U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 02–4142

NATIONAL WATER–QUALITY ASSESSMENT PROGRAM

Nutrient Dynamics in Five Off-Stream 

Reservoirs in the Lower South Platte 

River Basin, March–September 1995
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 

Nutrient Dynamics in Five Off-Stream 

Reservoirs in the Lower South Platte 

River Basin, March–September 1995

By Lori A. Sprague

Denver, Colorado

2002

02–4142
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Denver Federal Center

Denver, CO 80225

from:

District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225-0046
FOREWORD

The U.S. Geological Survey (USGS) is committed 

to serve the Nation with accurate and timely scientific 

information that helps enhance and protect the overall 

quality of life, and facilitates effective management of 

water, biological, energy, and mineral resources. Information on the quality of the Nation’s water resources 

is of critical interest to the USGS because it is so integrally linked to the long-term availability of water that 

is clean and safe for drinking and recreation and that is 

suitable for industry, irrigation, and habitat for fish and 

wildlife. Escalating population growth and increasing 

demands for the multiple water uses make water availability, now measured in terms of quantity and quality, 

even more critical to the long-term sustainability of 

our communities and ecosystems.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program to support 

national, regional, and local information needs and 

decisions related to water-quality management and 

policy. Shaped by and coordinated with ongoing 

efforts of other Federal, State, and local agencies, the 

NAWQA Program is designed to answer: What is the 

condition of our Nation’s streams and ground water? 

How are the conditions changing over time? How do 

natural features and human activities affect the quality 

of streams and ground water, and where are those 

effects most pronounced? By combining information 

on water chemistry, physical characteristics, stream 

habitat, and aquatic life, the NAWQA Program aims to 

provide science-based insights for current and emerging water issues. NAWQA results can contribute to 

informed decisions that result in practical and effective 

water-resource management and strategies that protect 

and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 50 

of the Nation’s most important river basins and aquifers, referred to as Study Units. Collectively, these 

Study Units account for more than 60 percent of the 

overall water use and population served by public 

water supply, and are representative of the Nation’s 

major hydrologic landscapes, priority ecological 

resources, and agricultural, urban, and natural sources 

of contamination. 

Each assessment is guided by a nationally consistent study design and methods of sampling and analysis. The assessments thereby build local knowledge 

about water-quality issues and trends in a particular 

stream or aquifer while providing an understanding of 

how and why water quality varies regionally and 

nationally. The consistent, multiscale approach helps 

to determine if certain types of water-quality issues are 

isolated or pervasive and allows direct comparisons of 

how human activities and natural processes affect 

water quality and ecological health in the Nation’s 

diverse geographic and environmental settings. Comprehensive assessments on pesticides, nutrients, volatile organic compounds, trace metals, and aquatic 

ecology are developed at the national scale through 

comparative analysis of the Study-Unit findings. 

The USGS places high value on the communication and dissemination of credible, timely, and relevant 

science so that the most recent and available knowledge about water resources can be applied in management and policy decisions. We hope this NAWQA 

publication will provide you the needed insights and 

information to meet your needs, and thereby foster 

increased awareness and involvement in the protection 

and restoration of our Nation’s waters. 

The NAWQA Program recognizes that a national 

assessment by a single program cannot address all 

water-resource issues of interest. External coordination at all levels is critical for a fully integrated understanding of watersheds and for cost-effective 

management, regulation, and conservation of our 

Nation’s water resources. The Program, therefore, 

depends extensively on the advice, cooperation, and 

information from other Federal, State, interstate, 

Tribal, and local agencies, nongovernment organizations, industry, academia, and other stakeholder 

groups. The assistance and suggestions of all are 

greatly appreciated.

Robert M. Hirsch

Associate Director for Water

FOREWORD III
CONTENTS V

CONTENTS

Foreword ................................................................................................................................................................................ III

Abstract.................................................................................................................................................................................. 1

Introduction............................................................................................................................................................................ 2

Purpose and Scope....................................................................................................................................................... 2

Description of the South Platte River Basin ................................................................................................................ 3

Physical Setting ................................................................................................................................................. 3

Hydrology .......................................................................................................................................................... 4

Water Quality..................................................................................................................................................... 5

Acknowledgments ....................................................................................................................................................... 6

Methods of Data Collection and Analysis ............................................................................................................................. 6

Data Collection ............................................................................................................................................................ 6

Data Analysis............................................................................................................................................................... 7

Riverside Reservoir................................................................................................................................................................ 10

Water Temperature and Dissolved Oxygen ................................................................................................................. 11

pH and Specific Conductance...................................................................................................................................... 14

Nutrient Dynamics....................................................................................................................................................... 14

Water-Column Concentrations........................................................................................................................... 14

Bottom-Sediment Flux....................................................................................................................................... 18

Mass Balance ..................................................................................................................................................... 19

Algal Growth ..................................................................................................................................................... 20

Jackson Reservoir .................................................................................................................................................................. 21

Water Temperature and Dissolved Oxygen ................................................................................................................. 21

pH and Specific Conductance...................................................................................................................................... 23

Nutrient Dynamics....................................................................................................................................................... 25

Water-Column Concentrations........................................................................................................................... 25

Bottom-Sediment Flux....................................................................................................................................... 25

Mass Balance ..................................................................................................................................................... 27

Algal Growth ..................................................................................................................................................... 30

Prewitt Reservoir ................................................................................................................................................................... 31

Water Temperature and Dissolved Oxygen ................................................................................................................. 33

pH and Specific Conductance...................................................................................................................................... 35

Nutrient Dynamics....................................................................................................................................................... 35

Water-Column Concentrations........................................................................................................................... 35

Bottom-Sediment Flux....................................................................................................................................... 39

Mass Balance ..................................................................................................................................................... 40

Algal Growth ..................................................................................................................................................... 40

North Sterling Reservoir........................................................................................................................................................ 42

Water Temperature and Dissolved Oxygen ................................................................................................................. 42

pH and Specific Conductance...................................................................................................................................... 44

Nutrient Dynamics....................................................................................................................................................... 46

Water-Column Concentrations........................................................................................................................... 46

Bottom-Sediment Flux....................................................................................................................................... 46

Mass Balance ..................................................................................................................................................... 52

Algal Growth ..................................................................................................................................................... 52

Julesburg Reservoir................................................................................................................................................................ 53

Water Temperature and Dissolved Oxygen ................................................................................................................. 54

pH and Specific Conductance...................................................................................................................................... 54

Nutrient Dynamics....................................................................................................................................................... 56

Water-Column Concentrations........................................................................................................................... 56

Bottom-Sediment Flux....................................................................................................................................... 61

Mass Balance ..................................................................................................................................................... 62

Algal Growth ..................................................................................................................................................... 63

Summary and Conclusions .................................................................................................................................................... 64

References.............................................................................................................................................................................. 66

Appendix................................................................................................................................................................................ 69
VI CONTENTS

FIGURES

1. Map showing locations of study reservoirs and river water-quality sites in the South Platte River Basin............... 3

2. Graph showing daily mean discharge in the South Platte River from Kersey to Julesburg, 1995............................ 4

3. Graphs showing monthly median concentrations of nitrogen and phosphorus in the South Platte River 

from Kersey to Julesburg, 1993–95 .......................................................................................................................... 5

4. Map showing location of sampling sites in Riverside Reservoir .............................................................................. 11

5–8. Graphs showing:

5. Inflow, outflow, and end-of-month storage volume in Riverside Reservoir, March–September 1995 ............. 12

6. Depth profiles of water temperature, dissolved oxygen, pH, and specific conductance in Riverside 

Reservoir, March–September 1995................................................................................................................... 13

7. Nitrogen and phosphorus concentrations, euphotic and bottom samples combined in Riverside 

Reservoir, March–September 1995................................................................................................................... 17

8. Concentrations of dissolved inorganic nitrogen and orthophosphate in bottom-sediment pore water and 

the overlying water column in Riverside Reservoir, April 5 and August 30, 1995 .......................................... 18

9. Map showing location of sampling sites in Jackson Reservoir................................................................................. 22

10–13. Graphs showing:

10. Inflow, outflow, and end-of-month storage volume in Jackson Reservoir, March–September 1995 ............... 23

11. Depth profiles of water temperature, dissolved oxygen, pH, and specific conductance in Jackson Reservoir,

March–September 1995 .................................................................................................................................... 24

12. Nitrogen and phosphorus concentrations, euphotic and bottom samples combined in Jackson Reservoir, 

March–September 1995 .................................................................................................................................... 28

13. Concentrations of dissolved inorganic nitrogen and orthophosphate in bottom-sediment pore water 

and the overlying water column in Jackson Reservoir, February 28, March 8, and August 18, 1995.............. 29

14. Map showing location of sampling sites in Prewitt Reservoir. ................................................................................. 32

15–18. Graphs showing:

15. Inflow, outflow, and end-of-month storage volume in Prewitt Reservoir, March–September 1995................. 33

16. Depth profiles of water temperature, dissolved oxygen, pH, and specific conductance in Prewitt Reservoir,

March–September 1995 .................................................................................................................................... 34

17. Nitrogen and phosphorus concentrations, euphotic and bottom samples combined in Prewitt Reservoir, 

March–September 1995 .................................................................................................................................... 38

18. Concentrations of dissolved inorganic nitrogen and orthophosphate in bottom-sediment pore water and 

the overlying water column in Prewitt Reservoir, March 9 and August 17, 1995 ............................................ 39

19. Map showing location of sampling sites in North Sterling Reservoir ...................................................................... 43

20–23. Graphs showing:

20. Inflow, outflow, and end-of-month storage volume in North Sterling Reservoir, March–September 1995 ..... 44

21. Depth profiles of water temperature, dissolved oxygen, pH, and specific conductance in North Sterling 

Reservoir, March–September 1995................................................................................................................... 45

22. Nitrogen and phosphorus concentrations in euphotic and bottom samples in North Sterling Reservoir, 

March–September 1995 .................................................................................................................................... 49

23. Concentrations of dissolved inorganic nitrogen and orthophosphate in bottom-sediment pore water and 

the overlying water column in North Sterling Reservoir, March 21 and August 16, 1995............................... 51

24. Map showing location of sampling sites in Julesburg Reservoir .............................................................................. 55

25–28. Graphs showing:

25. Inflow, outflow, and end-of-month storage volume in Julesburg Reservoir, March–September 1995 ............. 56

26. Depth profiles of water temperature, dissolved oxygen, pH, and specific conductance in Julesburg 

Reservoir, March–September 1995................................................................................................................... 57

27. Nitrogen and phosphorus concentrations, euphotic and bottom samples combined in Julesburg Reservoir,

March–September 1995 .................................................................................................................................... 60

28. Concentrations of dissolved inorganic nitrogen and orthophosphate in bottom-sediment pore water 

and the overlying water column in Julesburg Reservoir, March 22, April 4, and August 15, 1995 ................. 61
CONTENTS VII

TABLES

1. End-of-month storage, monthly median outflow, and residence time in Riverside Reservoir, March–

September 1995 ...................................................................................................................................................... 12

2. Statistical summary of nutrient concentrations at each site and results of Kruskal-Wallis test of site 

concentration differences in Riverside Reservoir, March–September 1995........................................................... 15

3. Statistical summary of nutrient concentrations in euphotic and bottom samples and results of Wilcoxon 

Rank-Sum test of euphotic and bottom concentration differences in Riverside Reservoir, March–

September 1995 ...................................................................................................................................................... 16

4. Mass-balance calculations for total nitrogen and total phosphorus in Riverside Reservoir, March–

September 1995 ...................................................................................................................................................... 19

5. Concentrations of biologically available nitrogen and phosphorus and N:P values for Riverside Reservoir, 

March–September 1995.......................................................................................................................................... 20

6. Chlorophyll-a concentrations, Secchi-disk depths, total phosphorus concentrations, and associated 

trophic-state indices for Riverside Reservoir, March–September 1995 ................................................................. 21

7. End-of-month storage, monthly median outflow, and residence time in Jackson Reservoir, March–

September 1995 ...................................................................................................................................................... 23

8. Statistical summary of nutrient concentrations at each site and results of Kruskal-Wallis test of site 

concentration differences in Jackson Reservoir, March–September 1995 ............................................................. 26

9. Statistical summary of nutrient concentrations in euphotic and bottom samples and results of Wilcoxon 

Rank-Sum test of euphotic and bottom concentration differences in Jackson Reservoir, March–

September 1995 ...................................................................................................................................................... 27

10. Mass-balance calculations for total nitrogen and total phosphorus in Jackson Reservoir, March–

September 1995 ...................................................................................................................................................... 30

11. Concentrations of biologically available nitrogen and phosphorus and N:P values for Jackson Reservoir, 

March–September 1995.......................................................................................................................................... 31

12. Chlorophyll-a concentrations, Secchi-disk depths, total phosphorus concentrations, and associated trophicstate indices for Jackson Reservoir, March–September 1995................................................................................. 31

13. End-of-month storage, monthly median outflow, and residence time in Prewitt Reservoir, March–

September 1995 ...................................................................................................................................................... 33

14. Statistical summary of nutrient concentrations at each site and results of Kruskal-Wallis test of site 

concentration differences in Prewitt Reservoir, March–September 1995............................................................... 36

15. Statistical summary of nutrient concentrations in euphotic and bottom samples and results of Wilcoxon 

Rank-Sum test of euphotic and bottom concentration differences in Prewitt Reservoir, March–

September 1995 ...................................................................................................................................................... 37

16. Mass-balance calculations for total nitrogen and total phosphorus in Prewitt Reservoir, March–

September 1995 ...................................................................................................................................................... 40

17. Concentrations of biologically available nitrogen and phosphorus and N:P values for Prewitt Reservoir, 

March–September 1995.......................................................................................................................................... 41

18. Chlorophyll-a concentrations, Secchi-disk depths, total phosphorus concentrations, and associated trophicstate indices for Prewitt Reservoir, March–September 1995.................................................................................. 41

19. End-of-month storage, monthly median outflow, and residence time in North Sterling Reservoir, March–

September 1995 ...................................................................................................................................................... 44

20. Statistical summary of nutrient concentrations at each site and results of Kruskal-Wallis test of site 

concentration differences in North Sterling Reservoir, March–September 1995 ................................................... 47

21. Statistical summary of nutrient concentrations in euphotic and bottom samples and results of Wilcoxon 

Rank-Sum test of euphotic and bottom concentration differences in North Sterling Reservoir, March–

September 1995 ...................................................................................................................................................... 48

22. Mass-balance calculations for total nitrogen and total phosphorus in North Sterling Reservoir, March–

September 1995 ...................................................................................................................................................... 52

23. Concentrations of biologically available nitrogen and phosphorus and N:P values for North Sterling Reservoir, 

March–September 1995.......................................................................................................................................... 53

24. Chlorophyll-a concentrations, Secchi-disk depths, total phosphorus concentrations, and associated trophic-state 

indices for North Sterling Reservoir, March–September 1995............................................................................... 54
VIII CONTENTS

25. End-of-month storage, monthly median outflow, and residence time in Julesburg Reservoir, March–

September 1995....................................................................................................................................................... 56

26. Statistical summary of nutrient concentrations at each site and results of Kruskal-Wallis test of site 

concentration differences in Julesburg Reservoir, March–September 1995 ........................................................... 58

27. Statistical summary of nutrient concentrations in euphotic and bottom samples and results of Wilcoxon 

Rank-Sum test of euphotic and bottom concentration differences in Julesburg Reservoir, March–

September 1995....................................................................................................................................................... 59

28. Mass-balance calculations for total nitrogen and total phosphorus in Julesburg Reservoir, March–

September 1995....................................................................................................................................................... 62

29. Concentrations of biologically available nitrogen and phosphorus and N:P values for Julesburg Reservoir, 

March–September 1995 .......................................................................................................................................... 63

30. Chlorophyll-a concentrations, Secchi-disk depths, total phosphorus concentrations, and associated trophicstate indices for Julesburg Reservoir, March–September 1995 .............................................................................. 64

31. Comparison of morphological, operational, and nutrient characteristics of Riverside, Jackson, Prewitt, 

North Sterling, and Julesburg Reservoirs, March–September 1995 ....................................................................... 65

APPENDIX ........................................................................................................................................................................... 69

Nutrient concentrations in inflow and outflow canals for Riverside, Jackson, Prewitt, North Sterling, and Julesburg 

Reservoirs, March–September 1995.............................................................................................................. 71

CONVERSION FACTORS AND WATER-QUALITY UNITS

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (° F) as follows:

° F = 9/5 (°C) + 32

The following abbreviations also are used in this report: 

milligrams per liter (mg/L) 

micrograms per liter (μg/L)

micrometer (μm)

microsiemens per centimeter at 25 degrees Celsius (μS/cm)

milligrams per meter squared per day (mg/m2/d)

Multiply By To obtain

acre-foot (acre-ft) 1,233 cubic meter

acre-foot per day (acre-ft/d) 0.5042 cubic feet per second

cubic foot per second (ft3/s) 0.02832 cubic meter per second

inch 2.540 centimeter (cm)

meter 3.281 foot (ft)

acre 4,047 square meter

square mile (mi2) 2,590 square kilometer

pound (lb) 0.4536 kilogram

liter (L) 0.2642 gallon

mile (mi) 1.609 kilometer
Abstract 1

Nutrient Dynamics in Five Off-Stream Reservoirs

in the Lower South Platte River Basin, 

March–September 1995

By Lori A. Sprague

Abstract

In 1995, the U.S. Geological Survey 

conducted a study to characterize nutrient concentrations in five off-stream reservoirs in the lower 

South Platte River Basin—Riverside, Jackson, 

Prewitt, North Sterling, and Julesburg. These 

reservoirs are critical sources of irrigation water 

for agricultural areas, and several also are used for 

fishing, boating, swimming, hunting, and 

camping. Data collected for this study include 

depth profiles of water temperature, dissolved 

oxygen, pH, and specific conductance; nutrient 

species concentrations in the water column, 

bottom sediment, and inflow and outflow canals; 

and chlorophyll-a concentrations in the water 

column. Data were collected during the irrigation 

season from March through September 1995 at 

five sites each in Riverside, Jackson, Prewitt, and 

Julesburg Reservoirs and at six sites in North 

Sterling Reservoir.

