concentrations in the reservoir that exceed
either the 1,200 or the 2,000 mg/L criteria (Table 3.19). All concentrations are well below 1,000
Nine Mile Reservoir Delisting February 2008

54
mg/L. TDS in the springs discharging to the reservoir exceed the TDS criteria of 1,200 mg/L
17% of the time (Table 3.20).
3.5.2.7 Total Phosphorus
Total phosphorus concentrations are generally quite low in Nine Mile Reservoir (Table 3.21).
More than half of the phosphorus data collected in Nine Mile Reservoir were recorded as
nondetect, and therefore assumed to be half of the detection limit for the purposes of analysis.
The prevalence of nondetect values in the reservoir brings the estimated median concentration
down to 0.005 mg/L (which is equal to half of the detection limit for most of the phosphorus data
collected in the reservoir). Mean concentrations are also very low and range from 0.01 mg/L at
the Dam Site to 0.023 mg/L at the Mid-reservoir Site. The maximum concentrations of 0.257
mg/L at the Mid-reservoir Site and 0.068 mg/L at the Dam Site are correlated with low water
levels in the reservoir. (See Section 3.4.2.2.2) As the reservoir is drawn down for irrigation,
phosphorus associated with the sediment-water interface becomes available for algal growth due
to shallow water conditions and more access to light. Phosphorus associated with the sediment-
water interface with the reservoir full is generally unavailable for algal growth because light does
not typically reach to such depths.

Table 3.21 Summary of Current Total Phosphorus Data (1999 – 2006) in Nine Mile
Reservoir Watershed.
Site Name N Mean Maximum Minimum Standard
Deviation
Inflow 15 0.033 0.261 0.005 0.067
Dam Site 35 0.009 0.068 0.005 0.012
Mid-reservoir Site 20 0.023 0.257 0.005 0.057
Six Mile Creek above San
Pitch confluence
15 0.017 0.070 0.005 0.021
Springs discharging to the
reservoir
3 0.011 0.016 0.006 0.005
Reservoir outflow 1 0.083 0.083 0.083 --
The State of Utah has not identified a criterion for phosphorus concentration. State water quality
guidance has established an indicator value of 0.025 mg/L total phosphorus concentration in
Table 3.20 Percent of Total Samples in Exceedance of Total Dissolved Solids Criteria
(<1200 mg/L)
Site Name 2000 2001 2002 2004 2005 2006 Mean
Inflow 0% 0% 0% 0% 0% 0%
Dam Site 0% 0% 0% 0% 0% 0%
SIX MILE CK AB CNFL / SAN
PITCH R NW OF STERLING
-- 0% 0% -- -- 0% 0%
Outflow -- -- -- -- -- 0% 0%
Springs discharging to the
reservoir
-- -- -- -- -- 17% 17%
Nine Mile Reservoir Delisting February 2008

55
lakes and reservoirs as a trigger for further, in-depth assessment of water body condition and
needs. A threshold of 0.05 mg/L has been established for streams in Utah.
Total phosphorus concentrations observed in both reservoir datasets contain isolated threshold
value. Approximately 6% of the data collected at the Mid-reservoir Site exceeds the 0.025 mg/L
indicator value and approximately 10% of the data collected near the dam shows values greater
than the indicator. Total phosphorus concentrations measured in Six Mile Creek exceed the
stream phosphorus indicator value of 0.05 mg/L 13% of the time. Diversion inflow data and
spring inflow data do not exceed the indicator concentration of 0.025 mg/L. However, 20% of
the data collected at the inflow to the reservoir exceed this threshold concentration.
3.5.3 TREND ANALYSIS
3.5.3.1 Chlorophyll a
Chlorophyll a is a pigment found in plants for use in photosynthesis. The amount of chlorophyll
a contained in the reservoir is an indicator of phytoplankton production and is used as an
indicator of eutrophication in waters. While the State of Utah does not publish criteria for
acceptable levels of chlorophyll a, one review regarding nuisance thresholds and chlorophyll a
standards reported that chlorophyll a concentrations of 10–15 µg/L protect waters inhabited by
salmonids (Pilgrim et al. 2001). Several states in the U.S. and provinces in Canada have stated
that a range of 15 to 50 µg/L maximum chlorophyll a concentrations is ideal for protecting
aesthetic value and recreational uses. Data on water discoloration (Rashke 1994) show that a
level of discoloration deemed acceptable to the average recreational user commonly occurs at
chlorophyll a concentrations between 10–15 µg/L. Above this concentration, deep discoloration
is observed to occur, along with the formation of algal scum, reducing aesthetics and recreational
use.
Current median in-reservoir chlorophyll a concentrations range from 0.7 µg/L at the Mid-
reservoir Site to 1 µg/L at the Dam Site. Mean concentrations are slightly higher ranging from
2.5 µg/L at the Mid-reservoir Site to 3.8 µg/L at the Dam Site, based on data collected from 1996
to 2006 (Figure 3.8).
Table 3.22 Percent of Total Samples in Exceedance of Total Phosphorus Indicator
Threshold (<0.05 mg/L for streams and <0.025 mg/L for reservoirs)
Site Name 2000 2001 2002 2004 2005 2006 Mean
Inflow 50% -- 0% 0% 20% 20% 20%
Dam Site 33% -- 20% 0% 0% 0% 6%
Mid-reservoir Site 25% -- 0% 0% 17% 0% 10%
Six Mile Creek above San Pitch
confluence
-- 0% 25% -- -- 17% 13%
Springs discharging to the
reservoir
-- -- -- -- -- 0% 0%
Nine Mile Reservoir Delisting February 2008

56
Table 3.23 Summary of Current (1999–2006) Chlorophyll a Data (µg/L) in Nine Mile
Reservoir.
Site Name N Mean Median Maximum Minimum Standard
Deviation
Dam Site 12 3.8 1 17.6 0.4 5.4
Mid-reservoir Site 8 2.5 0.7 15.5 0.4 5.3
With the exception of a sampling period in 2006, all of the chlorophyll a data collected during
recent monitoring of Nine Mile Reservoir were below 10 µg/L at both the Dam Site and the Mid-
reservoir Site. Until 2006, mean chlorophyll a levels had been decreasing.
Figure 3.8 Chlorophyll-a trend data for Nine Mile Reservoir.
3.5.3.2 Secchi Depth
Secchi depth is a measurement of the clarity or transparency of surface waters. Secchi depths are
measured using a disk with alternating black and white sections that is lowered into the water.
When the disk is no longer visible, the “Secchi depth” is recorded. For example, a Secchi depth
of three feet indicates that the disk was last visible at three feet below the surface. High Secchi
depth readings indicate that the water is relatively clear and will allow sunlight to penetrate to
greater depths. Low readings indicate turbid water (due to algae growth, suspended sediment, or
other causes), which can reduce the depth to which sunlight can penetrate. Limited light at lower
depths can result in decreased growth of aquatic plants. Secchi depth measurements, in meters,
were reported for the 1996 through 2006 period. According to the reported values taken at both
the Dam Site and Mid-reservoir Site, the Secchi depth readings have generally stayed constant
with a slight improvement in transparency observed since 1998.
In most cases, the Secchi depths recorded for Nine Mile Reservoir (Figure 3.9) show an
increasing trend over time during the summer growing season. Data collected in 2006 are
slightly lower than in past years and are likely related to higher chlorophyll a values during this
same sampling period. Low Secchi depth values also reflect non-algal turbidity associated with
dissolved organic carbon or carbonate alkalinity (Carlson 1992).
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De
c
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n
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c
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5
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Ch
l
o
r
o
p
h
y
l
l

a

u
g
/
L
Mid-Reservoir
Dam Site
Nine Mile Reservoir Delisting February 2008

57

Figure 3.9 Secchi depth trend data for Nine Mile Reservoir.

3.5.3.3 Total Phosphorus
The numeric criterion for total phosphorus is 0.025 mg/L in lakes and reservoirs. This is not
considered a water quality standard but a pollution level indicator that is used along with other
water quality parameters to assist in the determination of the reservoir impairment. The levels of
total phosphorus have been at levels which are primarily lower than the indicator threshold for
total phosphorus, especially in the later portion of the reporting period (Figure 3.10). Total
phosphorus concentrations show a generally decreasing trend in the reservoir.

Figure 3.10 Total phosphorus trend data for Nine Mile Reservoir.
0
1
2
3
4
5
6
7
8
9
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Se
c
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h
i

D
e
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(
m
)
Mid-Reservoir
Dam Site
0
0.05
0.1
0.15
0.2
0.25
0.3
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To
t
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P
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(
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)
Mid-Reservoir
Dam Site
Nine Mile Reservoir Delisting February 2008

58
3.5.4 RESERVOIR WATER COLUMN DATA ASSESSMENT
3.5.4.1 Data and Analytical Methods
This section describes a more in-depth analysis of reservoir stratification and dynamics as well as
water column habitat and fishery health. Water column data for temperature, pH, and dissolved
oxygen were evaluated using the percentage-based criteria established by the State of Utah
specifically for dissolved oxygen. The dataset used in this assessment was the in-reservoir depth
integrated monitoring information provided by UDWQ. Depth integrated data are available for
both the Mid-reservoir Site and the Dam Site and are presented in Table 3.24.
Table 3.24 Depth-integrated Reservoir Monitoring Data
Mid-reservoir Site Dam Site
2000 2002 2004 2005 2006 2000 2002 2004 2005 2006
May 9 9
June 9 9 9 9 9 9 9 9
July 9 9 9 9
August 9 9 9 9 9
September 9 9
3.5.4.2 Reservoir Stratification
Representative depth profile plots of dissolved oxygen, temperature, and pH are displayed for
spring, summer, and fall conditions (2005) observed in Nine Mile Reservoir at the Mid-reservoir
and Dam Sites (Figures 3.11 to 3.14). Depth increases down the vertical axis of each of the plots
displayed. To read the plots, assume that the lower horizontal axis represents the bottom (or
floor) of the reservoir and the top of the plot represents the water surface.