The five reservoirs studied are located in 

similar geographic, climatic, and land-use areas 

and, as a result, have a number of similarities in 

their internal nutrient dynamics. Nitrogen concentrations in the reservoirs were highest in March 

and decreased through September as a result of 

dilution from river inflows and biological activity. 

From March through June, decreases in nitrogen 

concentrations in the river and biological activity 

contributed to decreases in reservoir concentrations. From July through September, inflows from 

the river were cut off, and biological activity in 

the reservoirs led to further decreases in nitrate 

concentrations, which fell to near or below detectable levels. Phosphorus concentrations in the 

reservoirs did not show the same consistent 

decrease from March through September. Phosphorus likely was recycled continuously back to 

algae during the study period through processes 

such as excretion from fish, decay of aquatic 

plants and animals, and release of orthophosphate 

from bottom sediment during periods of low 

oxygen. With the exception of phosphorus in 

Jackson Reservoir, the reservoirs acted as a sink 

for both nitrogen and phosphorus; the percentage 

of the total mass (initial storage plus inflows) 

trapped in the reservoirs during the study period 

ranged from 49 to 88 percent for nitrogen and 

from 20 to 86 percent for phosphorus. 

The nutrient loading, morphology, and 

operation of the five reservoirs differed, however, 

leading to several important differences in 

nutrient dynamics among the reservoirs. Mean 

nutrient concentrations during the study period 

decreased in a downstream direction from Riverside Reservoir to Julesburg Reservoir because 

concentrations in the source water—the South 

Platte River—decreased downstream as a result of 

increased distance from wastewater loading 

upstream from Kersey, Colorado, and the replacement of diverted river water with more dilute 

ground-water return flow. North Sterling was an 

exception to this decrease; the strong stratification 

and resulting anoxia that developed in the reservoir led to nutrient release from the bottom sediments that offset the decrease in external nutrient 

loading.
2 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Variations in nutrient loading also contributed to differences in the nutrient limiting algal 

growth in the reservoirs, as indicated by mass 

nitrogen:phosphorus ratios. In Riverside and 

Jackson Reservoirs, nitrogen became the potential 

limiting nutrient by midsummer as biological 

activity depleted the available supply of nitrogen 

while the high initial phosphorus load was recycled. Prewitt, North Sterling, and Julesburg Reservoirs, with lower initial loadings of phosphorus, 

were phosphorus-limited throughout the study 

period, with additional colimitation of nitrogen as 

biological uptake reduced nitrogen concentrations 

to near or below laboratory detection limits. The 

percentage of the total nitrogen and phosphorus 

mass lost through outflow and trapped in the 

reservoir due to processes such as biological 

uptake and sedimentation varied between reservoirs. Generally, reservoirs with short residence 

times such as North Sterling and Julesburg lost a 

higher percentage of the total mass in the outflow, 

whereas reservoirs with longer residence times 

like Jackson and Prewitt trapped more of the total 

mass within the reservoir. 

Algal biomass, as measured by chlorophyll-a concentrations, generally increased in all 

reservoirs during the study period as nutrient 

concentrations decreased. Mean values of 

Carlson’s trophic-state index ranged from 59 in 

Riverside Reservoir to 72 in Jackson and North 

Sterling Reservoirs, indicating that all five reservoirs were eutrophic. The results of this study 

demonstrate that the practice of storing South 

Platte River water in off-stream reservoirs 

substantially decreases dissolved nitrogen 

concentrations during the irrigation season. Associated with the decreased nitrogen concentrations, 

however, is an increase in algal biomass, which 

could adversely affect the recreational use of the 

reservoirs.

INTRODUCTION

In 1991, the U.S. Geological Survey (USGS) 

began a study of the South Platte River Basin as part of 

the National Water-Quality Assessment (NAWQA) 

Program. The goals of the NAWQA Program are to 

assess the quality of surface water, ground water, and 

aquatic ecosystems throughout the Nation and to 

understand the primary natural and anthropogenic 

factors that affect the quality of these resources. This 

information supports national, regional, State, and 

local decision making for water-quality management.

One issue of concern in the South Platte River 

Basin is nutrient enrichment in off-stream reservoirs. 

Although nutrients are necessary for plant and animal 

life, in excessive quantities they can accelerate the 

growth of aquatic plants and cause algal blooms that 

deprive deeper waters of the sunlight needed by 

aquatic plants and animals (Wetzel, 1983). The subsequent decomposition of algae and other aquatic plants 

consumes dissolved oxygen, which can adversely 

affect fish and other aquatic life. Algal blooms and 

excessive aquatic growth also may result in unsuitable 

habitat and recreation conditions.

In 1995, the USGS conducted a study to characterize nutrient concentrations in five off-stream reservoirs in the lower South Platte River Basin—

Riverside, Jackson, Prewitt, North Sterling, and Julesburg—during the irrigation season from March 

through September. These reservoirs serve as critical 

sources of irrigation water for agricultural areas in the 

basin. Several also are used for fishing, hunting, 

boating, and swimming, and all are habitats for waterfowl, including bald eagles, white pelicans, and a 

variety of ducks and geese. 

Purpose and Scope

The purposes of this report are to (1) describe 

the spatial and temporal variation in nutrient concentrations in five off-stream reservoirs in the lower South 

Platte River Basin, (2) determine whether the reservoirs are a source or a sink of nutrients in irrigationsupply water, and (3) determine how the nutrient 

dynamics in the reservoirs differ from one another. 

Data collected for this study include depth profiles of 

water temperature, dissolved oxygen, pH, and specific 

conductance; nutrient species concentrations in the 

water column, bottom sediments, and inflow and 

outflow canals; and chlorophyll-a concentrations. 

These data were collected during the irrigation season 

from March through September 1995 at five sites each 

in Riverside, Jackson, Prewitt, and Julesburg Reservoirs and at six sites in North Sterling Reservoir.
INTRODUCTION 3

1

1

2

3

4

2

3

4

0 50 100 MILES

0 100 KILOMETERS 50

Balzac Kersey

DENVER

Weldona

Boulder

Greeley

Cheyenne

Loveland

Sterling

Julesburg

Fort Morgan

North Platte

Fort Collins

South Platte River

Kiowa

Lodgepole

North Fork

Bijou 

Creek

Creek

Creek

Bear Creek

Elder Creek Henderson

WELD

PARK

LARIMER

LOGAN

LARAMIE

ADAMS

ELBERT

MORGAN

KIMBALL

DOUGLAS

BOULDER

CHEYENNE

ARAPAHOE

KEITH

WASHINGTON

JEFFERSON

DEUEL

LINCOLN

TELLER

CLEAR CREEK

GILPIN

SEDGWICK

EL PASO

South Platte River

Cache

Thompson

Creek

Riverside

Reservoir

Big Dry Creek

Clear Creek

Big

River

La Poudre River

Creek

Box

St.

Vrain

Creek

North

Fork

Cherry

Plum

Creek

Beaver Creek

39°

41°

Tarryall

Lonesome Creek

Crow Creek

105°

103°

101°

COLORADO

WYOMING

NEBRASKA

Sterling

Reservoir Julesbu rg 

Reservoir

Jackson 

Reservoir

Prewitt 

Reservoir

WYOMING NEBRASKA

COLORADO

Study area

Base from U.S. Geological Survey and U.S. Census Bureau

digital data, 1974 to 1993

North

EXPLANATION

06754000—South Platte River near Kersey, Colorado

06758500—South Platte River near Weldona, Colorado

06759910—South Platte River near Cooper Bridge, near

 Balzac, Colorado

06764000—South Platte River at Julesburg, Colorado

STREAMFLOW-GAGING AND WATER-QUALITY STATION

x Mt. Lincoln

Description of the South Platte River 

Basin

Physical Setting

The South Platte River Basin has a drainage area 

of about 24,300 mi2 and is located in parts of three 

States (fig.1)—Colorado (79 percent of the basin), 

Nebraska (15 percent of the basin), and Wyoming 

(6 percent of the basin). The South Platte River originates in the Rocky Mountains of central Colorado at 

the Continental Divide and flows about 450 mi northeast across the Great Plains to its confluence with the 

North Platte River at North Platte, Nebr. Altitude in 

the basin ranges from 14,286 ft at Mt. Lincoln on the 

Continental Divide to 2,750 ft at the confluence of the 

South Platte and North Platte Rivers at North Platte, 

Nebr. (Dennehy and others, 1993).

The basin has a continental-type climate modified by topography, resulting in large temperature 

ranges and irregular seasonal and annual precipitation 

(Gaggiani and others, 1987). Mean temperatures 

increase from west to east and on the plains from north 

to south. Areas along the Continental Divide average 

30 inches or more of precipitation annually, which 

includes snowfall in excess of 300 inches. In contrast, 

annual precipitation on the plains east of Denver, 

Colo., and in the South Park area in the southwest part 

of the basin, ranges from 7 to 15 inches. Most precipitation on the plains results from rainfall, which 

primarily occurs between April and September, while 

Figure 1. Locations of study reservoirs and river water-quality sites in the South Platte River Basin.
4 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

06754000—South Platte River near Kersey, Colorado

06758500—South Platte River near Weldona, Colorado

06759910—South Platte River near Cooper Bridge, 

 near Balzac, Colorado

06764000—South Platte River at Julesburg, Colorado

25,000

20,000

15,000

10,000

5,000

0

DAILY MEAN DISCHARGE, IN CUBIC FEET PER SECOND 

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

1995

most precipitation in the mountains results from snowfall, which primarily occurs during the winter.

Land use in the South Platte River Basin is 

dominated by rangeland and agriculture, at 41 percent 

and 37 percent, respectively (Fegeas and others, 1983). 

Rangeland is present across all areas of the basin 

except over the high mountain forests. Agricultural 

land is present primarily in the plains areas and Park 

County. Forested area is the third largest land cover, at 

16 percent, and is present in a north-south band in the 

mountains. Urban areas cover 3 percent of the basin, 

primarily in the Front Range urban corridor along the 

base of the Rocky Mountains. The remaining land uses 

in the basin include barren lands, tundra, and perennial 

snow and ice. 

Hydrology

Perennial flow in the South Platte River and its 

major tributaries that originate in the Rocky Mountains is derived primarily from snowmelt runoff. 

Smaller tributaries in the plains are ephemeral and 

contribute little flow to the South Platte River except 

during spring and summer thunderstorms. The natural 

hydrology of the South Platte River Basin, however, 

has been modified considerably by water regulation. 

Each year, more than 3 million acre-ft of water is 

removed from the river through a complex network of 

ditches and pipes, more than 2 million acre-ft of water 

is stored in reservoirs, 400,000 acre-ft of water is 

imported through interbasin transfers from the Colorado, Arkansas, and North Platte River Basins, and an 

estimated 1 million acre-ft of water is pumped by 

ground-water wells (Dennehy and others, 1993). The 

total storage capacity of the five reservoirs monitored 

in this study equals approximately one-fourth of the 

annual streamflow in the South Platte River at Kersey, 

Colo.

An examination of flow conditions during 1995 

gives a general picture of water routing in the lower 

South Platte River from Kersey to Julesburg (fig. 2). 

Flow during January through April was low, as there 

was little precipitation and water was being diverted 

for storage in reservoirs. In May, flow increased 

substantially due to snowmelt runoff from the mountains. During this period, flow generally decreased in a 

downstream direction from Kersey to Julesburg as 

water was diverted all along the river for irrigation. 

From June through August, flow at all sites decreased 

Figure 2. Daily mean discharge in the South Platte River from Kersey to Julesburg, 1995.
INTRODUCTION 5

Kersey

Weldona

Balzac

0

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14

0

2

4

6

8

10

12

14

CONCENTRATION, IN MILLIGRAMS PER LITER AS NITROGEN

Julesburg

Kersey

Weldona

Balzac

Julesburg

Total organic nitrogen

Ammonia

Nitrite

Nitrate ND No data available

ND ND ND

ND ND

ND ND ND

Total organic phosphorus

Orthophosphate

0.2

0.4

0.6

0.8

1.0

1.2

CONCENTRATION, IN MILLIGRAMS PER LITER AS PHOSPHORUS

0

0

0

0.2

0.4

0.6

0.8

1.0

1.2

0

0.2

0.4

0.6

0.8

1.0

1.2

0.2

0.4

0.6

0.8

1.0

1.2

EXPLANATION

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

as snowmelt runoff ended and more water was 

diverted for irrigation. During the overall decrease, 

several small peaks occurred as runoff from seasonal 

storms offset water lost to diversions. The general 

downstream decrease in flow was still present, 

however. Beginning around September, irrigation 

diversions ended and irrigation return flows from the 

alluvial aquifer led to a slight increase in river flow. By 

mid-October, flows leveled off to near those during in 

January as water once again was diverted for storage in 

the off-stream reservoirs. 

Water Quality

Nutrients are delivered to the South Platte River 

both from point and nonpoint sources. Point sources of 

nutrients can be attributed to a single location, such as 

a wastewater discharge pipe. Nonpoint sources of 

nutrients are widely distributed over an area, such as 

storm runoff from agricultural fields and urban areas, 

atmospheric deposition, and ground-water discharge to 

the river. About 67 percent of the total treated wastewater discharge to the South Platte River occurs 

upstream from Henderson, Colo., where the river 

flows through the Denver metropolitan area; about 

another 28 percent occurs between Henderson and 

Kersey (Litke, 1996). Because the monitoring station 

at Kersey is closest to the wastewater discharges, total 

nitrogen and total phosphorus concentrations are 

higher than those measured at downstream stations 

(fig. 3). In particular, concentrations of nitrate, 

ammonia, and orthophosphate, the predominant 

nutrient species in wastewater effluent, are high. 

Ammonia and orthophosphate concentrations decrease 

during the late spring and summer due to biologically 

mediated transformations to nitrate and organic phosphorus, respectively. Overall, total nitrogen and total 

phosphorus concentrations at Kersey are lowest during 

the spring and summer, when wastewater discharges 

are diluted by snowmelt and biological activity is 

greatest.

Downstream from Kersey, the influence of 

wastewater discharges decreases and water is derived 

primarily from nonpoint agricultural sources in the 

plains. Nutrient concentrations in the river between 

Figure 3. Monthly median concentrations of nitrogen and phosphorus in the South Platte River from Kersey to Julesburg, 1993–95.
6 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Kersey and Julesburg decrease in a downstream direction (fig. 3), mainly because of the replacement of 

diverted river water with more dilute ground-water 

return flow. Plant uptake, incorporation into the soil, 

and microbial removal of a substantial portion of 

nitrate in returning ground water (McMahon and 

Böhlke, 1996) contribute to this dilution effect 

between diversion and return flow. Nutrient concentrations at stations downstream from Kersey also show 

more temporal variation than those at Kersey, largely 

because of seasonal agriculture diversions and groundwater return flow. 

Phosphorus commonly is the nutrient that 

controls eutrophication in reservoirs, and the U.S. 

Environmental Protection Agency (USEPA) has 

recommended a limit of 0.1 mg/L for total phosphorus 

in streams and less than 0.05 mg/L where streams 

enter lakes and reservoirs (Horne and Goldman, 1994; 

U.S. Environmental Protection Agency, 1986). Total 

phosphorus concentrations in the South Platte River 

from Kersey to Julesburg commonly exceeded both 

limits throughout the year. The USEPA also has a set a 

maximum contaminant level (MCL) of 10 mg/L for 

nitrate in drinking water (U.S. Environmental Protection Agency, 1990). Nitrate concentrations did not 

exceed the standard at these sites.

A more detailed discussion of water-quality 

conditions in the South Platte River Basin is presented 

in Dennehy and others (1998) and Litke and 

Kimbrough (1998).

Acknowledgments

The author thanks Robert Kimbrough of the 

USGS for his extensive planning and field work in this 

study. Additional field help was provided by Dennis 

Smits, Patrick Edelmann, Kevin Dennehy, Peter 

McMahon, David Litke, Jim Collins, and Jonathan 

Evans. The author also acknowledges the Riverside 

Irrigation District for providing access to sample 

Riverside Reservoir. 

METHODS OF DATA COLLECTION AND 

ANALYSIS

Water-quality data were obtained from the reservoir water column, the inflow and outflow canals, and 

the reservoir bottom sediment. The methods of sample 

collection from each area and the major computations 

performed on these data are described in the following 

paragraphs.

Data Collection

Water-quality samples were collected from the 

water column of each reservoir four times from March 

through September 1995. Five sites within Riverside, 

Jackson, Prewitt, and Julesburg Reservoirs and six 

sites within North Sterling Reservoir were selected for 

sampling to provide adequate coverage of the spatial 

variability throughout each reservoir. Immediately 

prior to sample collection, depth profiles of water 

temperature, dissolved oxygen, pH, and specific 

conductance were obtained at each site by using a 

multiparameter meter, and water transparency was 

measured with a black and white 20-cm-diameter 

Secchi disk. Generally, depth-profile measurements 

were taken every 0.5 ft for reservoir depths less than 

10 ft, every 2 ft for reservoir depths between 10 and 

20 ft, and every 5 ft for reservoir depths greater than 

20 ft. Two water-quality samples then were collected 

at each site—a near-surface sample collected to characterize conditions in the euphotic zone (the zone of 

light penetration, defined as twice the Secchi-disk 

depth) and a near-bottom sample collected to characterize conditions at a depth where light penetration and 

dissolved oxygen concentrations may have been very 

low. All reservoir water-column samples were 

collected from discrete depths by using a horizontally 

suspended 4-L acrylic Van Dorn sampler. Near-surface 

samples were collected at the water surface and at 

intervals of about every 2 ft to twice the Secchi-disk 

depth and were composited in a 14-L polyethylene 

churn splitter. Near-bottom samples were collected at 

a depth of about 3 ft above the reservoir bottom. When 

the depth of the euphotic zone extended to less than 

3 ft above the reservoir bottom, only one sample was 

collected. 

Water-quality samples also were collected from 

the inflow and outflow canals of each reservoir 

approximately monthly from March through 

September to quantify nutrient loads entering and 

leaving the reservoir. Measurements were obtained for 

water temperature, dissolved oxygen, pH, specific 

conductance, and discharge. When sufficient water 

was present, depth- and width-integrated samples were 

collected by using the equal-width-increment method 
METHODS OF DATA COLLECTION AND ANALYSIS 7

(Edwards and Glysson, 1988); otherwise, grab 

samples were obtained in the centroid of flow. At 

times, no water was present in the canals, and no 

samples were collected. 

Samples of the interstitial pore water in the 

reservoir bottom sediment were collected during the 

early spring and the summer to determine the potential 

for nutrients to move between the bottom sediment 

and the overlying water column. Cores were obtained 

with a gravity-driven coring device outfitted with 

acrylic core tubes at sites in the deepest parts of each 

reservoir, as potential for development of anaerobic 

conditions at the sediment/water interface was greatest 

at the deepest locations. Two sites were cored in each 

reservoir to compare and contrast results; however, 

only one site was cored at Riverside Reservoir because 

it has a sandy bottom that made retaining material in 

the coring device difficult. The cores, with a diameter 

of about 7 cm and ranging in length between 35 and 

70 cm, were kept upright and transported to an 

onshore laboratory for processing. The top 5 cm of 

each core was extruded and sectioned into 1-cm 

samples within a nitrogen gas atmosphere to prevent 

oxidation, and the pore water in each sample was 

removed by centrifuging. The pore-water samples 

were filtered through 0.45-μm syringe filters.

Samples collected from the reservoir water 

column, bottom sediment, and inflows and outflows 

were treated and preserved onsite using methods 

described in Horowitz and others (1994) and Shelton 

(1994). The samples were analyzed for dissolved 

ammonia, dissolved nitrite, dissolved nitrite plus 

nitrate, total kjeldahl nitrogen, dissolved kjeldahl 

nitrogen, total phosphorus, dissolved phosphorus, and 

dissolved orthophosphate at the USGS National Water 

Quality Laboratory in Denver, Colo., by using colorimetric methods described in Fishman and Friedman 

(1989) and Fishman (1993). Samples for chlorophyll-a 

were analyzed by using a high-pressure liquid chromatography/fluorescence spectroscopy method described 

in Britton and Greeson (1989). Chlorophyll-a analysis 

was performed only on near-surface samples collected 

from the water column of three sites in each reservoir 

from May through September. 

Instantaneous discharge in the inflow and 

outflow canals was measured at the time of waterquality sampling by using standard USGS streamgaging techniques described in Rantz (1978). For each 

reservoir, records of daily inflow and outflow 

discharge estimated from stage-discharge relationships 

and end-of-month storage volumes estimated from 

area-capacity curves were obtained from the Office of 

the State Engineer. The estimated daily inflow and 

outflow discharges recorded on each sampling date 

were compared to the corresponding USGS discharge 

measurements. Because the estimated inflow 

discharges were higher than the measured inflow 

discharges for both Jackson and North Sterling Reservoirs (about 25 percent and 15 percent higher, respectively), a correction factor was applied to the estimated 

daily inflow records for these two reservoirs.

Data Analysis

Reservoir water quality and productivity are 

affected to a large extent by the amount of external and 

internal nutrient loading to the reservoir. External 

nutrient loads reflect the climate, morphology, soil 

type, and land use of the contributing area; internal 

loads reflect the amount of nutrients released from 

reservoir bottom sediment. To estimate the nutrient 

mass entering and leaving each reservoir in surfacewater inflows and outflows, and to determine if the 

reservoirs were a source or a sink of nutrients to 

outflow irrigation-supply water, nutrient mass 

balances were calculated. In addition, the flux of nutrients from reservoir bottom sediment was calculated to 

estimate the internal loading to each reservoir.

Total nitrogen and total phosphorus mass 

balances were calculated for the period from March 1 

through September 30, 1995. The general massbalance equation is:

(1)

where 

RS is the final reservoir mass in storage in 

September, in kilograms;

RM is the initial reservoir mass in storage in 

March, in kilograms;

IMS is the mass in the surface-water inflow from 

March through September, in kilograms;

OMS is the mass in the surface-water outflow from 

March through September, in kilograms; 

and

UMS is the portion of the total mass that is unaccounted for from March through 

September, in kilograms.

RS RM IMS OMS UMS = + – ±
8 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

The final constituent mass in reservoir storage in 

September was assumed to be equal to the initial mass 

in storage in March plus inflow mass minus outflow 

mass plus any inflow or minus any outflow mass that 

was not accounted for directly in equation 1. Inflows 

as reported here are those in the major inflow canals, 

and outflows are those in the major irrigation-supply 

canals. “Sources” that are unaccounted for include 

inflow from ephemeral streams, release from reservoir 

bottom sediment, runoff from surrounding land area, 

atmospheric deposition to the reservoir surface, and 

ground-water inflow; “sinks” that are unaccounted for 

include ground-water outflow, sedimentation, and 

biological uptake. 

Inflow and outflow mass was calculated as the 

product of the concentration in a sample collected on a 

given date and the volume of water entering or leaving 

the reservoir between that date and the date of the 

previous sample collected. Therefore, each concentration was assumed to be representative of the period 

between when the sample was collected and when the 

previous sample had been collected. This was not 

always a good assumption, as inflow and outflow 

volumes, degrees of stratification, and levels of biological activity changed between sampling dates. In some 

cases, there was no inflow or outflow on a given 

sampling date, even though there had been substantial 

inflow or outflow during the period between that date 

and the date when the previous sample had been 

collected; as a result, no inflow or outflow mass for 

that period could be determined. Also, for Riverside 

and Jackson Reservoirs, there was a period of several 

weeks between collection of the September reservoir 

sample and collection of the last outflow sample that 

could not be accounted for appropriately in the mass 

balance. These uncertainties led to an unquantified, 

but likely fairly large, amount of error in the mass 

balance for each reservoir. Therefore, these mass 

balances should be used with caution.

The rate and direction of nutrient flux from the 

bottom sediment were estimated using Fick’s first law, 

which states that the rate of flux is directly proportional to the concentration gradient and to the porosity 

of the sediments (Freeze and Cherry, 1979; 

Kimbrough, 1999):

(2)

where

J is the constituent flux, in milligrams per square 

centimeter per day;

φ is the porosity of the bottom sediment;

DS is the bulk sediment diffusion coefficient, in 

square centimeters per day; and

δC/δχ is the concentration gradient, in grams per liter 

per centimeter.

Flux estimates were not included in the massbalance equation (1) because the small number of 

bottom-sediment samples collected may not 

adequately represent the variations in nutrient flux 

rates throughout the entire study period or throughout 

the entire reservoir bottom area. 

Nutrient loading to the reservoir is an important 

regulator of productivity, the rate of biomass formation. The primary productivity of phytoplankton, 

commonly known as algae, can increase markedly 

with increased nutrient loading and lead to extensive 

algal blooms. Primary producers fix light energy with 

chlorophyll to convert carbon dioxide to organic 

matter through photosynthesis (Smith, 1990). Chlorophyll-a is the primary pigment in algae that produces 

chemical energy during photosynthesis (Horne and 

Goldman, 1994). Therefore, chlorophyll-a was used in 

this study as a measure of the quantity of algae present 

in each reservoir. Because chlorophyll-a concentrations can be affected by various environmental factors 

that do not affect algal biomass (Britton and Greeson, 

1989), they should be considered as only an approximation of the true algal biomass. 

Chlorophyll-a concentrations also were used to 

provide an indication of the trophic state of each 

reservoir, a relative classification that reflects the 

extent of nutrient enrichment in a water body. Higher 

concentrations of chlorophyll-a (in the range from 

2.7 to 78 μg/L) are characteristic of eutrophic reservoirs with high nutrient concentrations, low light penetration, frequent algal blooms, and high densities of 

phytoplankton and zooplankton; whereas lower 

concentrations (in the range from 0.3 to 4.5 μg/L) are 

indicative of oligotrophic reservoirs with low nutrient 

concentrations, high light penetration, and low densities of phytoplankton and zooplankton (Organization 

for Economic Cooperation and Development, 1982). 

Mesotrophic reservoirs are those in transition between 

oligotrophic and eutrophic conditions and generally 

contain chlorophyll-a concentrations in the range from 

3 to 11 μg/L. Eutrophication may result in deterioration of water quality and aquatic life in a reservoir, 

J φDs = – ( ) δC ⁄ δχ
METHODS OF DATA COLLECTION AND ANALYSIS 9

which in turn may negatively affect its recreational, 

municipal, or agricultural use.

The chlorophyll-a concentration ranges characteristic of the trophic states overlap somewhat; therefore, there is not a clear distinction between eutrophic, 

mesotrophic, and oligotrophic conditions. Water 

bodies with the same chlorophyll-a concentrations 

could be classified into more than one trophic state. To 

address this limitation, Carlson (1977) refined the 

trophic classification by creating a biomass-based 

trophic-state index (TSI). The TSI falls on a continuous scale from 0 to 100, and every major trophic division of 10 represents a doubling of algal biomass 

concentration. A reservoir with a TSI value less than 

40 is classified as oligotrophic; a reservoir with a TSI 

value between 40 and 50 is classified as mesotrophic; 

and a reservoir with a TSI value between 50 and 100 is 

classified as eutrophic. Carlson’s TSI was estimated 

using the equation:

(3)

where

TSI(Chl) is the trophic-state index computed from 

chlorophyll-a concentrations;

ln is the natural logarithm function; and

Chl is the chlorophyll-a concentration, in micrograms per liter.

Although Carlson (1977) suggested that chlorophyll-a values often may provide the best indicator of 

trophic state, he also developed equations to compute 

the TSI from Secchi-disk depth and total phosphorus 

concentration: 

(4)

and

(5)

where

TSI(SD) is the trophic-state index computed from 

Secchi-disk depth;

TSI(TP) is the trophic-state index computed from 

total phosphorus concentration;

ln is the natural logarithm function;

SD is the Secchi-disk depth, in meters; and

TP is the total phosphorus concentration, in 

micrograms per liter.

The relationship between the three TSI values 

serves as a check on assumptions about the interactions between various components of the reservoir 

ecosystem (Carlson, 1977). The three variables should 

transform to approximately the same TSI value; any 

deviation may provide additional information on 

nutrient dynamics and biological activity in the reservoirs.

The primary factors that control algal biomass in 

most reservoirs are nitrogen and phosphorus concentrations, although other factors such as light and traceelement concentrations can be important as well; the 

nutrient in shortest supply will limit algal biomass. 

The chemical needs of phytoplankton during photosynthesis (Stumm and Morgan, 1996) are characterized by the formula:

106CO2 + 16NO3

– + HPO42– + 122H2O + 18H+

(+ trace elements and light energy) →

←

C106H263O110N16P1 + 138O2

algal protoplasm

The exact stoichiometry of this formula may vary from 

one aquatic habitat to another, but the general relationship will remain the same. During photosynthesis, 

nitrogen and phosphorus are taken up with carbon in 

the ratio 106C:16N:1P (Redfield and others, 1963). 

Transport of carbon dioxide from the atmosphere to 

the water column normally is rapid and, as a result, 

carbon limitation typically is not observed in reservoirs (Ryding and Rast, 1989). 

To determine the potential limiting nutrient in 

each reservoir, mass N:P ratios were calculated and 

compared to the reference N:P ratio. Only samples 

from the near surface, where light was available for 

photosynthesis, were used in the calculations. Also, 

only biologically available forms of nitrogen 

(dissolved nitrite plus nitrate and dissolved ammonia) 

and phosphorus (dissolved orthophosphate) were used 

in the calculations; if dissolved orthophosphate, nitrite 

plus nitrate, or ammonia concentrations were below 

the detection level, total nitrogen and total phosphorus 

concentrations were used instead. Because concentration values were reported in milligrams per liter, the 

TSI Chl ( ) 10 6 ( ) 2.04 0.68 – lnChl

ln2 = – ----------------------------------------------

TSI SD ( ) 10 6 lnSD

ln2 = – ------------

TSI TP ( ) 10 6

ln 48

TP

------    

ln2 = – ------------------
10 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

reference mass ratio of 7.2N:1P was used in place of 

the corresponding atomic ratio of 16N:1P. If the ratio 

of N:P was greater than 7.2, phosphorus was the 

potential limiting nutrient; if the ratio of N:P was less 

than 7.2, nitrogen was the potential limiting nutrient. If 

the ratio was near 7.2, then both nutrients or some 

other factor may have been limiting. In reservoirs 

where concentrations of both nitrogen and phosphorus 

were lower than algal-growth-limiting levels 

(0.020 mg/L nitrogen and 0.010 mg/L phosphorus), it 

is likely that both nutrients were limiting, even though 

the ratio of the two nutrients may have suggested 

otherwise (Ryding and Rast, 1989).

An additional influence on nutrient dynamics 

and primary production in reservoirs is water residence time. Residence time is the time necessary for a 

volume of water in a reservoir to be replaced by 

inflowing water or removed by outflowing water. 

Nutrient retention and stratification may be limited in 

reservoirs with short residence times and rapid 

flushing. Algal productivity also may be restricted 

when residence times are less than 7 days because 

advective cell loss may exceed the phytoplankton 

doubling rate (Kimmel and others, 1990). Monthly 

residence times for each reservoir were calculated 

using the equation:

(6)

where

T is the residence time, in days;

V is the end-of-month reservoir volume, in acrefeet; and

Q is the monthly median reservoir outflow, in 

acre-feet per day.

During periods of strong stratification and 

minimal mixing, the calculated value may not represent accurately the true residence time, as water may 

selectively enter into or leave from only one layer of 

water. Residence times in that layer would be shorter 

than the calculated residence times, whereas residence 

times in the remaining water would be longer than the 

calculated residence times. Additionally, an assumption of equation 6 is that the reservoirs were at steady 

state, where inflow and outflow rates are identical 

(Hornberger and others, 1998). This was not always a 

valid assumption; as a result, residence times 

presented in this report are only estimates intended for 

semiquantitative comparison among the reservoirs.

RIVERSIDE RESERVOIR

Riverside Reservoir is in Weld County, Colo., 

about 25 mi east of Greeley (fig. 1). It is owned by 

Riverside Irrigation District and is the only one of the 

five reservoirs studied that does not serve as a State 

recreational area. The reservoir has an estimated 

maximum storage capacity of 65,000 acre-ft and 

receives most of its surface-water inflow from Riverside Canal (fig. 4), which originates in the South Platte 

River. Additional surface water enters the reservoir 

during storms through Sanborn Draw, an ephemeral 

stream on the north side of the reservoir. Water is 

released from the reservoir through Riverside Canal 

and is used primarily for irrigation; other uses include 

ground-water augmentation and recharge. Riverside 

Reservoir occasionally releases water under a directflow decree, which requires that inflow water be put to 

immediate beneficial use rather than being stored in 

the reservoir; during these periods, inflow must equal 

outflow (Mae Cunning, Assistant Division Engineer, 

Colorado Division of Water Resources, written 

commun., 2001). 

During the study period, reservoir storage 

ranged from 63,113 acre-ft in March to 12,529 acre-ft 

in September (table 1, fig. 5) and maximum depth 

ranged from about 24 ft in March to about 12 ft in 

September. The reservoir normally is filled during the 

fall and winter, reaching maximum volumes in early 

spring. In 1995, the reservoir reached maximum 

storage in March and water was released for irrigation 

beginning in April. The outflow generally increased 

from April through August as demand for irrigation 

water increased; maximum outflow occurred during 

July and August. Correspondingly, storage volumes 

dropped sharply during July through September. 

Measurable residence times in the reservoir ranged 

from 259 days in April to 33 days in September as 

outflow increased and storage decreased (table 1).

Water-quality samples were collected at five 

sites throughout the reservoir (fig. 4) on March 31, 

June 7, July 26, and September 11, 1995. Samples for 

chlorophyll-a were collected only on the last three 

dates from sites 1, 2, and 5. Additional water-quality 

samples were collected from Riverside Canal at the 

inflow and the outflow of the reservoir (when flowing) 

on May 9, June 7, July 12, August 30, and September 

28, 1995. Bottom-sediment samples were collected 

from site 2 on April 5 and August 30, 1995.

T V

Q = ---
RIVERSIDE RESERVOIR 11

Reservoir sampling site and number

Inflow/outflow sampling site

EXPLANATION 0

0 2.5 5 KILOMETERS

2-1/2 5 MILES

Base from U.S. Geological Survey and

U.S. Census Bureau digital data,

1974 to 1993

5

3

4

1

2

Riverside Canal

Riverside Canal

South Platte River

Sanborn Draw

Riverside Reservoir

1

Riverside

Reservoir

South Platte Study Area

N

Water Temperature and Dissolved Oxygen

The temperature profile for March 31 indicates 

that the reservoir was well mixed and isothermal at all 

sites in early spring (fig. 6). If any stratification had 

existed during the winter, as is common for reservoirs 

in the temperate zone (Wetzel, 1983), it had been 

disrupted by the end of March due to loss of ice cover 

and warming and circulation of surface water. As 

spring progressed, the reservoir became moderately 

stratified at sites 2, 3, and 5 due to increased solar 

heating of surface water and the inflow of warmer 

water from Riverside Canal. When inflow water is 

warmer, and therefore less dense than the reservoir 

water, it will enter as overflow at the reservoir surface 

Figure 4. Location of sampling sites in Riverside Reservoir.
12 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

RESERVOIR VOLUME, IN ACRE-FEET

Inflow 

Outflow 

End-of-month volume

RESERVOIR INFLOW OR OUTFLOW, IN 

ACRE-FEET PER DAY

0

200

400

600

800

1,000

March April May June July August September

1995

EXPLANATION

(Elder and Garrison, 1965). The temperature difference between the inflow and reservoir water is reduced 

with distance from the inflow due to heat transfer at 

the air/water interface; therefore, the reservoir was 

only weakly stratified in June at sites 4 and 5, located 

farther from inflows. In July, site 3 was still weakly 

stratified due to its proximity to warmer inflow water, 

but all other sites showed minimal stratification. 