Figure 3.11 Spring (15 June 2005) depth profile plots for dissolved oxygen, temperature,
and pH observed in Nine Mile Reservoir.

Ninemile Reservoir at mid-lake 15 June 2005
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
DO (mg/L) T (degrees C) pH (units)
De
p
t
h

(
m
)
Dissolved Oxygen Temperature pH
Ninemile Reservoir at Dam 15 June 2005
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
DO (mg/L) T (degrees C) pH (units)
De
p
t
h

(
m
)
Dissolved Oxygen Temperature pH
Nine Mile Reservoir Delisting February 2008

59

Figure 3.12 Summer (28 July 2005) depth profile plots for dissolved oxygen, temperature,
and pH observed in Nine Mile Reservoir.
Dissolved oxygen, temperature, and pH data are plotted on separate curves on each of the
figures. Data are displayed for both in-reservoir sites. Depths at the Mid-reservoir Site are
generally shallower than those taken at the Dam Site. Depth integrated data from 2005 were
selected for display here as they represent the year with the best overall seasonal coverage and
relatively average flow conditions (Figures 3.11 to 3.14).
The 2005 water year followed an extended period of drought in the watershed and the Sevier
River basin, and may be representative of in-channel purge/flush conditions where sedimentation
and deposition processes occur to a greater extent upstream in low flow (drought) conditions,
and higher flow events associated with average water conditions result in a surge of deposited
material being delivered to downstream water bodies. These data cannot therefore be considered
completely representative of average water year conditions in the Nine Mile Reservoir watershed
but provide a good illustration of seasonal changes in dissolved oxygen, temperature, and pH
within the reservoir.

Figure 3.13 Summer (11 August 2005) depth profile plots for dissolved oxygen,
temperature and pH observed in Nine Mile Reservoir.

Ninemile Reservoir at mid-lake 28 July 2005
0
1
2
3
4
5
6
7
8
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
DO (mg/L) T (degrees C) pH (units)
De
p
t
h

(
m
)
Dissolved Oxygen Temperature pH
Ninemile Reservoir at Dam 28 July 2005
0
1
2
3
4
5
6
7
8
9
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
DO (mg/L) T (degrees C) pH (units)
De
p
t
h

(
m
)
Dissolved Oxygen Temperature pH
Ninemile Reservoir at mid-lake 11 August 2005
0
1
2
3
4
5
6
7
8
9
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
DO (mg/L) T (degrees C) pH (units)
De
p
t
h

(
m
)
Dissolved Oxygen Temperature pH
Ninemile Reservoir at Dam 11 August 2005
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
DO (mg/L) T (degrees C) pH (units)
De
p
t
h

(
m
)
Dissolved Oxygen Temperature pH
Nine Mile Reservoir Delisting February 2008

60

Figure 3.14 Fall (08 September 2005) depth profile plots for dissolved oxygen, temperature
and pH observed in Nine Mile Reservoir.
Drought or low water year conditions can be assumed to result in warmer water temperatures and
lower dissolved oxygen levels, while high water year conditions can be expected to result in
deeper water levels and lower water temperatures. As a managed system, these year-to-year
variations are likely not as noticeable as in free-flowing, non-impounded systems like natural
lakes.
Figure 3.11, displaying June 2005, depth-integrated data shows some stratification occurring
within the reservoir (a condition where dissolved oxygen and temperature change specific to
depth; lower water layers are generally cooler while upper water layers experience higher
temperatures). Stratification is noticeably stronger at the Dam Site with a marked thermocline
occurring between 4 and 5 meters deep. A thermocline is a location in the water body where
temperature changes by more than 1°C within a 1 meter change in depth. When strongly
established, thermoclines can act to resist mixing, and can lead to low dissolved oxygen in the
lower layers of a reservoir as decomposition removes oxygen from the water column and thermal
inertia discourages mixing of the better aerated surface layers.
Dissolved oxygen concentrations and water temperatures at the Mid-reservoir Site are not in
exceedance of water quality criteria, while dissolved oxygen in the lower depths near the dam
(below 9 meters) show concentrations below 4.0 mg/L, though these occur in less than 25% of
the water column. Because fish and most other aquatic life species are mobile and can relocate to
areas of suitable habitat in the event of a localized criteria exceedance, the State of Utah has
defined the support status of game fish populations relative to the percentage of the total water
column experiencing depressed dissolved oxygen concentrations. In terms of dissolved oxygen, a
water body is defined to be non-supporting for cold water game fish when more than 75% of the
water column exceed dissolved oxygen criteria, partially supporting if 50 to 75% of the water
column depth exceed criteria, and fully supporting where less than 50% of the water column
exceed criteria. These criteria were assessed for dissolved oxygen in Section 3.5.2.1.
Conditions in July 2005 (Figure 3.12) present a marked contrast to the June profiles.
Stratification is noticeably stronger at the Dam Site, with a marked thermocline between 6 and 7
meters deep, while the Mid-reservoir Site is noticeably mixed. Dissolved oxygen concentrations
at the Mid-reservoir Site are indicative of in-reservoir growth processes, and water temperatures
are noticeably higher than the cold water criteria through much of the water column. Dissolved
oxygen concentrations in the lower depths near the dam (below about 7 meters) show conditions
Ninemile Reservoir at mid-lake 08 September 2005
0
1
2
3
4
5
6
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
DO (mg/L) T (degrees C) pH (units)
De
p
t
h

(
m
)
Dissolved Oxygen Temperature pH
Ninemile Reservoir at Dam 08 September 2005
0
1
2
3
4
5
6
7
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
DO (mg/L) T (degrees C) pH (units)
De
p
t
h

(
m
)
Dissolved Oxygen Temperature pH
Nine Mile Reservoir Delisting February 2008

61
that are not supportive of cold water game fish. These low dissolved oxygen conditions are
directly correlated with high water temperatures (above 20°C) in the overlying water column and
act to reduce viable habitat. Fish trying to move deeper to escape warm water temperatures will
encounter low dissolved oxygen concentrations in the lower levels of the reservoir. While the
water column at the Mid-reservoir Site contains sufficient dissolved oxygen, there is no available
depth at which temperatures are appropriate for cold water species.
Conditions in the August 2005 (Figure 3.13) water column for the reservoir present a similar
situation to that observed for July, low dissolved oxygen concentrations in the reservoir depths
are correlated with high water temperatures in the overlying layers and cumulatively act to
reduce viable habitat for cold water species.
September 2005 conditions (Figure 3.14) show that substantial mixing and cooling has occurred
within the water column at both in-reservoir sites. Dissolved oxygen concentrations, water
temperatures, and pH values are nearly static from surface to depth and show that the shorter
daylight hours and cooler air temperatures are affecting both reservoir waters and inflows. While
pH exceedances are still occurring, dissolved oxygen and temperature are within criteria.
Dissolved oxygen concentrations are still indicative of in-reservoir growth, but are not as critical
as water temperatures are lower, allowing greater dissolution to occur.
3.5.4.3 Habitat Viability
Since multiple stressors can have an added detrimental impact on aquatic life (for example both
temperature and dissolved oxygen being in exceedance of the defined criteria simultaneously),
an additional assessment was completed examining the occurrence of two or more water quality
exceedances in the water column at the same time and place. Assuming that viable habitat is
defined as no observed exceedances of dissolved oxygen, temperature, or pH criteria, and
applying the depth distribution of percent water column (established for dissolved oxygen) out of
compliance with water quality criteria, the Mid-reservoir Site was shown to have an average of
more than 50% of the water column of viable habitat during the months of May and June for all
years (except 2006) for which profiles were available. Depth-integrated data collected at the
Mid-reservoir Site show that none of the water column, from surface to depth, could be defined
as viable habitat for the months of June 2006, July 2004, and August 2005 and 2006. Data
collected at this site show 100% of the water column experienced at least 1 parameter in
exceedance of water quality criteria during the above months. Data collected during July 2005
show that 95% of the water column at this site experienced at least 1 parameter out of
compliance. Annual average conditions compiled using available profile data show that 100% of
the water column experienced at least one parameter out of compliance during the month of
August (Table 3.25).
Nine Mile Reservoir Delisting February 2008

62

Depth-integrated data collected at the Dam Site show that none of the water column, from
surface to depth, was in full compliance with water quality criteria for the months of July 2005,
and August 2004, 2005 and 2006. Data collected at this site show 100% of the water column
experienced at least 1 parameter in exceedance of water quality criteria during the above months.
Data collected during June 2006 show that 76% of the water column at this site experienced at
least 1 parameter out of compliance, also indicative of non-support. Data collected during
September 2005 show that 62% of the water column at this site experienced at least 1 parameter
out of compliance, indicating a partial support status. Annual average conditions compiled using
available profile data show that 100% of the water column experiences at least one parameter out
of compliance during the month of August (Table 3.25).
Figure 3.15 presents plots of the relative percent of the water column experiencing a single
exceedance, multiple criteria excursions, and the relative amount of viable habitat available to
fish for summertime monitoring years of 2000, 2002, 2004, 2005 and 2006. Figure 3.16 shows
the relative percent of the water column that is viable habitat for coldwater fish species during
the same time period.
During the months of July and August of all years for which there are profile data, nearly 100%
of the water column at both sites is in exceedance of at least one criterion. The lack of viable
habitat during these months is almost entirely attributed to naturally high pH values and elevated
temperatures associated with the climate and hydrology of the reservoir. Low dissolved oxygen
generally occurs in less than 15% of the water column. (Fig. 3.15)
Table 3.25 Nine Mile Viable Habitat
Site Name Month 2000 2001 2002 2004 2005 2006 Average
May -- -- 100% -- -- -- 100%
June 75% -- -- 92% 87% 24% 70%
July -- -- -- 14% 0% -- 7%
Aug -- -- -- 0% 0% 0% 0%
Dam Site
Sept -- -- -- -- 38% -- 38%
May -- -- 84% -- -- -- 84%
June 98% -- -- 94% 100% 0% 73%
July -- -- -- 0% 5% -- 3%
Aug -- -- -- -- 0% 0% 0%
Mid-reservoir Site
Sep -- -- -- -- 27% -- 27%
Nine Mile Reservoir Delisting February 2008

63
Figure 3.15 Relative percent of the water column at Mid-reservoir Site (upper plot) and near the Dam Site (lower plot)
experiencing one or more exceedances of water quality criteria.
Average Percent Water Column Criteria Exceedances at Site 594324 - Ninemile Reservoir Above Dam 01 (2000-2006)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
ma
y
ju
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ma
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ju
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ju
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au
g
se
p
Low DO High Temp+pH High Temp only
2000 2002 2004 2005 2006 Avg
Average Percent Water Column Criteria Exceedances at Site 594325 - Ninemile Reservoir Mid-Lake 02 (2000-2006)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
ma
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Low DO High Temp+pH High Temp only
2000 2002 2004 2005 2006 Avg
Nine Mile Reservoir Delisting February 2008

64
Figure 3.16 Relative percent of the water column at Mid-reservoir Site (upper plot) and near the Dam Site (lower plot)
exhibiting viable habitat conditions. For the above plots, viable habitat condition was defined as that portion of the water
column where no exceedances of water quality criteria (joint or single) were observed.
Average Percent Water Column Viable Fish Habitat at Site 594324 - Ninemile Reservoir Above Dam 01 (2000-2006)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
ma
y
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ma
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se
p
Viable habitat Non-Viable Habitat
Average Percent Water Column Viable Fish Habitat at Site 594325 - Ninemile Reservoir Mid-Lake 02 (2000-2006)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
ma
y
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Viable habitat Non-Viable Habitat
2000 2002 2004 2005 2006 Avg
2000 2002 2004 2005 2006 Avg
Nine Mile Reservoir Delisting February 2008

65
3.5.5 COMPARISON WITH PALISADES LAKE
3.5.5.1 Characteristics of Palisades Lake
Palisades Lake, a reservoir located about 2.3 miles northeast of Nine Mile Reservoir, receives
water from Six Mile Creek and occupies soils similar to the majority of the watershed. Palisades
Lake does not overlay the extremely alkaline soils associated with the southwest portion of Nine
Mile Reservoir and does not receive appreciable spring water. All of the water in Palisades Lake
is diverted from Six Mile Creek.
Palisades Lake is smaller than Nine Mile Reservoir, holding 1,728 acre-feet of water
(approximately 1/2 the volume of Nine Mile Reservoir), and has a surface area of 28 acres
(approximately 1/3 of the surface area of Nine Mile Reservoir). Palisades Lake is shallower than
Nine Mile Reservoir with a maximum depth at Palisades Lake of 9.5 meters (31 feet) and a
maximum depth at Nine Mile of 11 meters (36 feet). Dissolved oxygen data at the dam for
Palisades Lake extend to 7.7 meters while dissolved oxygen data is available to a depth of 9.3
meters in Nine Mile Reservoir at the dam. Palisades Lake is not listed as impaired for low
dissolved oxygen, although it is listed as impaired for high temperature on the State of Utah’s
303(d) 2006 list of impaired waters. Therefore, comparison between the two water bodies
provides additional evidence for recommending that Nine Mile Reservoir be removed form the
303(d) list of impaired waters.
The operation of the two reservoirs differs substantially, as Palisades Lake experiences much
greater flow-through conditions than Nine Mile Reservoir. Therefore, a comparison of
instantaneous water quality characteristics between the two reservoirs can help to identify water
quality effects that are the result of watershed-based conditions and Six Mile Creek inflows. It
cannot, however, be used to distinguish between water quality effects that result from flow-
management and spring water inflows.
3.5.5.2 Water Quality Comparison
A direct comparison of the data available to the study is included in Table 3.26. Average water
quality conditions in Palisades Lake closely mimic average conditions occurring in Six Mile
Creek and Nine Mile Reservoir in all cases except alkalinity, minimum dissolved oxygen, pH at
the dam, and salinity.
Average alkalinity observed in Palisades Lake is approximately 28% less than that in Nine Mile
Reservoir. This is likely a direct result of the high-alkaline soils present in a portion of Nine Mile
Reservoir.
Average dissolved oxygen levels observed at the dams are similar between the two systems, with
Nine Mile Reservoir concentrations slightly higher. Although minimum dissolved oxygen
concentrations observed at the dams are lower for Nine Mile Reservoir than Palisades Lake, it
must be noted that this could be related to the greater depth of Nine Mile Reservoir than
Palisades Lake. At almost every depth, when compared directly, Nine Mile Reservoir maintains
a comparable and sometimes higher dissolved oxygen profile than Palisades Lake (Figure 3.17).
This comparison holds down to the maximum depth (7.8 meters) for which data is available in
both systems. Low dissolved oxygen at greater depths in Nine Mile Reservoir can not be directly
compared with Palisades. Low dissolved oxygen in Nine Mile Reservoir is isolated to the
sediment-water interface, a pattern that is exhibited by most reservoirs. The fact that the low
dissolved oxygen profiles do not extend upward into the water column suggests that the
hypolimnetic oxygen demand in Nine Mile Reservoir is not excessive.
Nine Mile Reservoir Delisting February 2008

66
Average inflow nitrogen concentrations to Nine Mile Reservoir, including the springs that
discharge to the reservoir, represent the greatest difference between the two systems, with
concentrations in Nine Mile Reservoir inflows observed to be nearly 4 times greater than those
observed in Six Mile Creek downstream of the diversion to Nine Mile Reservoir.
Table 3.26 Summary of Comparative Data Between Palisades Lake and Nine Mile
Reservoir
Nine Mile 1999–2006

Palisades
at Dam
1999–2006 Inflow At Dam Mid-
reservoir
Six Mile at
San Pitch
2001–2006
Springs
at
Discharge
2006
Alkalinity (mg/L)
Average 183 -- 269 -- 271 --
Max 216 -- 364 -- 405 --
Dissolved Oxygen (mg/L)
Average 8.07 7.53 8.84 9.82 9.40 6.79
Min 2.36 5.85 0.28 1.37 4.50 6.36
% Exceed 5% 0% 14% 5% 0% 0%
Dissolved Oxygen Sat (%)
Average 105.3% 90.9% 118 % 132 % 94 % --
Max 163.0% 113.9% 194 % 196 % 112 % --
% Exceed 47% 13% 64% 77% 5% --
NO2+NO3 (mg/L)
Average 0.21 3.41 0.33 0.32 0.95 2.26
Max 0.57 6.52 2.13 1.18 2.80 6.70
% Exceed 0% 0% 0% -- -- 17%
pH
Average 8.60 8.25 9.01 9.10 8.41 8.11
Max 9.10 8.73 10.07 9.86 8.97 8.44
% Exceed 13% 0% 54% 60% 0% 0%
Total Phosphorus (mg/L)
Average 0.02 0.030 0.010 0.020 0.02 0.010
Max 0.14 0.261 0.068 0.257 0.07 0.016
% Exceed 24% 20% 6% 10% 13% 0%
Temperature (°C)
Average 18.40 18.33 20.04 20.22 10.21 18.06
Max 24.25 26.41 25.8 24.36 21.81 22.3
% Exceed 32% 33% 20% 14% -- --
Salinity (ppt)
Average 0.20 0.43 0.42 0.39 0.35 --
Max 0.24 0.68 0.70 0.50 0.79 --

Nine Mile Reservoir Delisting February 2008

67

Figure 3.17 Dissolved oxygen profiles in Nine Mile Reservoir and Palisades Lake for Mean
and Minimum Dissolved Oxygen.
While maximum pH values observed at the dams show violations of water quality criteria (9.0)
occurring in both cases, the magnitude of the exceedance observed in Palisades Lake is 10 times
lower than that for Nine Mile Reservoir (0.1 units and 1.1 units, respectively). Given the
differences soil pH surrounding the two reservoirs the greater magnitude of pH exceedance is
most likely the result of both more alkaline soils on the southwest portion of Nine Mile
Reservoir.
Average and maximum total phosphorus concentrations are similar in both systems, while
average and maximum concentrations in Nine Mile Reservoir are at the dams 2 times lower than
those observed at Palisades Lake. However, the highest phosphorus value in the dataset was
observed at the Mid-reservoir Site in Nine Mile.
Average salinity observed in Nine Mile Reservoir is approximately 2 times higher than that
observed in Palisades Lake and maximum salinity is approximately 3 times higher. This is likely
due in part to the high-alkaline soils present in a portion of Nine Mile Reservoir.
Depth integrated data are available for the Dam Site at Palisades Lake and show similar trends as
observed in Nine Mile with the exception that pH exceedances are minor and dissolved oxygen
levels do not drop as low as observed in Nine Mile Reservoir.
0 5 10 15
0
0.5
1
1.5
2
2.5
3
3.5
4.1
4.6
5.1
5.6
6.1
6.6
7.1
7.6
8.1
8.6
9.1
De
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f
r
o
m

r
e
s
e
r
v
o
i
r

s
u
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f
a
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e
Mean dissolved oxygen (mg/L)
Reservoir at Dam
Reservoir at Midlake
Palisades Lake
0 5 10 15
0
0.5
1
1.5
2
2.5
3
3.5
4.1
4.6
5.1
5.6
6.1
6.6
7.1
7.6
8.1
8.6
9.1
De
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f
r
o
m

r
e
s
e
r
v
o
i
r

s
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f
a
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e
Minimum dissolved oxygen (mg/L)
Reservoir at Dam
Reservoir at Midlake
Palisades Lake
Nine Mile Reservoir Delisting February 2008