Removal of a large volume of water during July 

resulted in a weaker temperature and density gradient 

between surface water and bottom water, increasing 

mixing and reducing the degree of stratification at sites 

1, 2, 4, and 5. By September, the reservoir volume had 

dropped substantially, and the reservoir had become 

relatively shallow. Shallow water is less stable and 

requires less work to distribute heat uniformly with 

depth (Wetzel, 1983); as a result, the reservoir was 

well mixed and isothermal again at all sites by late 

summer.

The dissolved-oxygen profile for March 31 also 

indicates the reservoir was well mixed and that oxygen 

was uniformly distributed with depth at all five sites 

(fig. 6). Because oxygen solubility is inversely proportional to water temperature, dissolved oxygen concentrations during the study period were highest in March 

when temperatures were lowest. Slight supersaturation 

of dissolved oxygen at site 3 indicated some photosynthetic activity in this shallow area. Reduced dissolved 

oxygen concentrations first were measured in early 

June at site 3, likely as a result of biological decomposition of the organic matter contained in the inflow 

water and sinking algae that had been produced in the 

Table 1. End-of-month storage, monthly median outflow,

and residence time in Riverside Reservoir, 

March–September 1995

[acre-ft, acre-feet; acre-ft/d, acre-feet per day; --, could not be calculated]

Month

End-ofmonth 

storage

(acre-ft)

Monthly 

median 

outflow

(acre-ft/d)

Residence 

time

(days)

March 63,113 0 --

April 59,300 229 259

May 63,113 190 332

June 60,479 373 162

July 49,754 768 65

August 28,600 664 43

September 12,529 379 33

Figure 5. Inflow, outflow, and end-of-month storage volume in Riverside Reservoir, March–September 1995.
RIVERSIDE RESERVOIR 13

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

EXPLANATION

Median euphotic 

 zone depth

Site 1 

Site 2 

Site 3 

Site 4 

Site 5 

WATER TEMPERATURE, IN DEGREES CELSIUS

DEPTH, IN FEET

25

20

15

10

5

0

25

20

15

10

5

0

25

20

15

10

5

0

25

20

15

10

5

0

March 31 June 7 July 26 September 11

0 2 4 6 8 10 12 0 2 4 6 8 10 12 0 2 4 6 8 10 12 0 2 4 6 8 10 12

0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25

0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER

pH, IN STANDARD UNITS

0 400 800 1,200 0 400 800 1,200 0 400 800 1,200 0 400 800 1,200

Figure 6. Depth profiles of water temperature, dissolved oxygen, pH, and specific conductance in Riverside Reservoir, 

March–September 1995.
14 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

euphotic zone during previous months. However, most 

of the organic matter in the inflow likely traveled to 

the main sedimentation zone near the outlet and did 

not settle out at site 3. This is evidenced by the zone of 

nearly depleted dissolved oxygen that developed in the 

bottom water of site 2 by July 26. The development of 

this anoxic zone was accelerated by large outflows 

during June and July, which increased mixing with 

warmer and less dense surface water. Mixing increased 

the rate of biological oxygen uptake and the delivery 

of organic matter and decreased oxygen solubility in 

the deeper water at site 2. The zone of low dissolved 

oxygen had spread upreservoir from site 2 to sites 4 

and 5 as inflow volumes decreased during July. Even 

though these sites were only weakly stratified and 

there was only a small density gradient to impede 

mixing of bottom water with the oxygenated surface 

water, the anoxic zone in the bottom water remained. 

It appears that the rate of consumptive metabolism at 

the sediment/water interface, where settling organic 

matter accumulated, exceeded photosynthetic inputs in 

the euphotic zone during this period. By September 

11, the reservoir was shallow and well mixed at all 

sites, and oxygen once again was uniformly distributed 

with depth.

pH and Specific Conductance

Most lakes and reservoirs have a pH of 6 to 9 

(Horne and Goldman, 1994). In Riverside Reservoir, 

pH ranged seasonally between 8.0 and 9.0 (fig. 6). On 

all sampling dates, pH was constant from surface to 

bottom at all five sites. The pH value in reservoirs is 

primarily controlled by carbon dioxide dissolution in 

water, and photosynthesis and respiration are two 

major factors that influence the amount of carbon 

dioxide in water. The use of carbon dioxide during 

photosynthesis should increase pH in the euphotic 

zone, whereas the release of carbon dioxide during 

biological decomposition of organic matter should 

decrease pH at depth. However, the constant depth 

profiles of pH in Riverside Reservoir indicate that the 

system was well buffered by bicarbonate and 

carbonate, preventing large changes in pH, and that it 

likely was well mixed by wind.

The specific conductance of most lakes and 

reservoirs is proportional to concentrations of major 

ions (salinity) and can be used as a measure of inorganic water quality (Wetzel, 1983). The use of reservoir water with high specific conductance for 

irrigation can be detrimental to crop quality and yield, 

as saline water reduces the ability of plant roots to take 

up water (Maas, 1987). Yields of corn, the major irrigated crop in the lower South Platte River Basin, can 

begin to decline when irrigation water is above a 

specific conductance of 1,100 μS/cm, depending on 

local soil types, climate, and land-use practices (Aryes 

and Westcot, 1976). Specific conductance during the 

study period exceeded 1,200 μS/cm on March 31 and 

June 7 but had declined to near 1,000 μS/cm on July 

26 and September 11 (fig. 6). The specific conductance in Riverside Reservoir was influenced largely by 

inflow from Riverside Canal. The highest specificconductance values were measured on March 31 and 

June 7, when inflow volumes were greatest. Specificconductance values decreased through July and 

September, when inflows were relatively small. The 

highest values in June and July were observed at sites 

3 and 5, closest to the inflow. There was no spatial 

variation throughout the reservoir by September, as all 

inflow had ceased.

Nutrient Dynamics

Water-Column Concentrations

A statistical summary of nutrient concentrations 

at the five sites sampled in Riverside Reservoir, 

including euphotic and bottom samples, is provided in 

table 2. Median concentrations of total nitrogen during 

the study period ranged from 2.4 mg/L at site 1 to 

4.4 mg/L at sites 2 and 4, and median concentrations 

of total phosphorus during the study period ranged 

from 0.53 mg/L at site 4 to 0.59 mg/L at site 5. A 

statistical analysis of nutrient concentrations using the 

Kruskal-Wallis test indicates that the differences in 

nutrient concentrations among the sites were not 

significant (α = 0.05). Minimal longitudinal gradients 

developed, likely because the outflow volumes were 

substantial throughout most of the study period.

Additionally, nitrogen and phosphorus concentrations at all sites showed little difference between 

euphotic and near-bottom samples throughout the 

study period (table 3). Concentrations of nutrient 

species that are most readily available to biota—

dissolved inorganic nitrogen, which includes 

ammonia, nitrate, and nitrite; and dissolved orthophosphate—were slightly lower near the surface during 
RIVERSIDE RESERVOIR 15

Table 2. Statistical summary of nutrient concentrations at each site and results of Kruskal-Wallis test of site concentration 

differences in Riverside Reservoir, March–September 1995

[All concentrations are in milligrams per liter; p-value represents the probability that site concentrations have identical distributions; differences not significant at p-values greater than 0.05]

Constituent Site

(figure 4)

Number of 

analyses Mean Standard deviation

Twenty-fifth 

percentile Median Seventy-fifth percentile p-value

Total nitrogen 1 7 3.4 2.0 1.9 2.4 5.2

2 7 3.7 2.0 1.9 4.4 5.1

3 4 3.3 2.2 1.9 3.1 4.5

4 7 3.7 2.1 1.9 4.4 5.3

5 5 3.9 2.2 2.4 4.3 5.8 0.9725

Dissolved ammonia 1 7 0.25 0.21 0.087 0.18 0.43

2 7 0.18 0.18 0.042 0.090 0.31

3 4 0.17 0.17 0.026 0.16 0.30

4 7 0.20 0.18 0.057 0.13 0.33

5 5 0.21 0.14 0.18 0.18 0.27 0.9488

Dissolved nitrite 1 7 0.10 0.062 0.070 0.13 0.14

2 7 0.10 0.062 0.070 0.13 0.14

3 4 0.095 0.057 0.085 0.12 0.13

4 7 0.10 0.062 0.070 0.13 0.14

5 5 0.11 0.055 0.13 0.13 0.13 0.9274

Dissolved nitrate 1 7 1.7 1.7 0.41 0.81 3.4

2 7 2.1 1.7 0.43 3.1 3.5

3 4 1.9 1.7 0.75 1.9 3.1

4 7 2.1 1.7 0.43 3.1 3.5

5 5 2.3 1.7 0.87 2.9 3.8 0.9799

Total organic nitrogen 1 7 1.3 0.38 1.0 1.2 1.5

2 7 1.3 0.42 0.99 1.1 1.5

3 4 1.1 0.51 0.85 0.90 1.1

4 7 1.3 0.42 0.99 1.2 1.5

5 5 1.3 0.44 0.99 1.0 1.7 0.6251

Total phosphorus 1 7 0.54 0.095 0.51 0.56 0.61

2 7 0.52 0.096 0.47 0.54 0.58

3 4 0.54 0.017 0.53 0.54 0.55

4 7 0.52 0.091 0.47 0.53 0.59

5 5 0.56 0.062 0.51 0.59 0.60 0.8814

Dissolved orthophosphate 1 7 0.41 0.14 0.31 0.39 0.53

2 7 0.39 0.12 0.31 0.37 0.49

3 4 0.41 0.12 0.34 0.41 0.49

4 7 0.40 0.12 0.31 0.38 0.49

5 5 0.42 0.096 0.39 0.39 0.45 0.9782

Total organic phosphorus 1 7 0.13 0.075 0.060 0.13 0.20

2 7 0.13 0.075 0.075 0.12 0.20

3 4 0.13 0.13 0.040 0.13 0.22

4 7 0.12 0.085 0.040 0.12 0.18

5 5 0.14 0.077 0.060 0.17 0.20 0.9745
16 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

June and July, likely due to biological activity in the 

euphotic zone. Overall, however, results of a Wilcoxon 

Rank-Sum test indicate that there were no statistically 

significant differences (α=0.05) between euphotic and 

near-bottom nutrient concentrations during the study 

period. The lack of variation with depth likely is 

because the reservoir never developed strong stratification to limit mixing between surface water and bottom 

water.

Although there was little longitudinal and 

vertical variability in nutrient concentrations, results of 

a Kruskal-Wallis test indicate that there was significant (α=0.05) temporal variability in total nitrogen 

and total phosphorus concentrations from March 

through September (fig. 7). Median concentrations of 

total nitrogen for all data combined decreased from 

6.0 mg/L in March to 1.2 mg/L in September. The 

decrease in total nitrogen was driven largely by a 

decrease in nitrate; median concentrations of nitrate 

dropped from 3.9 mg/L in March to below laboratory 

detection levels by September. During periods of 

substantial inflow, reservoir water quality was influenced by inflowing water from the South Platte River. 

From March through June, concentrations of both 

nitrate and total nitrogen decreased at the South Platte 

River at Kersey (fig. 3), the monitoring station nearest 

to the Riverside Canal intake. The decreases in river 

concentrations contributed to a corresponding 

decrease in reservoir concentrations during that 

period. However, concentrations of total nitrogen at 

Kersey increased in July, whereas concentrations in 

the reservoir continued to decrease. This suggests that 

biological activity also was contributing to the decline 

in nitrogen concentrations. Nitrate is the form of 

Table 3. Statistical summary of nutrient concentrations in euphotic and bottom samples and results of Wilcoxon Rank-Sum 

test of euphotic and bottom concentration differences in Riverside Reservoir, March – September 1995

[e, euphotic; b, bottom; all concentrations are in milligrams per liter; p-value represents the probability that e and b concentrations have identical distributions; differences not significant at p-values greater than 0.05]

Constituent Location

Number 

of

analyses

Mean Standard 

deviation

Twentyfifth 

percentile

Median

Seventyfifth 

percentile

p-value

Total nitrogen e 20 3.5 1.9 2.0 3.3 5.0

b 10 3.9 2.1 1.5 4.5 5.9 0.7574

Dissolved ammonia e 20 0.22 0.18 0.026 0.23 0.34

b 10 0.18 0.17 0.034 0.15 0.28 0.5784

Dissolved nitrite e 20 0.10 0.055 0.085 0.13 0.13

b 10 0.099 0.062 0.040 0.13 0.14 0.7246

Dissolved nitrate e 20 1.9 1.6 0.62 1.9 3.3

b 10 2.2 1.8 0.22 3.1 3.7 0.7722

Total organic nitrogen e 20 1.2 0.40 0.91 1.0 1.4

b 10 1.4 0.41 1.0 1.2 1.8 0.4809

Total phosphorus e 20 0.54 0.069 0.51 0.55 0.59

b 10 0.52 0.098 0.44 0.55 0.59 0.7576

Dissolved orthophosphate e 20 0.42 0.12 0.34 0.42 0.51

b 10 0.38 0.11 0.29 0.37 0.47 0.4150

Total organic phosphorus e 20 0.12 0.087 0.057 0.11 0.20

b 10 0.14 0.070 0.075 0.14 0.20 0.6912
RIVERSIDE RESERVOIR 17

NH3

NH3—Dissolved ammonia

NO2

NO2—Dissolved nitrite

NO3 NO2 NO3 NO2 NO3 NO2 NO3

NO3—Dissolved nitrate

TN

TN—Total nitrogen

TOrgN

TOrgN—Total organic nitrogen

TN NH3 TOrgN NH3

TN TOrgN TN TOrgN NH3

0

1

2

3

4

5

6

7

CONCENTRATION, IN MILLIGRAMS PER 

LITER AS NITROGEN

March 31 June 7 July 26 September 11

0

0.2

0.4

0.6

0.8

CONCENTRATION, IN MILLIGRAMS PER 

LITER AS PHOSPHORUS

OP

OP—Dissolved orthophosphate

TOrgP

TOrgP—Total organic phosphorus

TP

TP—Total phosphorus

TP OP TOrgP OP TOrgP TP OP TOrgP TP

EXPLANATION

Outlier data value greater than 1.5

times the interquartile range

outside the quartile

Data value less than or equal

to 1.5 times the interquartile range

outside the quartile

75th percentile

25th percentile

Median

Figure 7. Nitrogen and phosphorus concentrations, euphotic and bottom samples combined in 

Riverside Reservoir, March–September 1995.
18 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995 DISTANCE ABOVE AND BELOW SEDIMENT/WATER INTERFACE, IN CENTIMETERS

5

4

3

2

1

0

1

Sediment/Water interface

Site 2 measured concentrations 

DISSOLVED INORGANIC NITROGEN CONCENTRATION, IN MILLIGRAMS PER LITER

April 5 August 30

April 5

5

4

3

2

1

0

0 1 2 3 4 5 0 2 4 6 8 10

0 1 2 3 4 5 0 2 4 6 8 10 12 14

1

DISSOLVED ORTHOPHOSPHATE CONCENTRATION, IN MILLIGRAMS PER LITER

August 30

Data missing

Data missing

Data missing

EXPLANATION

nitrogen most readily used by phytoplankton during 

growth after ammonia (Horne and Goldman, 1994), 

and the rate of nitrate assimilation by algae likely had 

exceeded the rate of nitrate inflow by July. After reservoir inflows had ended entirely in early September, 

continued biological activity resulted in assimilation 

of all remaining nitrate, and only total organic nitrogen 

was left in measurable quantities. 

Concentrations of total phosphorus did not have 

the same consistent decrease during the study period 

as concentrations of total nitrogen. Median concentrations of total phosphorus for all data combined were 

lowest in September, at 0.42 mg/L, and highest in July, 

at 0.62 mg/L (fig. 7). The temporal pattern of total 

phosphorus concentrations in the reservoir did not 

follow that of the South Platte River at Kersey (fig. 3), 

indicating that internal processes had a greater effect 

on concentrations of phosphorus in the reservoir than 

inflow loading. Dissolved orthophosphate is the only 

biologically available form of phosphorus (Horne and 

Goldman, 1994), but concentrations of this phosphorus species did not decline consistently as was 

observed with nitrate. Instead, orthophosphate may 

have been recycled continuously back to algae through 

processes such as excretion from zooplankton and fish, 

decay of aquatic plants and animals, and release from 

bottom sediment (Wetzel, 1983).

Bottom-Sediment Flux

Estimates of nutrient flux from the bottom sediment indicate that the sediment was, at times, a potential source of nutrients to the reservoir. On April 5, 

concentrations of dissolved inorganic nitrogen 

measured in the first centimeter of the sediment were 

higher than in the overly-ing water column at site 2 

(fig. 8). The estimated flux rate for inorganic nitrogen 

was 21 mg/m2/d. Concentrations of dissolved orthoFigure 8. Concentrations of dissolved inorganic nitrogen and orthophosphate in bottom-sediment pore water and the overlying 

water column in Riverside Reservoir, April 5 and August 30, 1995.
RIVERSIDE RESERVOIR 19

phosphate measured in the first centimeter of the sediment also were higher than in the overlying water 

column at site 2 on April 5. The estimated flux rate for 

orthophosphate was 3.8 mg/m2/d. On August 30, 

concentrations of dissolved inorganic nitrogen 

measured in the first centimeter of the sediment were 

higher than in the overlying water column at site 2. 