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3.5.5.3 Implications for Nine Mile Reservoir impairment status
The differences in management between Palisades Lake and Nine Mile Reservoir do not allow a
direct comparison of water quality characteristics on a seasonal basis. However, the general
trends identified in this comparison demonstrate that soil alkalinity in Nine Mile Reservoir and
the immediate watershed area contributes to elevated pH levels and pH criteria exceedances in
the reservoir. In addition, this comparison indicates that dissolved oxygen profiles are generally
slightly higher in Nine Mile Reservoir than in Palisades Lake. This comparison corresponds to
lower nutrient concentrations, on average, in Nine Mile Reservoir when compared with Palisades
Lake. Neither system exhibits routine exceedances of the water quality criteria established for
dissolved oxygen nor the threshold identified for total phosphorus in reservoirs. Since Palisades
Lake is not listed as impaired for dissolved oxygen and total phosphorus, based on the
comparison between the two systems Nine Mile Reservoir should be removed from the 303(d)
list for the low dissolved oxygen impairment.
3.5.6 IMPAIRMENT ADJUSTMENTS
Up to this point, all of the evidence presented indicates that Nine Mile Reservoir is in full
support status for all of the beneficial uses designated for the water body. Since this
determination is made on the basis of exceedances of physical and chemical water quality
parameters, the State of Utah allows for the initial support status determination to be modified
based on biological indicators of water quality. For reservoirs, this includes an evaluation of the
Trophic State Index (TSI), winter dissolved oxygen conditions with reported fish kills, and the
presence of significant blue-green algal populations in the phytoplankton community. In order to
complete a comprehensive review of the support status of Nine Mile Reservoir, these criteria are
evaluated in the following sections.
3.5.6.1 Trophic State Index (TSI)
The health and support status of a water body can be assessed using a TSI, a measurement of the
biological productivity or growth potential of a body of water. The basis for trophic state
classification is algal biomass (estimation of how much algae is present in the water body). The
calculation of a TSI generally includes the relationship between chlorophyll (the green pigment
in algae, where chlorophyll a is used as a surrogate measure of algal biomass), transparency
using Secchi depth measurements, and total phosphorus (commonly the nutrient in shortest
supply for algal growth) as follows (Carlson and Simpson 1996):
• Chlorophyll a: TSI CHL = 9.81 Ln (Chl a) + 30.6
• Secchi depth: TSI SD = 60– 14.41 Ln (SD)
• Total Phosphorus: TSI TP = 14.42 Ln (TP) + 4.15
Waterbodies with very low TSI values (less than 30) are generally transparent, have low algal
population densities, and have adequate dissolved oxygen throughout the water column.
Waterbodies with these characteristics are generally supportive of cold-water fisheries and are
identified as oligotrophic.
Waterbodies with low to midrange TSI values (40-50) are moderately clear, and have an
increasing chance of hypolimnetic anoxia in summer. Waterbodies with these characteristics are
generally supportive of warm water fisheries and are identified as mesotrophic.
Waterbodies with midrange TSI values (50–70) commonly experience more turbidity (the water
is not as clear) and higher algal population densities than oligotrophic waterbodies. These
waterbodies often exhibit low dissolved oxygen levels in mid- to late-summer, with the most
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extreme conditions observed in the hypolimnetic (deeper) water column. Waterbodies with these
characteristics often experience some macrophyte problems (excessive growth) and are generally
supportive of warm water fisheries only. These waterbodies are identified as being eutrophic.
Waterbodies with high TSI values (70 and greater) are generally observed to have heavy algal
blooms, dense macrophyte growth, and extensive dissolved oxygen problems that often occur
throughout the water column. Fish kills are often common and recreation is limited under such
conditions. Fish populations are generally confined to rough fish species. Such waterbodies are
identified as hypereutrophic.
Table 3.27 identifies generally accepted trophic state values derived from this relationship. In
most cases, the greater the TSI value a water body has, based on collected data, the more
eutrophic the water body is said to be.
Table 3.27 TSI Values and Status Indicators
TSI Trophic Status and Water Quality Indicators
< 30 Oligotrophic; clear water; high DO throughout the year in the entire hypolimnion.
30–40 Oligotrophic; clear water; possible periods of limited hypolimnetic anoxia (DO =0).
40–50 Mesotrophic; moderately clear water; increasing chance of hypolimnetic anoxia in summer;
cold water fisheries “threatened”; supportive of warm water fisheries.
50–60 Mildly eutrophic; decreased transparency; anoxic hypolimnion; macrophyte problems; generally
supportive of warm water fisheries only.
60–70 Blue-green algae dominance; scums possible; extensive macrophyte problems.
70–80 Heavy algal blooms possible throughout summer; dense macrophyte beds; hypereutrophic.
> 80 Algal scums; summer fish kills; few macrophytes due to algal shading; rough fish dominance.
Source: From Carlson and Simpson, 1996.
The trophic scale outlined in Table 3.27 illustrates these general classifications, as well as the
midrange conditions that occur between each major category. However, each water body is
unique and will exhibit site-specific characteristics based on the water quality conditions
identified within the lake or reservoir and over specific time periods, seasons, or water flow
conditions. The identification of TSI values for a specific waterbody allows a general
classification and may provide insight into overall water quality trends and seasonality.
Summer TSI values for Nine Mile Reservoir have been calculated using the data available for
chlorophyll a concentrations, Secchi depth, and total phosphorus concentrations. The resulting
values are displayed in Figures 3.18 and 3.19. Mean TSI values for Nine Mile Reservoir are
listed in Table 3.28.
Results of the TSI evaluation for Nine Mile Reservoir indicate that the reservoir is generally
mesotrophic with mean TSI values ranging from 29 (chl a TSI) at mid-reservoir to 45 (Secchi
depth TSI) at the Dam Site. Recently, TSI values at the Dam Site have been improving while
results at the Mid-reservoir Site have been more varied during the sampling period but are
indicative of improving conditions.

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Figure 3.18 Trophic state index trend data for Dam Site

Figure 3.19 Trophic state index trend data for Mid-reservoir Site.
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Table 3.28 Summary of Current (1999–2006) TSI Data in Nine Mile Reservoir.
Site Name Mean Chlorophyll a
TSI
Mean Secchi Depth
TSI
Mean Total Phosphorus
TSI
Reservoir at dam 33.9 45.5 30.6
Mid-reservoir 29.4 41.3 34.2
Determining the relationship between TSI values calculated for a specific water body is also
helpful in identifying factors that limit algal biomass and/or affect the measured water quality
parameters. Although every water body is unique, a number of common relationships between
Secchi depth, chlorophyll a, and total phosphorus have been identified (Carlson 1992). The
routine occurrence of lower chlorophyll a TSI than Secchi depth TSI, as shown in Figures 3.18
and 3.19, indicates reduced transparency from non-algal factors including clay or dissolved
organic matter (Carlson 1992). In Nine Mile Reservoir, this could indicate turbidity associated
with carbonate alkalinity. The data show that chlorophyll a TSI values and total phosphorus
levels rarely exceed mildly eutrophic levels. The majority of TSI values above the mesotrophic
condition are attributed to Secchi depth readings which are indicative of turbidity interference
associated with alkalinity.
3.5.6.2 Fish Kills
There have been no reported fish kills in Nine Mile Reservoir (UDWQ 2007). The reservoir is
stocked by DWR annually with rainbow trout and tiger trout (rainbow/brook trout hybrid).
Fishing usage within the reservoir has been reported as light.
3.5.6.3 Phytoplankton Composition
The presence of blue-green algae, or cyanobacteria, in the phytoplankton community has been
associated with the occurrence of toxins and mortality in local animal populations (Sabater and
Admiraal 2005). Although cyanobacteria may be of low toxicity, cyanotoxins can become highly
concentrated in the environment or through bioaccumulation where cyanobacterial overgrowth
occurs. The introduction and/or overgrowth of cyanobacterial species is a potential hazard to
water quality and the Nine Mile Reservoir aquatic ecosystem. Cyanobacteria can dominate
nitrogen-limited systems due to their ability to fix atmospheric nitrogen. As a result,
cyanobacteria can increase where low nitrogen limits the growth of other algal species (Sharpley
et al. 1984 and 1995; Tiessen 1995).
The relative densities of algal species and diversity of the algal community both serve as
surrogate measures of water quality by identifying overall species diversity, excessive algal
growth or eutrophication, and the presence and relative abundance of toxic blue-green algae.
This assessment is based on phytoplankton samples collected from Nine Mile Reservoir in 2000,
2002, and 2004. Species abundances were measured using counts for periphyton and number per
liter for phytoplankton. Detailed plankton data are available for the Dam Site at Nine Mile
Reservoir for August 9, 2000, August 8, 2002, and August 10, 2004 (Rushforth and Rushforth
2001, Rushforth and Rushforth 2003, Rushforth and Rushforth 2005). Algal taxa present at these
times were identified and grouped by taxon to show green algae (chlorophyta), blue-green algae
(cyanophyta), diatoms (bacillariophyta), and others.
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In 2000 and 2002, green algae dominated at 71.6% and 89.7% respectively of the total algal
population. Diatoms represented a much smaller population segment at 0.8% and 9.8%
respectively of the total. The 2004 sampling also proved green algae to be the dominant
population, representing 61.5% of the total algal population. Diatom populations were again
substantially smaller than the green algae population, representing 27.9% of the total. Blue-green
algae were not detected in 2000 or 2004. Small populations of blue-green algae were found in
2004 representing 5.4% of the total phytoplankton population.
3.5.7 SUPPORT STATUS SUMMARY
There have been no observed exceedances of the dissolved oxygen criteria established by the
State of Utah in Nine Mile Reservoir since 1999. More than 50% of the water column meets
dissolved oxygen criteria of greater than 4.0 mg/L during all months at both sites; therefore the
reservoir is in full support status for the cold water fishery–designated beneficial use, based on
the dissolved oxygen criteria. This conclusion is supported by an analysis of nutrient and
chlorophyll a data available for Nine Mile Reservoir.
Current mean and median total phosphorus concentrations are below the threshold established by
the State of Utah. This threshold of 0.025 mg/L is rarely exceeded in Nine Mile Reservoir, with
more than half of the data points available for the reservoir recorded as “nondetect.”
Exceedances appear to be associated primarily with low reservoir water level during which time
phosphorus in the sediment is more likely to be suspended throughout the water column. Since
no conservation pool has been established for this reservoir, the management of the reservoir
level is not negotiable. Chlorophyll a values are also well below the indicator values of 10 µg/L
identified in the literature of being protective of cold water fisheries and recreational uses.
Trend data for total phosphorus concentration, chlorophyll a, and Secchi depth indicate that
water quality in the reservoir has been improving since 2000, with the exception of one high total
phosphorus concentration identified in the summer of 2006.
Dissolved oxygen saturation data do show routine exceedances of the 110% saturation criteria
established for dissolved gases; however the majority of these exceedances occurred at depths
greater than 1 meter for which raw data had not been corrected for hydrostatic pressure. When
these data are corrected, mean and median dissolved oxygen saturation for Nine Mile Reservoir
are below the 110% criteria. Routine exceedances of 110% saturation at the surface of Nine Mile
Reservoir could be indicative of diurnal fluctuations in temperature and oxygen solubility. High
dissolved oxygen saturation values could also indicate in-reservoir algal growth, however
chlorophyll a data do not support this interpretation.
Elevated pH levels in Nine Mile Reservoir are associated with naturally alkaline soils in the
watershed that extend below the reservoir. This natural condition results in the exceedance of the
pH criteria established for cold water fisheries by the State of Utah. The reservoir also
experiences elevated temperature throughout the summer. However, this condition has been
associated with a recent drought throughout the state. UDWQ has determined that this explains
the temperature exceedances observed in the reservoir. Nine Mile Reservoir is not listed as
impaired for temperature on the 2006 State of Utah 303(d) list of impaired waters.
A comparison between Nine Mile Reservoir and Palisades Lake, both of which receive inflow
from Six Mile Creek, indicates that pH and alkalinity are significantly higher in Nine Mile
Reservoir than in Palisades Lake, providing another line of evidence that high pH in Nine Mile
results from the alkaline soils in the surrounding area. Mean total phosphorus and dissolved
oxygen are similar between the two systems, based on a detailed water column analysis,
indicating that Nine Mile Reservoir should not be listed as impaired for this water quality
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parameter. Water column dissolved oxygen values are comparable between the two reservoirs at
specific depths and often lower in Palisades Lake. The lowest dissolved oxygen measurement in
Nine Mile Reservoir is at a depth lower than the maximum depth for Palisades Lake making it
difficult to compare the two systems at such depths. It is expected that a larger hypolimnion
would form in a deeper reservoir and stratification would take place for a longer period of time
resulting in lower dissolved oxygen expected at the sediment-water interface.
Analysis of the TSI, fishery, and phytoplankton composition for Nine Mile Reservoir add further
support to the conclusion that Nine Mile is not impaired for low dissolved oxygen or elevated
total phosphorus. The reservoir is generally mesotrophic. Occurrences of reduced turbidity
measured in terms of Secchi depth generally indicate a non-algal source of light interference
associated with the high-alkaline water in the reservoir. There have been no documented fish
kills in the reservoir and blue-green algae species are not prevalent.
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4 WATERSHED AND RESERVOIR MODELING
The watershed and reservoir modeling approach chosen for Nine Mile Reservoir was used to
predict nutrient concentrations in Six Mile Creek, nutrient loads from the Nine Mile Reservoir
watershed, and reservoir water quality under average conditions. The Soil and Water Assessment
Tool (SWAT) model was used to simulate hydrologic and nutrient load output from the
watershed (Nietsch 2000) and the BATHTUB reservoir model (Walker 1999) was used to
simulate water quality in Nine Mile Reservoir. The linked SWAT and BATHTUB modeling
scheme provides a systematic method for modeling nutrient sources, transport, delivery, and
assimilation in a watershed-reservoir system.
4.1 WATERSHED MODEL: SOIL AND WATER ASSESSMENT TOOL (SWAT)
The SWAT model was used to simulate hydrologic and nutrient load output from the watershed.
Multiple SWAT simulations were executed in order to account for the variability in annual and
seasonal climatic patterns as well as input for reservoir management simulations which have
been completed with the BATHTUB model. The SWAT simulations were paired with
simulations of different reservoir management patterns (Figure 4.1) using the BATHTUB model.
The years 1997 through 2006 were used to estimate flow and runoff patterns during an average
hydrologic year. The SWAT simulation output was validated with existing water quality
monitoring data which had been collected within Six Mile Creek.
4.1.1 GENERAL MODEL DESCRIPTION
The USDA Agriculture Research Station (USDA ARS) developed SWAT to predict the effects
of management practices on water, sediment, nutrient, and pesticide yields at the watershed
scale. The tool uses a GIS environment to subdivide watersheds into smaller, spatially linked
units with Digital Elevation Models (DEM). To further divide subwatersheds into hydrologic
response units (HRUs), the tool breaks units by land use, management practices, and soil-type
GIS coverages. An HRU is not a spatial subdivision but rather a total area within a subwatershed
that possesses similar land uses and soils. Within SWAT, all HRUs are assumed to be
homogenous. Moreover, they simplify model simulations by combining land uses and soil types
that overlie each other in the GIS environment.
4.1.1.1 Model Components and Operation
The SWAT modeling tool incorporates climatic and physical watershed data and stream reach
routing to simulate hydrologic dynamics, crop growth, and nutrient dynamics. Overall, the
SWAT modeling tool provides the modeling environment necessary to simulate groundwater and
surface water hydrology and water quality at the watershed scale (Figure 4.1).
The model makes use of long-term continuous time period simulations using readily available
data for inputs. These data inputs include regional hydrology, DEMs, climatic data, soils, and
land uses. Most of these data are available from regional or national natural-resource agencies
without cost. Figure 4.1 summarizes the physical processes that SWAT simulates. The tool
simulates the hydrology of the watershed using several different physical processes including ET
and canopy storage for water that is intercepted by vegetation, infiltration, and redistribution.
The tool uses rainfall amounts to calculate surface runoff volumes, infiltration, and peak runoff
rates for each HRU. The model is capable of using either the STATSGO or the Soil Survey
Geographic (SSURGO) database for soils data. The model allows up to 10 soil layers where
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infiltration and water holding capacity, among other things, may be modified. Water held within
the soil profile is moved through the matrix by the storage routing method.
Not all areas within the state have complete SSURGO data coverage at this time, so STATSGO
data is applied. Land use and land cover (LULC) information is also used for a data layer. The
LULC coverage is overlain with the soil GIS layer to facilitate HRU development for the
application of the physical-based equations applied throughout the modeled watershed.
The model accounts for both saturated and unsaturated flows. Saturated flow is driven by gravity
and the movement is characterized by a storage routing method. The latter calculates the amount
of soil water percolating to an underlying soil layer on a given day. Water in excess of the
permanent wilting point, or soil field capacity, is available for plant growth or infiltration within
the soil profile. For unsaturated flow, movement occurs in any direction based on energy
gradients from areas of high to low water content. Only saturated flow is simulated; however,
water consumed by the plant during growth is simulated indirectly by the ET process associated
with the plants.
The model relies on climatic data for computer simulations and makes use of many different
sources of climatic data in equations associated with physical processes within the watershed.
The model also has a built-in weather generator that employs a network of weather stations
throughout the country to develop a climatic record. This climatic record is based on averaged
values, which demonstrate weather extremes that may have occurred within the watershed being
modeled during the simulation period.
Two daily climatic datasets (including temperature and precipitation), in most cases, should be
developed using local weather station data. The model will associate the local climate station
dataset to each subbasin within the watershed boundary and apply the climate data for the
simulation. During the simulations, any missing climate data are estimated by the model to
provide for a complete dataset. Other climate data required by the model, such as wind speed,
relative humidity, and daily solar radiation, are usually simulated by the model. On rare
occasions when actual measured data are available on a daily time step, these data are used in
lieu of simulated data.
The model has two methods for infiltration and runoff. The first method is the Green-Ampt
method, where water infiltration occurs through a wetted front routine. This method requires
subhourly precipitation data which is not available in this watershed and will therefore not be
discussed further. The other runoff method is the SCS curve number (CN), which is based on a
rainfall-runoff relationship where overland flow will not occur until all depressional storage
(surface storage, canopy interception, and infiltration) has occurred. The equation also looks at
soil permeability, land use, and antecedent soil moisture conditions to determine runoff. The
runoff rate is dependant upon empirical values that have been developed across the U.S. for
cover types associated with land uses present within the watershed. The CN influences the runoff
values and is accounted for by a CN value applied within model parameter settings. This number
is set initially within the model but may be changed to adjust runoff values during model
simulations.
The SWAT model also simulates shallow and deep groundwater aquifers. Shallow aquifers
contribute flow to the stream reach in the watershed and also reinfiltrate water into the soil
profile. The remaining infiltrated water may also be pumped out or may recharge the deeper
aquifer. This aquifer is confined and contributes water outside of the watershed. Waters of the
deep confined aquifer that are not pumped for irrigation purposes are considered lost to the
watershed.
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Figure 4.1. SWAT model schematic of water routing and processes (Neitsch 2002).
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To simulate erosion and sediment yield, SWAT uses the Modified Universal Soil Loss Equation
(MUSLE), which employs the amount of runoff derived from the runoff methods (listed above)
to calculate sediment yield. The sediment is delivered to the surface water system by overland
flow. The model uses two versions of the kinematic wave approximation (variable storage and
Muskingum approximation) to route waters through the stream channels. In-stream sediment
transport and channel erosion are also included. In-stream water quality processes are modeled
using built-in modified QUAL2E mathematical methods.
For nutrient simulation processes, SWAT models the water flow through the natural system to
determine the amount of nutrients transported from one source to another. For nitrogen
simulation, the basic nitrogen cycle and transformations are used. The SWAT tool monitors five
different pools of nitrogen in the soil (two inorganic and three organic). The loading function
estimates daily organic nitrogen (N) loss based on the organic N concentration in the uppermost
layer of soil, the sediment yield, and a N-enrichment ratio. Soluble and organic phosphorus are
also removed by transport with the water movement described above. Soluble phosphorus (P)
runoff is calculated using the solution P values in the upper 10 mm present in the soil, the runoff
volume, a soil-partitioning factor, as well as an enrichment ratio. The tool monitors six different
pools of phosphorus in the soil (three inorganic and three organic) and these pools are further
divided by rate of decay and mineralization into active and labile pools. Nutrient loads and water
flow rates will be used as inputs into a one-dimensional reservoir model (Neitsch et al. 2002).
4.1.2 MODEL DEVELOPMENT FOR NINE MILE RESERVOIR WATERSHED
A watershed model was produced for the Nine Mile Reservoir watershed using SWAT 2005.
Numerous steps are involved in the process of deriving and building the spatial model as input
into the SWAT model. A GIS interface is employed within SWAT to achieve this task. The
initial setup of the watershed included the processing of DEMs, hydrography, soils, LULC, as
well as connecting local climatic data to the watershed.
4.1.2.1 Digital Elevation Data for the Nine Mile Reservoir Watershed
The elevation gradient for the Nine Mile Reservoir watershed was obtained from the USGS
National Elevation Dataset (NED) website (http://ned.usgs.gov/). The NED is a 1:24,000-scale
DEM for the conterminous U.S.; it has a geographic projection with a one-arc second resolution
and elevation units in meters. The horizontal datum is NAD83, with a vertical datum of
NAVD88. The coverage is a continuous grid and overlays the entire watershed.
4.1.2.2 Hydrology Data for Nine Mile Reservoir Watershed
The Nine Mile Reservoir watershed is located in Sanpete County, within central Utah, about 90
miles south of Salt Lake City. The Hydrologic Unit Code (HUC) #16030004 is designated for
the San Pitch River watershed as defined by the United States Geological Survey (USGS), of
which the Six Mile Creek and Nine Mile Reservoir watersheds are part of. The Nine Mile
Reservoir watershed is located in the western margins of the Six Mile watershed. With the
construction of the Sterling Irrigation Diversion located near Sterling, Utah, a significant portion
of the Six Mile Creek flow can be diverted directly to Highland Canal for delivery and storage in
the Nine Mile Reservoir for use as irrigation water. The SWAT model for the Nine Mile
Reservoir watershed was developed with the knowledge of the trans-basin diversion. The
modeled watershed includes the Six Mile watershed as well as the much smaller Nine Mile
Reservoir watershed. The trans-basin diversion occurs from approximately March to the end of
May with one-half of the Six Mile Creek flow diverted into the Highland Canal for delivery to
the Nine Mile Reservoir. The remaining Six Mile Creek flow continues through the watershed to
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the San Pitch River or is diverted into the Palisades Lake. No historical measurement data is
available for any of the diversions. Only anecdotal information from local water resource
officials is available.
Hydrology for the Nine Mile Reservoir watershed was obtained from the USGS National
Hydrography (NHD) website (http://nhd.usgs.gov/data.html). Within the NHD, surface water
reaches link the surface water drainage network. The NHD is based on a digital line graph
(DLG) hydrography, with reach-related information from the EPA Reach File Version 3.0 (RF3).
Included within the NHD are Six Mile Creek and Nine Mile Reservoir.
4.1.2.3 LULC and Soils Data for the Nine Mile Reservoir Watershed
The Southwest Regional Gap Analysis Project (SWReGAP) LULC dataset was originally used
in the setup of the Nine Mile Reservoir watershed model. Coverage for SWReGAP is on a
regional, multiple-state scale (i.e., Arizona, Colorado, Nevada, New Mexico, and Utah). The
project focuses on mapping land cover for large geographic areas (Lowry et al. 2005). The
SWReGAP LULC dataset, though very detailed in land coverage description, is not compatible
with SWAT. The SWReGAP land use descriptions were therefore converted to Multi-Resolution
Land Characteristics Consortium (MRLC) National Land Cover Dataset (NLCD) descriptions.
These land cover data are based on land cover classes including various forest types, urban land
uses, surface water, wetlands, and agricultural lands, among others.
Table 4.1 Watershed Land Use and Area Breakdown Used for Load Calculations
Six Mile Watershed Nine Mile Watershed
Total Watershed
Combined
Acreage
Land Use Area (ha) Area % Area (ha) Area % Area
(ha) Area %
Urban 56.9 0.7% 29.5 1.4% 86.4 0.8%
Pasture 208.7 2.4% 173.0 8.1% 381.7 3.5%
Forest (deciduous) 3714.0 42.0% 421.0 19.7% 4135.0 37.7%
Forest (evergreen) 3166.0 35.8% 1065.1 49.8% 4231.1 38.5%
Range (brush) 599.8 6.8% 256.8 12.0% 856.6 7.8%
Range (grass) 1063.0 12.0% 182.2 8.5% 1245.2 11.3%
Wetland 31.6 0.4% 13.3 0.6% 44.9 0.4%
Total 8840.0 100.0% 2140.9 100.0% 10980.9 100.0%
The SWAT model uses MRLC NLCD land cover descriptions to assign plant growth properties
and land runoff potential based on a built-in database of physical properties associated with each
of the land uses. Together with the STATSGO data on soils coverage properties HRUs for the
watershed were calculated by setting a threshold level of minor land-use areas with land-use
areas less than the threshold level being ignored, and reapportioning it among the major land
uses and soil types. The default values for the threshold are usually set at 10% for land use and
20% for soil type. This means that any land use taking place in an area of less than 10% within
the watershed would be reassigned by the model to the most similar land use type. The same
would occur for soil coverage, which has an area threshold of less than 20% within the
watershed. Because of the detailed coverage supplied by the SWReGAP data, the threshold
levels for the HRU determination were set at 2% for land use and 4% for soils. This allowed for
a more detailed assessment associated with land use near Six Mile Creek and Nine Mile
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watershed for the application of irrigation events and grazing activity. Land uses assigned within
the model, including acreage and percent area within the watershed, are listed in Table 4.1.
Forested lands and rangelands make up approximately 76% and 19% of the watershed,
respectively.
4.1.2.4 Management Practices for the Nine Mile Reservoir Watershed
The SWAT model accommodates input for management practices associated with land uses.
These practices are imbedded in the model at the HRU level, facilitating input of spatial scale
land management data and, in turn, facilitating output results that may be evaluated within
specific areas within the watershed.
The grazing of cattle and sheep within the watershed occurs throughout the summer growing
season. Though the boundaries of the grazing allotments are known in the watershed, what is not
known is the specific number of cattle and sheep that are present within each subbasin for any
specified amount of time. The SWAT model allows for the inclusion of grazing animals within a
subbasin but evaluates the grazing within each subbasin based upon daily food uptake, animal
pressure on the range, and manure deposition. Other grazing controls also allow for the setting of
grazing days within a subbasin along with the establishment of a crop residual amount that
precludes the animals from overgrazing the grassland to levels below actual growth potential.
This factor forces the model to eliminate grazing from any subbasin before simulated
overgrazing occurs. The average grazing animal parameters have been set as follows:
Cattle Grazing Sheep Grazing
Daily Forage Uptake(dry weight) 80.1 lb/ac/day 28.8 lb/ac/day
Grazing Time Limit/subbasin 45 days 45 days
Trampling impact from hooves (dry weight) 0.9 lb/ac/day 0.29 lb/ac/day
Manure deposition (dry weight) 0.22 lb/ac/day 0.06 lb/ac/day
Biomass Residual 356 lb/acre 356 lb/acre