The estimated flux rate for inorganic nitrogen was 

38 mg/m2/d. Concentrations of dissolved orthophosphate measured in the first centimeter of the sediment 

were lower than in the overlying water column at site 2 

on August 30. The estimated flux rate for orthophosphate was –1.7 mg/m2

/d (into the sediment). Because 

of the limited number of samples collected, these flux 

estimates may not represent the spatial and temporal 

variations in flux throughout the reservoir and 

throughout the entire study period. Additionally, 

because bottom sediment release is redox-dependent, 

movement from the sediment into the overlying water 

column is restricted if the sediment/water interface is 

well oxygenated (Horne and Goldman, 1994). The 

near-bottom dissolved oxygen concentration at site 2 

on April 5 was close to 9.5 mg/L, measured on March 

31; concentrations on August 30 fell somewhere 

between 0.8 mg/L, measured on July 26, and 

5.3 mg/L, measured on September 11. Therefore, all 

of these flux calculations may overestimate the movement of nutrients from bottom sediment to the water 

column.

Mass Balance

Mass-balance calculations for nitrogen indicate 

that about 68 percent of the total mass of nitrogen—

the mass that had been in storage in March plus the 

mass that had entered in inflows through September—

was trapped in the reservoir (represented by the “unaccounted for” term), while 28 percent was released 

through the outflow (table 4). Only 3 percent of the 

mass remained in the water column by September. 

Mass-balance calculations for phosphorus indicate that 

about 25 percent of the total mass of phosphorus was 

trapped in the reservoir, while 65 percent was released 

through the outflow. Only 10 percent of the mass 

remained in the water column by September. Much of 

Table 4. Mass-balance calculations for total nitrogen and total phosphorus in Riverside Reservoir, 

March–September 1995

[From equation 1: Unaccounted for mass = Final storage mass – Initial storage mass – Inflow mass + Outflow mass]

Date

Total nitrogen mass, in kilograms

Initial 

storage Inflow Outflow Final 

storage

Unaccounted 

for

March 31 467,289

April 1–May 9 73,512 0

May 10 –June 7 27,504 52,539

June 8–July 12 32,147 63,175

July 13–August 30 0 54,677

September 11

Total 467,289 133,163 170,391 19,326 –410,735

Date Total phosphorus mass, in kilograms

Initial 

storage Inflow Outflow Final 

storage

Unaccounted 

for

March 31 45,950

April 1 – May 9 9,513 0

May 10 – June 7 5,015 5,954

June 8 – July 12 4,822 12,227

July 13 – August 30 0 24,058

September 11 6,416

Total 45,950 19,351 42,240 6,416 –16,645
20 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

the nitrogen was trapped in the reservoir through 

biological uptake and deposition of particulate organic 

nitrogen onto bottom sediment. Much of the phosphorus was trapped through sedimentation of orthophosphate adsorbed to inorganic particles, deposition 

of particulate organic phosphorus onto bottom sediment, and biological uptake. Toward the end of the 

study period, the reservoir had relatively short residence times and some of the recycled phosphorus still 

in the water column was flushed out; much of the 

nitrogen, however, had already been utilized by algae 

and was not available to be flushed out. As a result, a 

greater percentage of the phosphorus mass was 

released through the outflow. Overall, a substantial 

portion of the total nutrient mass was trapped in the 

reservoir during the study period, and the reservoir 

acted as a sink for both nitrogen and phosphorus. 

Algal Growth

The nutrient dynamics in the reservoir led to a 

change in the nutrient potentially limiting algal 

growth. In March, N:P values ranged from 10 to 11, 

indicating that phosphorus was the limiting nutrient in 

the early spring (table 5). In June, N:P values ranged 

from 6.8 to 7.3, indicating that both nutrients were 

potentially limiting. By July, N:P values had decreased 

to around 2.5, indicating that nitrogen had become the 

limiting nutrient. The nitrogen limitation continued 

into the late summer, with N:P values ranging from 1.8 

to 3.3 in September. Concentrations of dissolved inorganic nitrogen species were all below the detection 

limit in September, giving further support that nitrogen 

was limiting by the end of the study period. A change 

in the nutrient limiting algal growth can influence the 

algal community structure; low N:P values typically 

favor the growth of nitrogen-fixing blue-green algae 

(Ryding and Rast, 1989). 

Although chlorophyll-a measurements cannot 

provide information on the composition of the 

phytoplankton community, they can provide a measure 

of the quantity of algae present. Chlorophyll-a concentrations measured in the euphotic zone in June ranged 

from 1.4 μg/L at site 1 to 2.6 μg/L at site 2 near the 

outlet (table 6). By late July, chlorophyll-a concentrations ranged from 6.6 μg/L at site 1 to 7.8 μg/L at site 

2. The higher concentrations at site 2 during the late 

spring and summer support the theory that much of the 

organic matter in the inflow traveled to a sedimentation zone near the outlet, contributing to high primary 

productivity in the surface water and a zone of nearly 

depleted dissolved oxygen in the bottom water of 

site 2 by late July. By September, algal biomass had 

increased substantially; chlorophyll-a concentrations 

ranged from 19 μg/L at site 5 to 42 μg/L at site 1. 

Based on mean chlorophyll-a concentrations, Riverside Reservoir is classified as eutrophic. Carlson’s 

chlorophyll TSI values ranged from 43 to 49 in June, 

from 58 to 60 in July, and from 68 to 76 in September 

(table 6), indicating that the algal biomass doubled 

approximately three times from late spring to late 

summer. Chlorophyll and Secchi-disk TSI values were 

similar throughout the study period, though they 

consistently were lower than the phosphorus TSI 

values. The larger phosphorus TSI values support the 

conclusion that phosphorus was not limiting algal 

growth during much of the summer.

Table 5. Concentrations of biologically available nitrogen 

and phosphorus and N:P values for Riverside Reservoir, 

March–September 1995

[<, less than; DIN, dissolved inorganic nitrogen; DOP, dissolved 

orthophosphate; N:P, mass ratio of nitrogen to phosphorus; values of 

N:P calculated from total nitrogen and total phosphorus if DIN or DOP

concentrations below detection limits]

Site

(figure 4)

Concentration,

in milligrams per liter N:P

DIN DOP

March 31

1 4.1 0.39 11

2 4.1 0.37 11

3 4.0 0.36 11

4 4.1 0.38 10

5 4.1 0.39 11

June 7

1 3.6 0.50 7.3

2 3.5 0.48 7.3

3 3.2 0.47 6.8

4 3.5 0.48 7.3

5 3.3 0.45 7.3

July 26

1 1.5 0.57 2.5

2 1.4 0.55 2.6

3 1.4 0.54 2.7

4 1.5 0.58 2.5

5 1.4 0.56 2.5

September 11

1 < 0.065 0.23 2.9

2 < 0.065 0.24 3.1

3 < 0.065 0.27 1.8

4 < 0.065 0.26 3.3

5 < 0.065 0.30 2.0
JACKSON RESERVOIR 21

JACKSON RESERVOIR

Jackson Reservoir is in Morgan County, Colo., 

about 6 mi northwest of Weldona (fig. 1). It is owned 

by Jackson Lake Reservoir Company and also is operated as a State Park by the Colorado Division of Parks 

and Outdoor Recreation. The reservoir has an estimated maximum storage capacity of 35,629 acre-ft 

and receives most of its surface-water inflow from 

Jackson Lake Inlet (fig. 9), which originates in the 

South Platte River. Water released from the reservoir 

through Jackson Lake Outlet is used primarily for irrigation; other uses include ground-water augmentation, 

recharge, and recreation. 

During the study period, reservoir storage 

ranged from 26,719 acre-ft in March to 19,443 acre-ft 

by September (table 7, fig. 10) and maximum depth 

ranged from about 18 ft in March to about 15 ft in 

September. The reservoir normally is filled during the 

fall and winter, reaching maximum volumes in the 

early spring. In 1995, the reservoir reached maximum 

storage in April, and water was released for irrigation 

beginning in August. There were small inflows during 

March, April, and May and small outflows during 

August and September. Correspondingly, storage 

volumes remained fairly steady through the end of 

July then decreased slightly through the end of 

September. Measurable residence times in the reservoir ranged from 377 days in August to 560 days in 

September as outflow decreased slightly (table 7).

Water-quality samples were collected at five 

sites throughout the reservoir (fig. 9) on March 8, 

May 30, July 25, and September 12, 1995. Samples for 

chlorophyll-a were collected only on the last three 

dates from sites 2, 4, and 5. Additional water-quality 

samples were collected from Jackson Lake Inlet and 

Outlet (when flowing) on March 8, July 12, August 

30, and September 28, 1995. Bottom-sediment 

samples were collected from sites 1 and 2 in the late 

winter, on February 28 and March 8, and in the 

summer, on August 18.

Water Temperature and Dissolved Oxygen

The temperature profile for March 8 indicates 

that the reservoir was well mixed at all sites in late 

winter; temperatures were approximately 3oC from 

surface to bottom (fig. 11). In water near the temperature of maximum density (4oC), density differences 

per degree of temperature change are very small 

(Wetzel, 1983), so very little wind energy would have 

been necessary to mix the water column. As spring 

and summer progressed, the water column remained 

nearly isothermal and no stratification was observed. 

Stratification from warmer inflow water entering as 

overflow did not occur, as inflow volumes during the 

study period were small. Additionally, because the 

Table 6. Chlorophyll-a concentrations, Secchi-disk depths, total phosphorus concentrations, and associated trophic-state 

indices for Riverside Reservoir, March–September 1995

[μg/L, micrograms per liter; mg/L; milligrams per liter; TSI, Trophic State Index, Chl, chlorophyll-a; SD, Secchi disk; TP, total phosphorus]

Site

(figure 

4)

Chlorophyll-a

concentration

(μg/L)

TSI(Chl)

Secchi-disk 

depth

(feet)

TSI(SD)

Total

phosphorus 

concentration

(mg/L)

TSI(TP)

June 7

1 1.4 43 10 44 0.56 95

2 2.6 49 6.0 51 0.51 94

5 1.8 45 6.0 51 0.51 94

July 26

1 6.6 58 8.0 47 0.63 97

2 7.8 60 8.0 47 0.65 98

5 7.0 59 5.0 54 0.61 97

September 11

1 42 76 1.0 77 0.46 93

2 33 74 1.0 77 0.43 92

5 19 68 0.50 87 0.47 93
22 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Reservoir sampling site and number

Inflow/outflow sampling site

EXPLANATION 0

0 2.5 5 KILOMETERS

2-1/2 5 MILES

Base from U.S. Geological Survey and

U.S. Census Bureau digital data,

1974 to 1993

2 5

1

3

4

Jackson Lake Inlet

South Platte River

Jackson Reservoir

Jackson Lake

Outlet

Jackson 

Reservoir

South Platte Study Area

2

N

reservoir is relatively shallow and the surrounding 

terrain is flat, frequent strong winds likely helped keep 

the water column well mixed during the study period. 

The dissolved-oxygen profile for March 8 indicates that the reservoir was well mixed and that 

oxygen was uniformly distributed with depth at all five 

sites (fig. 11). Because oxygen solubility is inversely 

proportional to water temperature, dissolved oxygen 

concentrations during the study period were highest in 

March when temperatures were lowest. Slight supersaturation of dissolved oxygen at site 5 indicates some 

photosynthetic activity in the shallowest water close to 

the nutrient-rich inflow water (Appendix). Reduced 

dissolved oxygen concentrations with depth were first 

observed in late July at all sites, due to biological 

decomposition of sinking organic matter. Though the 

reservoir was well mixed at this time, rates of 

consumptive metabolism near the bottom were high 

Figure 9. Location of sampling sites in Jackson Reservoir.
JACKSON RESERVOIR 23

0

5,000

10,000

15,000

20,000

25,000

30,000

0

200

400

600

800

1,000

RESERVOIR VOLUME, IN ACRE-FEET

Inflow 

Outflow 

End-of-month volume

March April May June July August September

1995

EXPLANATION

RESERVOIR INFLOW OR OUTFLOW, IN 

ACRE-FEET PER DAY

enough to decrease bottom dissolved oxygen concentrations. Without any significant inflow during the 

summer to replenish nutrient loads in the reservoir, 

primary production likely had decreased markedly in 

the late summer. The concomitant decrease in decomposition reduced the dissolved oxygen gradient by 

September 12. 

pH and Specific Conductance

In Jackson Reservoir, pH ranged seasonally 

between 8.0 and 9.2 (fig. 11). On all sampling dates, 

pH was constant from surface to bottom at all five 

sites. The lack of a pH difference between zones of 

photosynthesis and decomposition indicate that the 

system was well buffered by bicarbonate and 

carbonate, preventing large changes in pH, and that it 

likely was well mixed by wind. On all sampling dates, 

specific conductance also was constant from surface to 

bottom at all five sites (fig. 11). The maximum specific 

conductance measured during the study period was 

approximately 1,500 μS/cm on March 8. At this time, 

specific conductance was influenced primarily by 

inflow water; specific conductance was measured at 

1,493 μS/cm in Jackson Lake Inlet that day. With little 

additional inflow after that date, specific conductance 

decreased slightly to near 1,400 μS/cm during the 

remainder of the study period. Because of the high 

salinity, use of this water for irrigation could have been 

detrimental to crop quality and yield (Aryes and 

Westcot, 1976). 

Table 7. End-of-month storage, monthly median outflow, and 

residence time in Jackson Reservoir, March–September 1995

[acre-ft, acre-feet; acre-ft/d, acre-feet per day; --, could not be calculated]

Month

End-ofmonth 

storage

(acre-ft)

Monthly 

median 

outflow

(acre-ft/d)

Residence 

time

(days)

March 26,719 0 --

April 27,257 0 --

May 27,257 0 --

June 26,611 0 --

July 24,893 0 --

August 20,956 56 377

September 19,443 35 560

Figure 10. Inflow, outflow, and end-of-month storage volume in Jackson Reservoir, March–September 1995.
24 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995 DEPTH, IN FEET

0 15 20 25 10 5 10 0 15 20 25 10 5 0 15 20 25 10 5 0 15 20 25 5

0 15 20 10 5 10 0 15 20 10 5 0 15 20 10 5 0 15 20 5

20

15

10

5

0

20

15

10

5

0

March 8 

WATER TEMPERATURE, IN DEGREES CELSIUS

May 30 July 25 September 12

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

Median euphotic 

 zone depth

Site 1 

Site 2 

Site 3 

Site 4 

Site 5 

EXPLANATION

0

0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14

200 600 1,000 1,400 0 200 600 1,000 1,400 0 200 600 1,000 1,400 0 200 600 1,000 1,400

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER

pH, IN STANDARD UNITS

20

15

10

5

0

20

15

10

5

0

Figure 11. Depth profiles of water temperature, dissolved oxygen, pH, and specific conductance in Jackson Reservoir, 

March–September 1995.
JACKSON RESERVOIR 25

Nutrient Dynamics

Water-Column Concentrations

A statistical summary of nutrient concentrations 

at the five sites sampled in Jackson Reservoir, 

including euphotic and bottom samples, is provided in 

table 8. Median concentrations of total nitrogen during 

the study period ranged from 1.9 mg/L at site 5 to 

2.8 mg/L at site 4, and median concentrations of total 

phosphorus during the study period ranged from 

0.17 mg/L at site 3 to 0.19 mg/L at sites 1, 4, and 5. A 

statistical analysis of nutrient concentrations using the 

Kruskal-Wallis test indicates that the differences in 

nutrient concentrations between the sites were not 

significant (α=0.05). Because the inflow and outflow 

volumes were small throughout most of the study 

period, there was little advective flow in the reservoir, 

resulting in minimal longitudinal gradients in concentration.

Samples collected near the reservoir surface and 

near the reservoir bottom show little difference 

between euphotic and bottom concentrations of 

nitrogen and phosphorus during most of the study 

period. Median concentrations of total phosphorus, 

orthophosphate, and dissolved inorganic nitrogen for 

all sites combined were slightly higher near the bottom 

during July, likely due to biological uptake in the 

euphotic zone. Overall, however, results of a Wilcoxon 

Rank-Sum test indicate that there were no statistically 

significant differences (α=0.05) between euphotic and 

bottom nutrient concentrations during the study period 

(table 9). The lack of variation with depth likely was 

because the reservoir never developed strong stratification to limit mixing between surface water and bottom 

water.

Although there was little longitudinal and 

vertical variability in nutrient concentrations, results of 

a Kruskal-Wallis test indicate that there was significant (α=0.05) temporal variability in total nitrogen and 

total phosphorus concentrations from March through 

September (fig. 12). Median concentrations of total 

nitrogen for all data combined decreased from 

4.9 mg/L in March to 1.7 mg/L in September. The 

decrease in total nitrogen was driven largely by a 

decrease in nitrate; median concentrations of nitrate 

dropped from 2.9 mg/L in March to below laboratory 

detection levels by July. During periods of inflow, 

reservoir water quality was influenced slightly by 

inflowing water from the South Platte River. From 

March through May, concentrations of both nitrate and 

total nitrogen decreased at the South Platte River at 

Kersey (fig. 3), the nearest monitoring station 

upstream from the Jackson Lake Inlet intake. The 

decreases in river concentrations contributed to a 

decrease in reservoir concentrations during that 

period, though the influence of the river water likely 

was minor compared to internal processes, as inflow 

volumes were very small compared to the volume in 

storage. After reservoir inflows had ended entirely at 

the end of May, continued biological activity resulted 

in assimilation of all remaining nitrate by the end of 

July, and only total organic nitrogen was left in 

measurable quantities. 

Concentrations of total phosphorus did not have 

the same consistent decrease during the study period 

as concentrations of total nitrogen. Median concentrations of total phosphorus for all data combined were 

lowest in May, at 0.11 mg/L, and highest in March and 

July, at 0.20 mg/L (fig. 12). The temporal pattern of 

total phosphorus concentrations in the reservoir 

appeared to follow that of the South Platte River at 

Kersey (fig. 3), indicating that inflow loading affected 

concentrations of phosphorus in the reservoir. 

However, as with nitrogen, the influence of the river 

water likely was minor compared to internal 

processes. After reservoir inflows had ended entirely 

at the end of May, biological activity continued, but 

phosphorus concentrations likely increased due to 

internal recycling.