The management inputs were scheduled within the simulation by using heat units (Neitsch
2002). The heat units are based upon the fact that any crop needs specific temperatures to start
production within a growing period. Any temperature value above the baseline minimum
temperature for a crop goes into the plant for growth. Specific management practices can be
scheduled in SWAT based upon a percentage of the total heat units required for the crop to grow
and be harvested, with a value of 1 for when growth is complete for the year. Based upon that
information, irrigation events were scheduled at the fractional values 0.3 and 0.75 heat units for
the crop while grazing was scheduled at a fraction value of 0.4 heat units.
Table 4.2 demonstrates locations within the watershed where irrigation water have been applied
as well as the amount of water applied. The irrigation water is applied to specific HRUs located
within the subbassin to limit the water application to defined land use areas. Irrigation of
individual fields or exclusive areas within an HRU is not possible within SWAT. Water applied
to an HRU is applied to the entire HRU.
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Table 4.2 Irrigation of Agricultural Lands of Six Mile and Nine Mile Watersheds
Subbasin HRU Area (km2) Acres Input(mm) Input (in) Volume (ac-ft)
70 217 0.50 123.6 18.82 0.74 7.62
95 295 0.06 14.83 3.05 0.12 0.15
97 302 0.02 4.94 8.21 0.32 0.13
99 308 0.78 192.74 11.18 0.44 7.07
100 311 0.25 61.77 34.10 1.34 6.90
102 316 0.01 2.47 1.03 0.04 0.008
102 317 0.01 2.47 53.68 2.11 0.43
128 398 0.08 19.77 33.82 1.33 2.19
138 426 1.04 256.99 44.62 1.75 37.48
187 568 0.19 46.95 44.62 1.75 6.85
Total -- 2.94 726.53 253.13 9.94 68.83
4.1.2.5 Climatic Data Inputs for the Nine Mile Reservoir Watershed
Climatic data for the watershed are required by SWAT. A built-in weather generator within the
model generates data by using climatic averages from nearby climatic stations. Data generated
from stations located in the watershed, or within close proximity, more closely represent existing
watershed conditions than data generated elsewhere. The Nine Mile Reservoir watershed is in an
isolated area with no climatic stations. The SWAT model assigned the New Harmony climatic
station using a “nearest subbasin-centroid” algorithm. In situations where no climatic stations are
present within the watershed being studied, interpolating between datasets from two nearby
climatic stations is ideal. However, in the case of the Nine Mile Reservoir watershed, only one
nearby climatic station is present. Minimum and maximum daily temperatures and precipitation
levels from the New Harmony climatic station were used as significant driving factors in the
model.
4.1.3 SIMULATION PERIOD FOR THE NINE MILE RESERVOIR WATERSHED
For the purposes of this analysis, the SWAT 2005 model simulation period was October 1995
through September 2006 (water year 1996 through water year 2006). The first water year of the
simulation output is considered a model “warm-up” period, with water levels and content within
the hydrologic system reaching equilibrium by filling the soil profile with water, supplying water
to the reservoir, and starting the physical processes occurring within the watershed. Data from
1995 is therefore excluded. Flow and nutrient concentrations from the SWAT model are used to
evaluate model output for the Nine Mile Reservoir watershed. Output results for SWAT were
summarized for the May–October period, in order to capture the critical season for potential
algal growth.
4.2 RESERVOIR MODEL: BATHTUB
4.2.1 GENERAL MODEL DESCRIPTION
The BATHTUB reservoir model was developed by the U.S. Army Corps of Engineers (USACE)
as a sophisticated empirical model for predicting eutrophication in reservoirs. The model can be
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81
used to predict nutrient concentrations, chlorophyll a, Secchi depth (transparency), and other
eutrophication indices in a spatially segmented reservoir under steady-state conditions.
Model inputs include reservoir morphometry (mean depth, length, width, mixed-layer depth),
hydraulic connectivity (between reservoir segments and tributaries), tributary water quality (total
nutrients, dissolved nutrients, and flow), and climatic parameters (precipitation and ET). The
model uses empirical equations for physical processes—advective transport, diffusive transport,
and nutrient sedimentation—to predict nutrient concentrations in reservoir water quality.
Within the BATHTUB model, various empirical models predict total phosphorus, total nitrogen,
chlorophyll a concentrations, and Secchi depth. The models summarized in Table 4.3 were found
to best fit Nine Mile Reservoir system conditions.