Bottom-Sediment Flux

Estimates of nutrient flux from the bottom sediment indicate that the sediment was, at times, a potential source of nutrients to the reservoir. In late winter, 

concentrations of dissolved inorganic nitrogen 

measured in the first centimeter of the sediment were 

higher than in the overlying water column at site 1 and 

lower than in the overlying water column at site 2 

(fig. 13). The estimated flux rates for inorganic 

nitrogen were 12 mg/m2/d at site 1 and –14 mg/m2/d 

(into the sediment) at site 2. Concentrations of 

dissolved orthophosphate measured in the first centimeter of the sediment were higher than in the overlying water column at sites 1 and 2 in late winter. The 

estimated flux rate for orthophosphate was 

7.5 mg/m2

/d at site 1 and 0.10 mg/m2/d at site 2. On 

August 18, concentrations of dissolved inorganic 

nitrogen measured in the first centimeter of the sedi-
26 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Table 8. Statistical summary of nutrient concentrations at each site and results of Kruskal-Wallis test of site concentration 

differences in Jackson Reservoir, March–September 1995

[All concentrations are in milligrams per liter; p-value represents the probability that site concentrations have identical distributions; differences not significant at p-values greater than 0.05]

Constituent Site

(figure 9)

Number of 

analyses Mean Standard deviation

Twentyfifth 

percentile

Median

Seventyfifth 

percentile

p-value

Total nitrogen 1 8 2.8 1.4 1.8 2.3 3.3

2 7 2.9 1.4 1.9 2.6 3.9

3 4 2.7 1.6 1.8 2.2 3.1

4 8 3.0 1.3 2.3 2.8 3.4

5 6 2.4 1.5 1.5 1.9 2.6 0.8546

Dissolved ammonia 1 8 0.18 0.23 0.015 0.073 0.29

2 7 0.11 0.19 0.015 0.015 0.10

3 4 0.15 0.25 0.015 0.025 0.15

4 8 0.19 0.24 0.015 0.040 0.38

5 6 0.23 0.24 0.015 0.18 0.42 0.8895

Dissolved nitrite 1 8 0.035 0.028 0.010 0.030 0.055

2 7 0.030 0.026 0.010 0.010 0.050

3 4 0.035 0.030 0.010 0.030 0.055

4 8 0.035 0.028 0.010 0.030 0.055

5 6 0.027 0.027 0.010 0.010 0.040 0.9437

Dissolved nitrate 1 8 0.97 1.3 0.040 0.043 1.4

2 7 0.98 1.4 0.040 0.040 1.9

3 4 0.97 1.4 0.040 0.43 1.4

4 8 0.96 1.3 0.040 0.43 1.4

5 6 0.66 1.2 0.040 0.040 0.64 0.9608

Total organic nitrogen 1 8 1.6 0.33 1.3 1.7 1.8

2 7 1.8 0.53 1.6 1.7 1.9

3 4 1.5 0.44 1.1 1.5 1.9

4 8 1.8 0.52 1.4 1.8 1.9

5 6 1.5 0.49 1.3 1.5 1.7 0.7284

Total phosphorus 1 8 0.17 0.043 0.13 0.19 0.20

2 7 0.18 0.053 0.17 0.18 0.20

3 4 0.16 0.060 0.12 0.17 0.21

4 8 0.19 0.044 0.16 0.19 0.20

5 6 0.18 0.041 0.15 0.19 0.21 0.9663

Dissolved orthophosphate 1 8 0.053 0.028 0.040 0.065 0.070

2 7 0.053 0.031 0.030 0.070 0.075

3 4 0.040 0.026 0.025 0.040 0.055

4 8 0.053 0.030 0.040 0.055 0.073

5 6 0.070 0.038 0.055 0.070 0.10 0.7064

Total organic phosphorus 1 8 0.11 0.059 0.057 0.12 0.15

2 7 0.13 0.050 0.11 0.13 0.17

3 4 0.12 0.067 0.085 0.12 0.15

4 8 0.13 0.055 0.090 0.12 0.18

5 6 0.11 0.061 0.077 0.11 0.13 0.9424
JACKSON RESERVOIR 27

ment were higher than in the overlying water column 

at sites 1 and 2. The estimated flux rates for inorganic 

nitrogen were 35 mg/m2/d at site 1 and 46 mg/m2/d at 

site 2. Concentrations of dissolved orthophosphate 

measured in the first centimeter of the sediment also 

were higher than in the overlying water column at sites 

1 and 2 on August 18. The estimated flux rates for 

orthophosphate were 9.0 mg/m2/d at site 1 and 

8.3 mg/m2/d at site 2. Clearly, flux measurements on 

one date or at one site would not have represented the 

spatial and temporal variations in flux throughout the 

reservoir. Additionally, because bottom sediment 

release is redox-dependent, movement from the sediment into the overlying water column is restricted if 

the sediment/water interface is well oxygenated 

(Horne and Goldman, 1994). The bottom dissolved 

oxygen concentrations at site 1 and site 2 near the 

beginning of March were close to 18 mg/L; concentrations on August 18 fell somewhere between 3 mg/L, 

the minimum measured on July 25, and 9 mg/L, 

measured on September 12. Therefore, these flux 

calculations may overestimate the movement of nutrients from bottom sediment to the water column.

Mass Balance

Mass-balance calculations for nitrogen indicate 

that about 63 percent of the total mass of nitrogen—

the mass that had been in storage in March plus the 

mass that had entered in inflows through September—

Table 9. Statistical summary of nutrient concentrations in euphotic and bottom samples and results of Wilcoxon Rank-Sum test 

of euphotic and bottom concentration differences in Jackson Reservoir, March–September 1995

[e, euphotic; b, bottom; all concentrations are in milligrams per liter; p-value represents the probability that e and b concentrations have identical distributions; differences not significant at p-values greater than 0.05]

Constituent Location Number of 

analyses Mean

Standard 

deviation

Twentyfifth 

percentile

Median

Seventyfifth 

percentile

p-value

Total nitrogen e 20 2.9 1.3 1.9 2.5 3.4

b 13 2.6 1.4 1.3 2.5 3.1 0.3277

Dissolved ammonia e 20 0.17 0.23 0.015 0.025 0.39

b 13 0.18 0.21 0.015 0.020 0.33 1.000

Dissolved nitrite e 20 0.035 0.027 0.010 0.030 0.055

b 13 0.028 0.025 0.010 0.010 0.050 0.4877

Dissolved nitrate e 20 0.96 1.2 0.040 0.43 1.4

b 13 0.83 1.2 0.040 0.040 0.84 0.6405

Total organic nitrogen e 20 1.7 0.40 1.4 1.8 1.9

b 13 1.6 0.55 1.3 1.6 1.8 0.3553

Total phosphorus e 20 0.18 0.047 0.14 0.19 0.20

b 13 0.18 0.044 0.13 0.17 0.20 0.7947

Dissolved orthophosphate e 20 0.049 0.028 0.025 0.050 0.070

b 13 0.062 0.032 0.060 0.070 0.080 0.1322

Total organic phosphorus e 20 0.13 0.053 0.097 0.12 0.17

b 13 0.11 0.059 0.070 0.12 0.16 0.5414
28 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

EXPLANATION

Outlier data value greater than 1.5

times the interquartile range

outside the quartile

Data value less than or equal

to 1.5 times the interquartile range

outside the quartile

75th percentile

25th percentile

Median

TP OP TOrgP OP TOrgP TP OP TOrgP TP OP TOrgP TP

0

0.2

0.4

0.6

0.8

CONCENTRATION, IN MILLIGRAMS PER 

LITER AS PHOSPHORUS

0

1

2

3

4

5

6

7

CONCENTRATION, IN MILLIGRAMS PER 

LITER AS NITROGEN

NH3 NO2 NO3 TN TOrgN NH3 NO2 TN TOrgN NH NO3 3 NO2 NO3 TN TOrgN NH3 NO2 TN TOrgN NO3

March 8 May 30 July 25 September 12

NH3—Dissolved ammonia

NO2—Dissolved nitrite

NO3—Dissolved nitrate

TN—Total nitrogen

TOrgN—Total organic nitrogen

TOrgP—Total organic phosphorus

OP—Dissolved orthophosphate

TP—Total phosphorus

Figure 12. Nitrogen and phosphorus concentrations, euphotic and bottom samples combined in 

Jackson Reservoir, March–September 1995.
JACKSON RESERVOIR 29

5

4

3

2

1

0

1

5

4

3

2

1

0

0

0 5 10 15 20 0 5 10 15 20 25 30 35

2 4 6 8 0 2 4 6 8 10 12

1

DISSOLVED INORGANIC NITROGEN CONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED ORTHOPHOSPHATE CONCENTRATION, IN MILLIGRAMS PER LITER

February 28 and March 8 August 18

February 28 and March 8 August 18

Sediment/Water interface

Site 1 measured concentrations 

Site 2 measured concentrations

EXPLANATION

DISTANCE ABOVE AND BELOW SEDIMENT/WATER

INTERFACE, IN CENTIMETERS

Data missing

Data missing

was trapped in the reservoir (represented by the “unaccounted for” term), while 12 percent was released 

through the outflow and 25 percent remained in the 

water column by September (table 10). Mass-balance 

calculations for phosphorus indicate that about 360 kg 

from sources other than inflow loading was gained and 

retained by the reservoir; input sources could have 

included surface runoff and loading from ephemeral 

streams during storms. Approximately 36 percent of 

the total mass of phosphorus was released through the 

outflow, while 69 percent remained in the water 

column by September. 

Much of the nitrogen likely was trapped in the 

reservoir through biological uptake and deposition of 

particulate organic nitrogen onto bottom sediment. 

Much of the phosphorus likely was internally recycled 

due to processes such as excretion from zooplankton 

and fish, decay of aquatic plants and animals, and 

release from bottom sediment (Wetzel, 1983). Because 

little outflow occurred, much of the recycled phosphorus was not flushed out and remained in the water 

column. Overall, a substantial portion of the total mass 

of nitrogen was trapped in the reservoir during the 

study period, and the reservoir acted as a sink for 

nitrogen. The very small gain in the total mass of 

phosphorus during the study period indicates that the 

reservoir may have acted as a minor source of phosphorus to the outflowing irrigation-supply water. It is 

possible also that the gain in phosphorus is attributable 

to error in the mass balance.

Figure 13. Concentrations of dissolved inorganic nitrogen and orthophosphate in bottom-sediment pore water and the overlying water column in Jackson Reservoir, February 28, March 8, and August 18, 1995.
30 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Algal Growth

The nutrient dynamics in the reservoir led to a 

change in the nutrient potentially limiting algal 

growth. In March, dissolved orthophosphate concentrations were below the algal-growth-limiting concentration of 0.010 mg/L, and N:P values ranged from 23 

to 27, indicating that phosphorus was the limiting 

nutrient in the late winter (table 11). In May, phosphorus remained the nutrient limiting algal growth, as 

N:P values ranged from 28 to 29. By the end of July, 

N:P values ranged from 1.6 to 3.6, indicating that 

nitrogen had become the limiting nutrient. The 

nitrogen limitation may have ended in some portions 

of the reservoir by late summer, with N:P values 

ranging from 1.1 to 10 in September; however, these 

N:P values were calculated with total nitrogen and 

total phosphorus and may overestimate the concentrations of biologically available nitrogen and phosphorus. Concentrations of dissolved inorganic nitrogen 

species were all below the detection limit in 

September, suggesting that nitrogen was limiting at the 

end of the study period. The nitrogen limitation may 

have promoted the growth of nitrogen-fixing bluegreen algae during the summer. 

Although chlorophyll-a measurements cannot 

provide information on the composition of the 

phytoplankton community, they can provide a measure 

of the quantity of algae present. Chlorophyll-a concentrations measured in the euphotic zone in May ranged 

from 2.4 μg/L at site 5 to 4.1 μg/L at site 4 (table 12). 

By late July, chlorophyll-a concentrations ranged from 

53 μg/L at site 5 to 150 μg/L at sites 2 and 4. Concentrations reached relatively high levels during the 

summer because the residence time was high and there 

was no advective outflow to cause phytoplankton 

washout. By September, algal growth had decreased; 

chlorophyll-a concentrations ranged from 50 μg/L at 

site 5 to 84 μg/L at site 4. Without any significant 

inflow during the summer to replenish nutrient loads 

in the reservoir, algal populations could not sustain 

growth. Additionally, it is possible that algal growth 

was restricted by zooplankton grazing or light limitation due to self-shading (Wetzel, 1983). Based on 

mean chlorophyll-a concentrations, Jackson ReserTable 10. Mass-balance calculations for total nitrogen and total phosphorus in Jackson Reservoir, 

March–September 1995

[From equation 1: Unaccounted for mass = Final storage mass – Initial storage mass – Inflow mass + Outflow mass]

Date

Total nitrogen mass, in kilograms

Initial 

storage Inflow Outflow Final 

storage

Unaccounted 

for

March 8 161,559

March 9–July 12 0 10,115

July 13–August 30 0 8,002

August 31–September 28 0 1,428

September 12 39,588

Total 161,559 0 19,545 39,588 –102,426

Date

Total phosphorus mass, in kilograms

Initial 

storage Inflow Outflow Final storage

Unaccounted 

for

March 8 6,594

March 9–July 12 0 850

July 13–August 30 0 1,429

August 31–September 28 0 114

September 12 4,559

Total 6,594 0 2,393 4,559 357
PREWITT RESERVOIR 31

voir is classified as eutrophic. Carlson’s chlorophyll 

TSI values ranged from 48 to 53 in May, from 79 to 89 

in July, and from 78 to 83 in September (table 12), 

indicating that the algal biomass doubled approximately four times during the spring and summer. In 

July and September, Secchi-disk TSI values were 

lower than the chlorophyll and phosphorus TSI values, 

further evidence that the reservoir may have been 

light-limited during this period.

PREWITT RESERVOIR

Prewitt Reservoir is in Washington County, 

Colo., on the border with Logan County, about 5 mi 

east of Balzac (fig. 1). It is owned by Logan Irrigation 

District and also is operated as a State Wildlife Area 

by the Colorado Division of Wildlife. The reservoir 

has an estimated maximum storage capacity of 

28,840 acre-ft and receives most of its surface-water 

inflow from Prewitt Inlet Canal and the agriculture 

drain Lower Platte and Beaver Ditch (fig. 14), which 

originate in the South Platte River. Water released 

from the reservoir through Prewitt Outlet Canal is used 

primarily for irrigation; other uses include groundwater augmentation, recharge, and recreation. 

During the study period, reservoir storage varied 

from 24,976 acre-ft in March to 28,600 acre-ft in May 

and June, and to 20,174 acre-ft in September (table 13, 

fig. 15); maximum depth varied from about 17 ft in 

March to about 22 ft in June and to about 16 ft in 

September. The reservoir normally is filled during the 

Table 11. Concentrations of biologically available nitrogen 

and phosphorus and N:P values for Jackson Reservoir, 

March–September 1995

[<, less than; DIN, dissolved inorganic nitrogen; DOP, dissolved

orthophosphate; N:P, mass ratio of nitrogen to phosphorus; values of 

N:P calculated from total nitrogen and total phosphorus if DIN or DOP 

concentrations below detection limits]

Site

(figure 9)

Concentration,

in milligrams per liter N:P

DIN DOP

March 8

1 3.0 < 0.010 25

2 3.0 < 0.010 27

3 3.0 < 0.010 23

4 3.0 < 0.010 26

5 3.0 < 0.010 24

May 30

1 1.4 0.050 29

2 1.4 0.050 29

3 1.4 0.050 28

4 1.4 0.050 29

5 1.4 0.050 29

July 25

1 0.18 0.070 2.6

2 0.13 0.080 1.6

3 0.070 0.030 2.3

4 0.11 0.050 2.2

5 0.40 0.11 3.6

September 12

1 < 0.065 0.070 9.2

2 < 0.065 0.070 8.3

3 0.080 0.070 1.1

4 < 0.065 0.070 10

5 < 0.065 0.070 8.8

Table 12. Chlorophyll-a concentrations, Secchi-disk depths, total phosphorus concentrations, and associated 

trophic-state indices for Jackson Reservoir, March–September 1995

[μg/L, micrograms per liter; mg/L; milligrams per liter; TSI, Trophic State Index, Chl, chlorophyll-a; SD, Secchi disk; TP, total phosphorus]

Site

(figure 

9)

Chlorophyll-a

concentration

(μg/L)

TSI(Chl)

Secchi-disk 

depth

(feet)

TSI(SD)

Total phosphorus 

concentration

(mg/L)

TSI(TP)

May 30

2 2.5 49 6.5 50 0.09 69

4 4.1 53 6.0 51 0.14 75

5 2.4 48 4.5 55 0.12 73

July 25

2 150 89 2.0 67 0.20 81

4 150 89 2.0 67 0.27 85

5 53 79 2.0 67 0.21 81

September 12

2 72 82 1.5 71 0.20 81

4 84 83 2.0 67 0.19 80

5 50 78 1.0 77 0.20 81
32 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

South Platte River

Prewitt Outlet Canal

Prewitt Reservoir

Prewitt 

Reservoir

South Platte Study Area

5

N

Reservoir sampling site and number

Inflow/outflow sampling site

EXPLANATION 0

0 1 2 KILOMETERS

Base from U.S. Geological Survey and 1 2 MILES

U.S. Census Bureau digital data,

1974 to 1993

5

4

3 1

2

Prewitt Inlet Canal

Lower Platte and Beaver Ditch

Figure 14. Location of sampling sites in Prewitt Reservoir.
PREWITT RESERVOIR 33

0

5,000

10,000

15,000

20,000

25,000

30,000

RESERVOIR VOLUME, IN ACRE-FEET

RESERVOIR INFLOW OR OUTFLOW, IN 

ACRE-FEET PER DAY

0

200

400

600

800

1,000

Inflow 

Outflow 

End-of-month volume

March April May June July August September

1995

EXPLANATION

fall and winter, reaching maximum volumes in the 

spring. In 1995, the reservoir reached maximum 

storage in May and water was released for irrigation 

beginning in June. Outflow volumes were small relative to inflow volumes, and outflow fluctuated from 

June through September with demand for irrigation 

water. Correspondingly, storage volumes dropped only 

about 30 percent from June through September. 