Table 4.3 Empirical Models Selected for Reservoir BATHTUB Model of Nine
Mile Reservoir
Parameter Model selected Justification
Conservative substance Not computed Default and no data
Total phosphorus 2nd Order, Available P Default
Total nitrogen 2nd Order, Available N Default
Chlorophyll-a P, N, Light, T Reservoir has low turbidity except
during algal blooms. N and P are
possibly co-limiting.
Transparency Chl-a and Turbidity Default
Longitudinal dispersion Fischer-numeric Default
Phosphorus calibration Decay rates (1) Default
Nitrogen calibration Decay rates (1) Default
4.2.2 MODEL INPUTS FOR NINE MILE RESERVOIR
4.2.2.1 Reservoir Morphometry
Model inputs were developed that describe average climatic and reservoir management
conditions. The physical description of the reservoir's morphometry was calculated by
correlating reservoir volume with average depth profiles throughout the reservoir and to area and
length calculations. Reservoir elevation, overflow and volume data do not exist for the entire
temporal range of water quality sampling activities nor are there specific elevation measurements
taken on the same day water quality sampling occurred. However, reservoir elevation levels were
correlated to reservoir volume using curve data provided by Rolan Beck (Gunnison Irrigation
Company) and originally obtained by the engineer at the time of the dam's construction. Length
of the reservoir was assumed to change by an equal percentage to area with changing depth.

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82
Nine Mile Reservoir Volume
0
500
1000
1500
2000
2500
3000
3500
4000
0 5 10 15 20 25 30
Water depth at dam (feet)
Vo
l
u
m
e

(
a
c
r
e
-
f
e
e
t
)

Figure 4.2 Correlation between reservoir volume and reservoir depth at dam.
Average reservoir volume during an average flow year was estimated to be 1,790 acre-feet with a
corresponding area of 133 acres (0.54 km2) and a mean depth of 4.08 meters. Mean reservoir
volume was estimated using a monthly reservoir hydrograph developed based on the following
assumptions.
• Spring discharge to the reservoir averages 200 acre-feet/month
• Average diversion from Six Mile Creek is 1000 acre-feet/year during the irrigation
season
• Diversion from Six Mile Creek occurs, on average, at a rate ranging from 6 to 10 acre-
feet/day between March and June, with higher diversion rates in warmer months.
• Reservoir drawdown begins on June 15 and goes until August 15 at an average rate of 50
acre-feet/day
Hypolimnetic depth was determined through examination of depth profiles of temperature and
dissolved oxygen collected between 2000 and 2005 at various times of the year at various
reservoir volumes. From these data the percent of the total depth that is represented by the
hypolimnion and metalimnion was determined for the reservoir (Table 4.4).

Table 4.4 Calculation of Hypolimnetic Depth for Nine Mile Reservoir Average Conditions

Reservoir layer
Average percent
water column Average seasonal depth (m)*
Mixed layer 68% 2.76
Metalimnion 11% 0.45
Hypolimnion 21% 0.87
TOTAL 100% 4.08
*Assuming volume of 1,790 acre-feet
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4.2.2.2 Tributary Inputs
Nutrient concentrations for Six Mile Creek were determined using SWAT for the May–October
period. The predicted mean total phosphorus concentration during this period is 0.013 mg/L
including 0.05 mg/L of orthophosphate. The predicted mean total nitrogen concentration during
the summer period was 1.43 mg/L with 0.9 mg/L of inorganic nitrogen. Flow diversion from Six
Mile Creek was assumed to be 1,000 acre-feet per year during the irrigation season.
Loads from the Nine Mile Reservoir direct drainage watershed were also estimated using the
SWAT model for the May – October period. The predicted total phosphorus concentration
during this period is 0.029 mg/L including 0.011 mg/L of orthophosphate. The predicted mean
total nitrogen concentration during the summer period was 1.44 mg/L with 1.0 mg/L of inorganic
nitrogen. Runoff from the direct drainage area is estimated to be 0.061 cms (cubic meters per
second).

Nutrient loads from the springs discharging to Nine Mile Reservoir were estimated using
monitoring data for nutrient concentrations (0.011 mg/L total phosphorus; 0.15 mg/L total
nitrogen) and an estimated flow of 200 acre-feet/month or 1.01 acre-feet/day. Based on these
inputs, the total nutrient load to Nine Mile Reservoir for the May–October season was calculated
and is summarized in Table 4.6.
Table 4.5 Summary of Phosphorus and Nitrogen Loads to Nine Mile Reservoir
Phosphorus (kg/day) Nitrogen (kg/day)
Six Mile Creek 0.08 5.89
Nine Mile direct drainage 0.39 1.72
Local springs 0.10 1.22
Precipitation 0.04 1.48
TOTAL 0.51 10.31

4.2.3 RESERVOIR MODEL RESULTS
4.2.3.1 Nutrients
Predicted average nutrient concentrations for Nine Mile Reservoir are 0.014 mg/L total
phosphorus and 0.638 mg/L of total nitrogen. The majority of the nitrogen is predicted to be
organic nitrogen (0.332 mg/L) and the majority of the phosphorus is predicted to be
orthophosphate (0.011 mg/L).
The BATHTUB model also predicts total nitrogen to phosphorus ratios and dissolved nitrogen to
phosphorus ratios. Values greater than 7 through 10 generally indicate a phosphorus limited
system. The predicted N:P in Nine Mile Reservoir is 35, indicating a phosphorus limited system.
This is supported by observed N:P discussed in Section 3.5.1.4.
4.2.3.2 Chlorophyll a and Secchi Depth
The predicted mean chlorophyll a concentration in Nine Mile Reservoir is 6 µg/L. Predicted
percent exceedance of various chlorophyll a concentrations indicate the frequency at which
nuisance algal levels are expected to occur in the reservoir under each condition. Exceedance of
a nuisance threshold of 30 µg/L occurs less than 1% of the time. Exceedance of 10 µg/L, a value
Nine Mile Reservoir Delisting February 2008