Measurable residence times in the reservoir ranged 

from 953 days in June to 761 days in August as 

storage decreased (table 13).

Water-quality samples were collected at five 

sites throughout the reservoir (fig. 14) on March 9, 

June 6, August 1, and September 13, 1995. Samples 

for chlorophyll-a were collected only on the last three 

dates from sites 1, 2, and 4. Additional water-quality 

samples were collected from Prewitt Inlet Canal, 

Lower Platte and Beaver Ditch, and the Prewitt Outlet 

Canal (when flowing) on March 9, May 10, June 6, 

July 11, August 17, and September 13, 1995. Bottomsediment samples were collected from sites 1 and 2 on 

March 9 and August 17.

Water Temperature and Dissolved Oxygen

The temperature profile for March 9 indicates 

that the reservoir was well mixed at all sites in late 

winter; temperatures were approximately 2–3oC from 

surface to bottom (fig. 16). The reservoir was closer to 

Table 13. End-of-month storage, monthly median outflow, and 

residence time in Prewitt Reservoir, March–September 1995

[acre-ft, acre-feet; acre-ft/d, acre-feet per day; --, could not be calculated]

Month

End-ofmonth 

storage

(acre-ft)

Monthly 

median 

outflow

(acre-ft/d)

Residence

time

(days)

March 24,976 0 --

April 26,766 0 --

May 28,600 0 --

June 28,600 30 953

July 27,675 0 --

August 22,830 30 761

September 20,174 0 --

Figure 15. Inflow, outflow, and end-of-month storage volume in Prewitt Reservoir, March–September 1995.
34 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Median euphotic 

 zone depth

Site 1 

Site 2 

Site 3 

Site 4 

Site 5 

EXPLANATION

25

20

15

10

5

0

25

20

15

10

5

0

25

20

15

10

5

0

25

20

15

10

5

0

0 15 20 25 30 10 5 10 0 15 20 25 30 10 5 0 15 20 25 30 10 5 0 15 20 25 30 5

WATER TEMPERATURE, IN DEGREES CELSIUS

March 9 June 6 August 1 September 13

DEPTH, IN FEET

0 15 20 25 10 5 10 0 15 20 25 10 5 0 15 20 25 10 5 0 15 20 25 5

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

pH, IN STANDARD UNITS

0

0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14

500 1,000 1,500 0 500 1,000 1,500 0 500 1,000 1,500 0 500 1,000 1,500

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER

Figure 16. Depth profiles of water temperature, dissolved oxygen, pH, and specific conductance in Prewitt Reservoir, 

March–September 1995.
PREWITT RESERVOIR 35

5oC at site 5, where it was shallowest and therefore 

warmed more quickly. At these temperatures, very 

little wind energy would have been necessary to mix 

the water column (Wetzel, 1983). As spring 

progressed, all sites became weakly stratified due to 

increased solar heating of surface water and the inflow 

of warmer water from Prewitt Inlet Canal and Lower 

Platte and Beaver Ditch as overflow at the reservoir 

surface. By the beginning of August, the degree of 

stratification had increased slightly at all sites. Little 

inflow or outflow occurred during this period to 

disrupt the stratification. By September, the reservoir 

volume had dropped, evaporative heat losses had 

increased, and water temperatures had decreased. The 

shallow water was less stable, and all sites were well 

mixed and isothermal by late summer.

On March 9, the reservoir was well mixed and 

oxygen was uniformly distributed with depth at all five 

sites (fig. 16). Because oxygen solubility is inversely 

proportional to water temperature, dissolved oxygen 

concentrations during the study period were highest in 

March when temperatures were lowest. Supersaturation of dissolved oxygen at sites 3, 4, and 5 indicates 

some photosynthetic activity in close proximity to the 

nutrient-rich inflow water (Appendix). Reduced 

dissolved oxygen concentrations were measured in 

early June and August at all sites, likely due to biological decomposition of sinking organic matter. By 

September 13, all sites were shallower and well mixed, 

and oxygen once again was uniformly distributed with 

depth. 

pH and Specific Conductance

In Prewitt Reservoir, pH ranged seasonally 

between 7.7 and 9.1 (fig. 16). On all sampling dates, 

pH was constant from surface to bottom at all five 

sites. The lack of a pH difference between zones of 

photosynthesis and decomposition indicates that the 

system was well buffered by bicarbonate and 

carbonate, preventing large changes in pH, and that it 

likely was well mixed by wind. On all sampling dates, 

specific conductance also was constant from surface to 

bottom at all five sites (fig. 16). The maximum specific 

conductance measured during the study period was 

approximately 1,680 μS/cm on March 9. At this time, 

specific conductance was influenced primarily by 

inflow water; specific conductance was measured at 

1,960 μS/cm in Prewitt Inlet Canal that day. With 

smaller inflow volumes after that date, specific 

conductance decreased slightly to near 1,500 μS/cm 

during the remainder of the study period. Because of 

the high salinity, use of this water for irrigation could 

have been detrimental to crop quality and yield (Aryes 

and Westcot, 1976).

Nutrient Dynamics

Water-Column Concentrations

A statistical summary of nutrient concentrations 

at the five sites sampled in Prewitt Reservoir, 

including both euphotic and bottom samples, is 

provided in table 14. Median concentrations of total 

nitrogen during the study period ranged from 1.1 mg/L 

at site 2 to 1.6 mg/L at site 5, and median concentrations of total phosphorus during the study period 

ranged from 0.050 mg/L at site 2 to 0.080 mg/L at 

sites 1, 3, 4, and 5. A statistical analysis of nutrient 

concentrations using the Kruskal-Wallis test indicates 

that the differences in nutrient concentrations between 

the sites were not significant (α=0.05). Because the 

inflow and outflow volumes were small throughout 

most of the study period, there was little advective 

flow in the reservoir, resulting in minimal longitudinal 

gradients in concentration.

Samples collected near the reservoir surface and 

near the reservoir bottom show little difference 

between euphotic and bottom concentrations of 

nitrogen and phosphorus during most of the study 

period. Results of a Wilcoxon Rank-Sum test indicate 

that there were no statistically significant differences 

(α=0.05) between euphotic and bottom nutrient 

concentrations during the study period (table 15). The 

lack of variation with depth likely was because the 

reservoir never developed strong stratification to limit 

mixing between surface water and bottom water.

Although there was little longitudinal and 

vertical variability in nutrient concentrations, results of 

a Kruskal-Wallis test indicate that there was significant (α=0.05) temporal variability in total nitrogen and 

total phosphorus concentrations from March through 

September (fig. 17). Median concentrations of total 

nitrogen for all data combined decreased from 

3.9 mg/L in March to 1.3 mg/L in September. The 

decrease in total nitrogen was driven largely by a 

decrease in nitrate; median concentrations of nitrate 

dropped from 1.6 mg/L in March to below laboratory 
36 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Table 14. Statistical summary of nutrient concentrations at each site and results of Kruskal-Wallis test of site concentration 

differences in Prewitt Reservoir, March–September 1995

[All concentrations are in milligrams per liter; p-value represents the probability that site concentrations have identical distributions; differences not significant at p-values greater than 0.05]

Constituent Site

(figure 14)

Number of 

analyses Mean Standard deviation

Twentyfifth 

percentile

Median

Seventyfifth 

percentile

p-value

Total nitrogen 1 7 2.0 1.3 1.2 1.3 2.8

2 7 1.9 1.4 1.0 1.1 2.8

3 7 2.0 1.3 1.2 1.3 2.9

4 6 1.8 1.0 1.2 1.5 1.8

5 4 2.0 1.4 1.2 1.6 2.3 0.9550

Dissolved ammonia 1 7 0.11 0.19 0.015 0.015 0.080

2 7 0.11 0.18 0.015 0.015 0.095

3 7 0.093 0.19 0.015 0.015 0.035

4 6 0.11 0.18 0.015 0.043 0.070

5 4 0.10 0.18 0.015 0.015 0.10 0.9970

Dissolved nitrite 1 7 0.016 0.015 0.010 0.010 0.010

2 7 0.016 0.015 0.010 0.010 0.010

3 7 0.016 0.015 0.010 0.010 0.010

4 6 0.015 0.012 0.010 0.010 0.010

5 4 0.013 0.017 0.010 0.010 0.017 0.9998

Dissolved nitrate 1 7 0.57 0.74 0.075 0.11 1.0

2 7 0.59 0.76 0.075 0.12 1.1

3 7 0.56 0.72 0.070 0.11 1.0

4 6 0.40 0.65 0.57 0.11 0.35

5 4 0.51 0.67 0.085 0.27 0.69 0.9816

Total organic nitrogen 1 7 1.3 0.60 0.89 1.2 1.6

2 7 1.2 0.66 0.81 0.89 1.5

3 7 1.4 0.61 0.93 1.2 1.7

4 6 1.2 0.46 0.92 1.2 1.4

5 4 1.3 0.83 0.84 1.1 1.6 0.8911

Total phosphorus 1 7 0.079 0.040 0.055 0.080 0.11

2 7 0.066 0.044 0.040 0.050 0.095

3 7 0.076 0.036 0.050 0.080 0.10

4 6 0.072 0.031 0.047 0.080 0.090

5 4 0.083 0.046 0.060 0.080 0.10 0.9653

Dissolved orthophosphate 1 7 0.013 0.0076 0.010 0.010 0.010

2 7 0.013 0.0076 0.010 0.010 0.010

3 7 0.013 0.0076 0.010 0.010 0.010

4 6 0.013 0.0082 0.010 0.010 0.010

5 4 0.015 0.010 0.010 0.010 0.015 0.9999

Total organic phosphorus 1 7 0.066 0.041 0.035 0.070 0.095

2 7 0.053 0.047 0.020 0.040 0.085

3 7 0.063 0.038 0.030 0.070 0.090

4 6 0.058 0.033 0.033 0.060 0.080

5 4 0.067 0.049 0.035 0.060 0.093 0.9488
PREWITT RESERVOIR 37

detection levels by early September. During periods of 

inflow, reservoir water quality was influenced slightly 

by inflowing water from the South Platte River. 

Between March and June, concentrations of both 

nitrate and total nitrogen decreased overall at the 

South Platte River at Balzac (fig. 3), the nearest monitoring station to the Prewitt Reservoir intakes. These 

decreases in river concentrations contributed to a 

decrease in reservoir concentrations during that 

period, though the influence of the river water likely 

was minor compared to internal processes during April 

through June, as inflow volumes during that period 

were small compared to the volume in storage. After 

reservoir inflows had ended entirely in mid-July, 

continued biological activity resulted in assimilation 

of all remaining nitrate by September, and only total 

organic nitrogen was left in measurable quantities. 

Median concentrations of total phosphorus for 

all data combined were lowest in August, at 

0.03 mg/L, and highest in March, at 0.12 mg/L 

(fig. 17). The temporal pattern of total phosphorus 

concentrations in the reservoir during periods of 

inflow appeared to follow that of the South Platte 

River at Balzac (fig. 3), indicating that inflow loading 

influenced concentrations of phosphorus in the reservoir. However, as with nitrogen, the influence of the 

river water likely was minor compared to internal 

processes. After reservoir inflows had ended entirely 

in mid-July, biological activity continued, but concentrations increased by September due to internal recycling.

Table 15. Statistical summary of nutrient concentrations in euphotic and bottom samples and results of Wilcoxon Rank-Sum 

test of euphotic and bottom concentration differences in Prewitt Reservoir, March–September 1995

[e, euphotic; b, bottom; all concentrations are in milligrams per liter; p-value represents the probability that e and b concentrations have identical distributions; differences not significant at p-values greater than 0.05]

Constituent Location Number of 

analyses Mean Standard deviation

Twenty-fifth 

percentile Median

Seventyfifth percentile

p-value

Total nitrogen e 20 2.0 1.2 1.1 1.6 2.3

b 11 1.9 1.3 1.1 1.3 2.6 0.6045

Dissolved ammonia e 20 0.14 0.20 0.015 0.015 0.15

b 11 0.040 0.038 0.015 0.015 0.060 0.4750

Dissolved nitrite e 20 0.019 0.017 0.010 0.010 0.017

b 11 0.010 0.001 0.010 0.010 0.010 0.0811

Dissolved nitrate e 20 0.55 0.65 0.085 0.27 0.70

b 11 0.51 0.74 0.040 0.11 0.85 0.5429

Total organic nitrogen e 20 1.3 0.59 0.87 0.99 1.5

b 11 1.3 0.59 0.90 1.2 1.7 0.6491

Total phosphorus e 20 0.077 0.034 0.040 0.070 0.095

b 11 0.070 0.044 0.030 0.080 0.11 0.6779

Dissolved orthophosphate e 20 0.015 0.0089 0.010 0.010 0.015

b 11 0.010 0.001 0.010 0.010 0.010 0.0803

Total organic phosphorus e 20 0.061 0.037 0.030 0.050 0.085

b 11 0.060 0.044 0.020 0.070 0.095 0.8355
38 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

EXPLANATION

Outlier data value greater than 1.5

times the interquartile range

outside the quartile

Data value less than or equal

to 1.5 times the interquartile range

outside the quartile

75th percentile

25th percentile

Median

CONCENTRATION, IN MILLIGRAMS PER 

LITER AS NITROGEN

CONCENTRATION, IN MILLIGRAMS PER 

LITER AS PHOSPHORUS

TP OP OP TOrgP TP OP TOrgP TP OP TOrgP TP TOrgP

0.2

0

0.4

0.6

0.8

NH3NO2 NO3 TN TOrgN NH3 NO2 TN TOrgN NH NO3 3 NO2 NO3 TN TOrgN NH3 NO2 TN TOrgN NO3

March 9 June 6 August 1 September 13

0

1

2

3

4

5

6

7

NH3—Dissolved ammonia

NO2—Dissolved nitrite

NO3—Dissolved nitrate

TN—Total nitrogen

TOrgN—Total organic nitrogen

TOrgP—Total organic phosphorus

OP—Dissolved orthophosphate

TP—Total phosphorus

Figure 17. Nitrogen and phosphorus concentrations, euphotic and bottom samples 

combined in Prewitt Reservoir, March–September 1995.
PREWITT RESERVOIR 39

DISSOLVED INORGANIC NITROGEN CONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED ORTHOPHOSPHATE CONCENTRATION, IN MILLIGRAMS PER LITER

5

0

0 2 4 6 8 10 0 2 4 6 8 10

10 20 30 40 50 60 0 10 20 30 40 50 60

4

3

2

1

0

1

5

4

3

2

1

0

1

August 17

August 17

March 9 

March 9 

Sediment/Water interface

Site 1 measured concentrations 

Site 2 measured concentrations

EXPLANATION

DISTANCE ABOVE AND BELOW SEDIMENT/WATER

 INTERFACE, IN CENTIMETERS

Bottom-Sediment Flux

Estimates of nutrient flux from the bottom sediment indicate that the sediment was, at times, a potential source of nutrients to the reservoir. On March 9, 

concentrations of dissolved inorganic nitrogen 

measured in the first centimeter of the sediment were 

higher than in the overlying water column at sites 1 

and 2 (fig. 18). The estimated flux rates for inorganic 

nitrogen were 35 mg/m2/d at site 1 and 88 mg/m2/d at 

site 2. Concentrations of dissolved orthophosphate 

measured in the first centimeter of the sediment also 

were higher than in the overlying water column at 

sites 1 and 2 on March 21. The estimated flux rate 

for orthophosphate was 1.7 mg/m2

/d at site 1 and 

4.3 mg/m2/d at site 2. On August 17, concentrations of 

dissolved inorganic nitrogen measured in the first 

centimeter of the sediment were higher than in the 

overlying water column at sites 1 and 2. The estimated 

flux rates for inorganic nitrogen were 142 mg/m2

/d at 

site 1 and 116 mg/m2/d at site 2. Concentrations of 

dissolved orthophosphate measured in the first centimeter of the sediment also were higher than in the 

overlying water column at sites 1 and 2 on August 17. 

The estimated flux rates for orthophosphate were 

9.9 mg/m2

/d at site 1 and 10 mg/m2/d at site 2. The 

bottom dissolved oxygen concentrations at site 1 and 

site 2 on March 9 were close to 15 mg/L; concentrations on August 17 fell somewhere between 5 mg/L, 

the minimum measured on August 1, and 7 mg/L, 

measured on September 13. Because bottom sediment 

release is redox-dependent, movement from the sediment into the overlying water column is restricted if 

the sediment/water interface is well oxygenated 

(Horne and Goldman, 1994). Therefore, these flux 

calculations may overestimate the movement of nutrients from bottom sediment to the water column.

Figure 18. Concentrations of dissolved inorganic nitrogen and orthophosphate in bottom-sediment pore water and the overlying water column in Prewitt Reservoir, March 9 and August 17, 1995.
40 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Mass Balance

Mass-balance calculations for nitrogen indicate 

that about 88 percent of the total mass of nitrogen—

the mass that had been in storage in March plus the 

mass that had entered in inflows through September—

was trapped in the reservoir (represented by the 

“unaccounted for” term), while less than 1 percent 

was released through the outflow (table 16). Only 

12 percent of the mass remained in the water column 

by September. Mass-balance calculations for phosphorus indicate that about 86 percent of the total mass 

of phosphorus was trapped in the reservoir, while less 

than 1 percent was released through the outflow. Only 

14 percent of the mass remained in the water column 

by September. 

Much of the nitrogen was trapped in the reservoir through biological uptake and deposition of 

particulate organic nitrogen onto bottom sediment. 