84
identified in the literature as protective of cold water fisheries and recreational uses is not
predicted to occur under average reservoir conditions (Figure 4.3). Predicted Secchi depth in
Nine Mile Reservoir is 2.9 meters.
4.2.3.3 Eutrophication Potential and Oxygen Depletion
The BATHTUB model outputs several metrics of eutrophication potential and oxygen depletion
that can also be used to assess the suitability of the reservoir for cold-water fish. The initial
results from a principal component analysis of reservoir response variables are expressed as an
index value. Values greater than 500 are believed to indicate high eutrophication potential
(Walker 1999). The value predicted for Nine Mile Reservoir is only 58. All of the predicted total
phosphorus, chlorophyll a, and Secchi depth concentrations indicate a mesotrophic system.
The hypolimnetic oxygen depletion (HOD) rate predicts oxygen depletion below the thermocline
and is related to the supply of organic matter from settling algae as well as to external organic
sediment loads and hypolimnetic depth. When HOD is above 0.10 mg/L/day, the oxygen supply
in the hypolimnion is usually depleted within 120 days (4 months) after stratification. Dissolved
oxygen depth profiles collected at the Dam Site on June 15, 2005 and July 28, 2005 were
compared and used to calculate actual oxygen depletion rates throughout the hypolimnion during
the period of stratification. The calculation is based on guidance from the PROFILE and
BATHTUB user manual (Walker 1999). Based on this analysis, HOD rates in Nine Mile
Reservoir in summer 2005 were 0.07 mg/L/day. This calculation supports the finding that
dissolved oxygen is greater than 4 mg/L throughout the majority of the water column throughout
that stratification period (see Section 3.5.4.2).
4.2.3.4 Linkage to Water Quality Criteria
Based on examination of depth profiles, Nine Mile Reservoir appears to stratify for two months,
in early June and mix again in early August. Initial hypolimnetic oxygen concentration at
stratification is assumed to be 8.5 mg/L which is typical of reservoirs at stratification and is
supported by the dissolved oxygen profiles observed in Nine Mile Reservoir. Assuming a
hypolimnetic oxygen depletion rate of 0.07 mg/L/day, the hypolimnion of Nine Mile Reservoir
would be reduced to only 4 mg/L in 120 days making the water quality criteria suitable for a cold
water fishery. Since the reservoir is only stratified for 80 to 95 days, the model indicates that the
reservoir is in full supporting status of the cold water fishery based on dissolved oxygen.
4.2.4 MODEL CALIBRATION AND VALIDATION
A reasonably robust dataset was available for validation of the Six Mile Creek portion of the
SWAT model. The mean total phosphorus concentration observed in Six Mile Creek was 0.015
mg/L whereas the median total phosphorus concentration was 0.005 mg/L due to the
overwhelming number of values that were recorded as Nondetects. The SWAT model predicts
mean total phosphorus concentration in the creek at 0.012 mg/L which is between the mean and
median values observed at this site.
Reservoir water quality data is not used directly in the BATHTUB model but is used to validate
the model assumptions and tributary input data used to configure the reservoir model. Figure 4.3
shows that water quality predicted by the reservoir model is similar to average observed water
quality in the reservoir.
Nine Mile Reservoir Delisting February 2008

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Figure 4.3 Model validation graphs for Nine Mile Reservoir BATHTUB model
Nine Mile Reservoir Delisting February 2008

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4.3 MODEL UNCERTAINTY AND VARIABILITY
Watershed models are simplified abstractions of a natural system. No measurement within nature
can be made without error, and the developed models cannot represent true spatial and temporal
variability. Model outputs therefore present some level of uncertainty. Specific causes of
uncertainty include lack of appropriate data pertaining to watershed output, conflicting data, data
ambiguity, and measurement uncertainty. Uncertainty in estimating nutrient loads produced
within the watershed comes primarily from the following.
1. Errors in weather station data
2. Errors related to weather station location
3. Errors in nutrient parameter adjustments
4. Errors associated with the SWAT or BATHTUB models themselves
5. Errors that result from combining the SWAT and BATHTUB models
Since the SWAT model is a spatially distributed and physically based model, output
modification is accomplished by parameter adjustment. Because of the lack of physical data
available for the watershed, computer parameter sensitivity methods are not available and
manual parameter modification and measurement become very tedious and time consuming, with
limited complexity. Uncertainty in model output is increased with the use of arbitrary parameter-
estimation methods. However, recent studies have demonstrated that a direct comparison of
model output for a complete dataset versus an incomplete dataset did not produce large
discrepancies in model performance (Wainwright 2004). Since SWAT produced results meeting
acceptable calibration measurement performance, the SWAT model outputs were determined to
be appropriate for use in this watershed analysis.
4.4 CONCLUSIONS
The combination of SWAT watershed modeling and the BATHTUB reservoir model provides
additional evidence for recommending that Nine Mile Reservoir be removed from the State of
Utah's 303(d) list for impaired waters. The model suite is quite conservative, slightly over-
predicting nutrient loads, concentrations, and trophic condition in all cases. Despite this over-
prediction, the model suggests that on average, Nine Mile Reservoir remains below the total
phosphorus threshold of 0.025 mg/L during the summer algal growth season and the chlorophyll
a concentrations remain below 10 µg/L during this same period. Oxygen depletion estimates
indicate that the reservoir will not deplete more than 50% of the water column is maintained at a
dissolved oxygen concentration above 4 mg/L when the reservoir is maintained at its average
volume of 1,790 acre-feet.
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5 SUMMARY OF EVIDENCE FOR DELISTING
5.1 WATER QUALITY SUMMARY
5.1.1 COMPLIANCE WITH WATER QUALITY CRITERIA
There have been no observed exceedances of the dissolved oxygen criteria established by the
State of Utah in Nine Mile Reservoir since 1999. More than 50% of the water column meets
dissolved oxygen criteria of greater than 4.0 mg/L during all months at both sites; therefore the
reservoir is in full support status for the cold water fishery designated beneficial use based on
dissolved oxygen criteria. This conclusion is supported by an analysis of nutrient and chlorophyll
a data available for Nine Mile Reservoir.
Current mean and median total phosphorus concentrations are below the indicator threshold
established by the State of Utah. This threshold of 0.025 mg/L is rarely exceeded in Nine Mile
Reservoir, with more than half of the data points available for the reservoir recorded as
nondetect, and therefore below the detection limit of 0.01 to 0.02 mg/L (depending on the
analytical method used). Exceedances appear to be associated primarily with low reservoir water
levels, during which time phosphorus in-bed sediment is more likely to be suspended throughout
the water column. Since no conservation pool has been established for this reservoir,
management of the reservoir level is not negotiable. Chlorophyll a values are also well below the
indicator values of 10 µg/L identified in the literature as being protective of cold water fisheries
and recreational uses.
5.1.2 TREND TOWARDS IMPROVING WATER QUALITY
Trend data for total phosphorus concentration, chlorophyll a, and Secchi depth indicate that
water quality in the reservoir has been improving since 2000, with the exception of one high total
phosphorus concentration identified in the summer 2006.
5.1.3 COMPARISON WITH PALISADES LAKE
A comparison between Nine Mile Reservoir and Palisades Lake, both of which receive inflow
from Six Mile Creek, indicates that pH and alkalinity are significantly higher in Nine Mile
Reservoir than in Palisades Lake. This provides another line of evidence that high pH in Nine
Mile results from the alkaline soils in the surrounding area. Mean total phosphorus and dissolved
oxygen, based on a detailed water column analysis, is similar between the two systems,
indicating that Nine Mile Reservoir should not be listed as impaired for this water quality
parameter. Water column dissolved oxygen values are comparable between the two reservoirs at
specific depths and often lower in Palisades Lake. The lowest dissolved oxygen measurement in
Nine Mile Reservoir is at a depth lower than the maximum depth for Palisades Lake, making it
difficult to compare the two systems at such depths. It is expected that a larger hypolimnion
forms in the deeper Nine Mile Reservoir and that stratification takes place for a longer period of
time, resulting in lower dissolved oxygen at the sediment-water interface.
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5.2 EXPLANATION OF OBSERVED WATER QUALITY EXCEEDANCES
5.2.1 NATURALLY ALKALINE SOILS
Elevated pH levels in Nine Mile Reservoir are associated with naturally alkaline soils in the
watershed and that extend below the reservoir. This natural condition results in the exceedance
of the pH criteria established for cold water fisheries by the State of Utah.
5.2.2 CORRELATION BETWEEN WATER QUALITY AND RESERVOIR LEVEL
There have been no observed exceedances of dissolved oxygen criteria in Nine Mile Reservoir
since 1999. The few incidences of elevated chlorophyll a and total phosphorus correlate well
with low reservoir water levels. Since no conservation pool has been established for Nine Mile
Reservoir, when the reservoir is drawn down to its minimum level it no longer can support fish
due to a lack of water. When the reservoir maintains only a relatively small pool of water it can
quickly become stagnant and warm, thereby reducing viable habitat for a cold water fishery and
promoting in-reservoir algal growth. Watershed level nutrient reductions would not have a
substantial improvement on this condition, which is primarily a function of reservoir level
management and water rights.
5.2.3 DROUGHT ASSOCIATED TEMPERATURE EXCEEDANCE
Nine Mile Reservoir has little natural cover and the watershed is located in an area experiencing
warm, dry climate conditions. The State of Utah recently conducted an assessment of
temperature inputs to several local water bodies and determined that the primary source of
temperature loading was from solar radiation and heat transfer. Exceedances of the cold water
temperature criteria occurring in the Six Mile Creek diversion inflow data, and spring inflow
data indicate that both inflow and in-reservoir heating processes are responsible to some degree
for the elevated water temperatures observed. Nine Mile Reservoir was not determined to be
impaired for temperature on the 2006 303(d) list.
5.3 OTHER INDICATORS OF TROPHIC STATE
Analysis of the trophic state index (TSI), fishery, and phytoplankton composition for Nine Mile
Reservoir add further support to a full support designation for Nine Mile Reservoir. The
reservoir is generally mesotrophic. Occurrences of reduced turbidity measured in terms of Secchi
depth generally indicate a non-algal source of light interference associated with the high-alkaline
water in the reservoir. There have been no documented fish kills in the reservoir and blue-green
algae species are not prevalent.
5.4 MODELED WATER QUALITY
The combination of SWAT watershed modeling and BATHTUB reservoir model provides
additional evidence for recommending that Nine Mile Reservoir be removed from the State of
Utah's 303(d) list for impaired waters. The model suite is quite conservative, slightly over-
predicting nutrient loads, concentrations, and trophic condition in all cases. Despite this over-
prediction, the model suggests that on average Nine Mile Reservoir remains well below the total
phosphorus threshold of 0.025 mg/L during the summer algal growth season and that chlorophyll
a concentrations remain below 10 µg/L during this same period. Oxygen depletion estimates
indicate that the reservoir will maintain dissolved oxygen concentrations above 4 mg/L in more
than 50% of the water column when the reservoir is maintained at its average volume of 1,790
acre-feet.
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