Though phosphorus likely was internally recycled due 

to processes such as excretion from zooplankton and 

fish, decay of aquatic plants and animals, and release 

from bottom sediment, much of the total mass was 

trapped due to sedimentation. The residence time of 

water in Prewitt Reservoir was on the order of 2 to 

3 years, and sedimentary losses of phosphorus and 

nitrogen typically are higher within reservoirs with 

longer residence times than in more rapidly flushed 

reservoirs (Kennedy and Walker, 1990). Additionally, 

because little outflow occurred during the study 

period, the mass lost in outflowing water was very 

small. Overall, a substantial portion of the total 

nutrient mass was trapped in the reservoir during the 

study period, and the reservoir acted as a sink for both 

nitrogen and phosphorus. 

Algal Growth

The nutrient dynamics in the reservoir may have 

led to a change in the nutrients limiting algal growth. 

In March, August, and September, dissolved orthophosphate concentrations were below the algal 

growth-limiting concentration of 0.010 mg/L 

(table 17), indicating that phosphorus was potentially 

the limiting nutrient in those months. In September, 

concentrations of dissolved inorganic nitrogen species 

Table 16. Mass-balance calculations for total nitrogen and total phosphorus in Prewitt Reservoir, March–September 1995

[From equation 1: Unaccounted for mass = Final storage mass – Initial storage mass – Inflow mass + Outflow mass]

Date

Total nitrogen mass, in kilograms

Initial storage Inflow Outflow Final storage Unaccounted 

for

March 9 120,122

March 10–May 10 115,347 0

May 11–June 6 9,355 0

June 7–July 11 11,324 0

July 12–August 17 821 784

August 18–September 13 0 0

September 11 31,118

Total 120,122 136,847 784 31,118 –225,067

Date

Total phosphorus mass, in kilograms

Initial storage Inflow Outflow Final storage Unaccounted

for

March 9 3,698

March 10–May 10 7,263 0

May 11–June 6 985 0

June 7–July 11 2,427 0

July 12–August 17 155 50

August 18–September 13 0 0

September 11 1,992

Total 3,698 10,829 50 1,992 –12,487
PREWITT RESERVOIR 41

and dissolved orthophosphate were all below the 

detection limit, suggesting that both phosphorus and 

nitrogen were limiting by the end of the study period. 

Values of N:P indicate that phosphorus was the 

limiting nutrient throughout the entire study period. In 

March, N:P values ranged from 29 to 34; in June, 

N:P values ranged from 28 to 33; in August, N:P 

values ranged from 23 to 27; and in September, 

N:P values ranged from 15 to 16. Because blue-green 

algae are less common when N:P values exceed 29 

(Ryding and Rast, 1989), green algae likely predominated over blue-green algae until September.

Although chlorophyll-a measurements cannot 

provide information on the composition of the 

phytoplankton community, they can provide a measure 

of the quantity of algae present. Chlorophyll-a concentrations measured in the euphotic zone in June ranged 

from 1.2 μg/L at site 2 to 1.6 μg/L at sites 1 and 4 

(table 18). By early August, chlorophyll-a concentrations ranged from 17 μg/L at site 2 to 23 μg/L at sites 

1 and 4. By September, chlorophyll-a concentrations 

ranged from 35 μg/L at site 1 to 40 μg/L at site 2. 

Based on mean chlorophyll-a concentrations, Prewitt 

Reservoir is classified as eutrophic. Carlson’s chlorophyll TSI values ranged from 41 to 44 in June, from 67 

to 70 in August, and from 74 to 76 in September (table 

18), indicating that the algal biomass doubled approximately three times during the spring and summer.

Table 17. Concentrations of biologically available nitrogen 

and phosphorus and N:P values for Prewitt Reservoir, 

March–September 1995

[<, less than; --, missing data; DIN, dissolved inorganic nitrogen; DOP, 

dissolved orthophosphate; N:P, mass ratio of nitrogen to phosphorus; 

values of N:P calculated from total nitrogen and total phosphorus if DIN or 

DOP concentrations below detection limits]

Site

(figure 14)

Concentration, 

in milligrams per liter N:P

DIN DOP

March 9

1 1.6 < 0.010 31

2 1.7 < 0.010 31

3 1.6 < 0.010 32

4 1.7 < 0.010 34

5 1.5 < 0.010 29

June 6

1 0.99 < 0.030 33

2 1.0 < 0.030 33

3 1.0 < 0.030 33

4 0.94 < 0.030 31

5 0.85 < 0.030 28

August 1

1 0.19 < 0.010 25

2 0.20 < 0.010 25

3 0.13 < 0.010 27

4 0.20 < 0.010 23

5 0.13 < 0.010 24

September 13

1 <0.065 < 0.010 16

2 <0.065 < 0.010 15

3 <0.065 < 0.010 16

4 <0.065 < 0.010 16

5 <0.065 < 0.010 15

Table 18. Chlorophyll-a concentrations, Secchi-disk depths, total phosphorus concentrations, and associated trophic-state 

indices for Prewitt Reservoir, March–September 1995

[μg/L, micrograms per liter; mg/L; milligrams per liter; TSI, Trophic State Index, Chl, chlorophyll-a; SD, Secchi disk; TP, total phosphorus]

Site

(figure 

14)

Chlorophyll-a

concentration

(μg/L)

TSI(Chl)

Secchi-disk

depth

(feet)

TSI(SD)

Total

phosphorus 

concentration

(mg/L)

TSI(TP)

June 6

1 1.6 44 7.5 48 0.070 65

2 1.2 41 10 44 0.040 57

4 1.6 44 4.5 55 0.070 65

August 1

1 23 70 3.0 61 0.040 57

2 17 67 3.0 61 0.040 57

4 23 70 2.5 64 0.040 57

September 13

1 35 74 2.0 67 0.080 67

2 40 76 2.0 67 0.070 65

4 37 75 2.0 67 0.090 69
42 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

NORTH STERLING RESERVOIR

North Sterling Reservoir is in Logan County, 

Colo., approximately 10 mi northwest of Sterling 

(fig. 1). It is owned by North Sterling Irrigation 

District and also is operated as a State Park by the 

Colorado Division of Parks and Outdoor Recreation. 

The reservoir has an estimated maximum storage 

capacity of 74,010 acre-ft and receives most of its 

surface-water inflow from North Sterling Canal 

(fig. 19), which originates in the South Platte River 

upstream from Sterling. Additional surface water 

enters the reservoir during storms through Cedar 

Creek, an ephemeral stream on the northwest side of 

the reservoir. Water released from the reservoir 

through North Sterling Outlet Canal is used primarily 

for irrigation; other uses include recreation.

During the study period, reservoir storage 

ranged from 70,883 acre-ft in March to 9,820 acre-ft 

in September (fig. 20 and table 19) and maximum 

depth ranged from about 52 ft in March to about 24 ft 

in September. The reservoir normally is filled during 

the fall and winter, reaching maximum volumes in the 

early spring. In 1995, the reservoir reached maximum 

storage in April, and water was released for irrigation 

beginning in May. Most of the outflow occurred from 

July through September as demand for irrigation water 

increased; maximum outflow occurred during July. 

Correspondingly, storage volumes dropped sharply 

from July through September. Measurable residence 

times in the reservoir ranged from 47 days in July to 

26 days in September as outflow increased and storage 

decreased (table 19).

Water-quality samples were collected at six sites 

throughout the reservoir (fig. 19) on March 10, June 1, 

July 27, and September 14, 1995 (site 6 could not be 

sampled on September 14 due to lack of water). 

Samples for chlorophyll-a were collected only on the 

last three dates from sites 1, 2, and 3. Additional 

water-quality samples were collected from North 

Sterling Canal and North Sterling Outlet (when 

flowing) on March 10, May 11, July 11, August 16, 

and September 14, 1995. Bottom-sediment samples 

were collected from sites 1 and 2 on March 21 and 

August 16, 1995.

Water Temperature and Dissolved Oxygen

The temperature profile for March 10 indicates 

that the reservoir was well mixed at all sites in the late 

winter, with the exception of slight surface warming at 

site 6 (fig. 21). At temperatures near 3oC from surface 

to bottom, very little wind energy would have been 

necessary to mix the water column. As spring 

progressed, all sites became weakly stratified due to 

increased solar heating of surface water and the inflow 

of warmer water from North Sterling Canal as overflow at the reservoir surface. By late July, the deeper 

sites (1, 2, 4, 5) were strongly stratified due to heating 

and density differences with depth. Although the 

surrounding terrain is relatively flat and strong winds 

are common, the reservoir likely was deep enough 

through August that there was insufficient energy to 

overcome stratification. Removal of a large volume of 

water and increased evaporative cooling during the 

period from July 27 to September 14 resulted in a 

weaker temperature and density gradient between 

surface water and bottom water by mid-September, 

increasing mixing and reducing the degree of stratification at all sites. As a result, all sites were well mixed 

and isothermal by late summer.

Dissolved oxygen concentrations during the 

study period were highest in March when water 

temperatures were lowest. On March 10, the reservoir 

was well mixed and oxygen was uniformly distributed 

with depth at all sites except site 6 (fig. 21). Supersaturation of dissolved oxygen at site 6 indicated photosynthetic activity in the shallow southern arm of the 

reservoir; enhanced primary productivity often occurs 

early in the growing season in reservoir embayments 

isolated from flow perturbations (Kimmel and others, 

1990). Reduced dissolved oxygen concentrations first 

were observed in early June at all sites due to biological oxidation of sinking organic matter. Oxygen in the 

hypolimnion became progressively more depleted and 

reached anoxic levels by the end of July as more 

organic matter sank to bottom depths. Summer 

hypolimnetic oxygen in highly productive lakes 

commonly is depleted after only a few weeks of stratification (Wetzel, 1983). By September, the reservoir 

was shallow and well mixed, though slightly reduced 

oxygen conditions remained in the bottom water, 

where the rate of consumptive metabolism continued 

to exceed photosynthetic inputs at the surface.
NORTH STERLING RESERVOIR 43

3

4

6

2

5

1

Sterling

North

Reservoir

3

North

Outlet Canal

Sterling

Sterling

Reservoir

South Platte Study Area

N

Cedar Creek

Cedar Creek

North Sterling Canal

North

Reservoir sampling site and number

Inflow/outflow sampling site

EXPLANATION 0

0 1 2 KILOMETERS

1 2 MILES Base from U.S. Geological Survey and

U.S. Census Bureau digital data,

1974 to 1993

Figure 19. Location of sampling sites in North Sterling Reservoir.
44 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Inflow 

Outflow

End-of-month volume

March April May June July August September

1995

EXPLANATION

RESERVOIR INFLOW OR OUTFLOW, IN 

ACRE-FEET PER DAY

0

500

1,000

1,500

0

20,000

40,000

60,000

80,000

RESERVOIR VOLUME, IN ACRE-FEET

pH and Specific Conductance

In North Sterling Reservoir, pH ranged seasonally between 7.4 and 8.6 (fig. 21). On March 10 and 

June 1, pH was constant from surface to bottom at all 

six sites. A slight gradient in pH values was present on 

July 27 and September 14 due to photosynthesis, 

which tends to increase pH near the surface, and 

decomposition, which tends to decrease pH near the 

bottom, though these effects were buffered by bicarbonate and carbonate. On all sampling dates, specific 

conductance was constant from surface to bottom at all 

six sites (fig. 21). The maximum specific conductance 

measured during the study period was approximately 

1,780 μS/cm on March 10. At this time, specific 

conductance was influenced primarily by inflow 

water; specific conductance was measured at 

1,827 μS/cm in North Sterling Canal that day. After 

inflows ended in late May, specific conductance 

decreased slightly. Because of the high salinity, use of 

this water for irrigation could have been detrimental to 

crop quality and yield.

Figure 20. Inflow, outflow, and end-of-month storage volume in North Sterling Reservoir, March–September 1995.

Table 19. End-of-month storage, monthly median 

outflow, and residence time in North Sterling 

Reservoir, March–September 1995

[acre-ft, acre-feet; acre-ft/d, acre-feet per day; --, could not be 

calculated]

Month

End-ofmonth 

storage

(acre-ft)

Monthly 

median 

outflow

(acre-ft/d)

Residence 

time

(days)

March 70,883 0 --

April 74,876 0 --

May 73,300 0 --

June 74,010 0 --

July 47,893 1,010 47

August 19,440 890 22

September 9,820 385 26
NORTH STERLING RESERVOIR 45

DEPTH, IN FEET

60

50

40

30

20

10

0

60

50

40

30

20

10

0

60

50

40

30

20

10

0

60

50

40

30

20

10

0

0 15 20 25 10 5 10 0 15 20 25 10 5 0 15 20 25 5

0 15 20 25 10 5 10 0 15 20 25 10 5 0 15 20 25 10 5 0 15 20 25 5

0 15 20 25 5 10

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

WATER TEMPERATURE, IN DEGREES CELSIUS

March 10 June 1 July 27 September 14

Median euphotic 

 zone depth

Site 1 

Site 2 

Site 3 

Site 4 

Site 5 

Site 6 

EXPLANATION

pH, IN STANDARD UNITS

0

0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14

500 1,000 1,500 0 500 1,000 1,500 0 500 1,000 1,500 0 500 1,000 1,500

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER

Figure 21. Depth profiles of water temperature, dissolved oxygen, pH, and specific conductance in North Sterling Reservoir, March–September 1995.
46 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Nutrient Dynamics

Water-Column Concentrations

A statistical summary of nutrient concentrations 

at the six sites sampled in North Sterling Reservoir, 

including both euphotic and bottom samples, is 

provided in table 20. Median concentrations of total 

nitrogen during the study period ranged from 2.7 mg/L 

at sites 2 and 4 to 3.5 mg/L at site 6. Median concentrations of total phosphorus during the study period 

ranged from 0.060 mg/L at site 3 to 0.15 mg/L at 

sites 1 and 2. A statistical analysis of nutrient concentrations using the Kruskal-Wallis test indicates that the 

differences in nutrient concentrations between the sites 

were not significant (α=0.05). Because the outflow 

volumes were large, residence times were short, 

resulting in a minimal longitudinal gradient.

Observed concentrations for all samples 

collected near the reservoir surface and near the reservoir bottom were significantly different for some 

constituents during the study period (table 21). Results 

of a Wilcoxon Rank-Sum test indicate that concentrations of nutrient species that are most readily available 

to biota—dissolved orthophosphate and dissolved 

ammonia—were significantly lower (α=0.05) near the 

surface due to biological activity in the euphotic zone 

and release from bottom sediment. Additionally, 

concentrations of total organic nitrogen were significantly higher near the surface, likely due to assimilation in plankton and secretion by aquatic plants and 

animals present in the euphotic zone. The variation 

with depth likely was because the reservoir developed 

strong stratification to limit mixing between surface 

water and bottom water during the summer.

In addition to the vertical variability in nutrient 

concentrations, results of a Kruskal-Wallis test indicate that there was significant (α=0.05) temporal variability in total nitrogen and total phosphorus 

concentrations from March through September 

(fig. 22). Median concentrations of total nitrogen in 

the euphotic zone decreased from 4.9 mg/L in 

March to 1.7 mg/L in September; median concentrations of total nitrogen near the bottom decreased from 

4.8 mg/L in March to 1.4 mg/L in September. The 

decrease in total nitrogen throughout the water column 

was driven largely by a decrease in nitrate; median 

concentrations of nitrate dropped from 3.8 mg/L in 

March to below laboratory detection levels by 

September in the euphotic zone and near the bottom.

Reservoir water quality was influenced by 

conditions in the South Platte River during periods of 

inflow, particularly in the euphotic zone, as warmer 

river water entered as overflow. In March and April, 

concentrations of both nitrate and total nitrogen 

decreased at the South Platte River at Balzac (fig. 3), 

the monitoring station nearest to the North Sterling 

Canal intake. The decreases in river concentrations 

contributed to a corresponding decrease in reservoir 

concentrations during March and April. However, 

concentrations of total nitrogen at Balzac remained 

level in May, whereas concentrations in the reservoir 

continued to decrease. This suggests that biological 

activity also was contributing to the decline in nitrogen 

concentrations and that the rate of nitrate assimilation 

by algae had exceeded the rate of nitrate inflow by 

May. After reservoir inflows had ended entirely at the 

end of May, continued biological activity resulted in 

assimilation of all remaining nitrate, and only total 

organic nitrogen was left in measurable quantities in 

the euphotic zone. Additionally, ammonia was present 

in the bottom water in the June, July, and September 

samples, likely due to release from the bottom sediment under anoxic conditions. Ammonia in the 

hypolimnion may have reached the epilimnion during 

storm-induced turbulence near the top of the thermocline or during hypolimnetic entrainment (Horne and 

Goldman, 1994) and thereby been available for biological uptake or nitrification in the euphotic zone.

Concentrations of total phosphorus did not have 

the same consistent decrease during the study period 

as concentrations of total nitrogen. Median concentrations of total phosphorus in the euphotic zone ranged 

from 0.15 mg/L in March to 0.18 mg/L in September; 

median concentrations of total phosphorus near the 

bottom ranged from 0.13 mg/L in March to 0.17 mg/L 

in September (fig. 22). The temporal pattern of total 

phosphorus concentrations in the euphotic zone from 

March through May appeared to follow that of the 

South Platte River at Balzac (fig. 3), indicating that 

inflow loading influenced concentrations of phosphorus in the reservoir. After reservoir inflows ceased 

at the end of May, biological activity continued, but 

concentrations increased in both euphotic and bottom 

water by September due to internal recycling.

Bottom-Sediment Flux

Estimates of nutrient flux from the bottom sediment indicate that the sediments were, at times, a 
NORTH STERLING RESERVOIR 47

Table 20. Statistical summary of nutrient concentrations at each site and results of Kruskal-Wallis test of site concentration 

differences in North Sterling Reservoir, March–September 1995

[All concentrations are in milligrams per liter; p-value represents the probability that site concentrations have identical distributions; differences not significant at p-values greater than 0.05]

Constituent Site

(figure 19)

Number of 

analyses Mean Standard deviation

Twenty-fifth 

percentile Median

Seventyfifth 

percentile

p-value

Total nitrogen 1 8 3.0 1.5 1.9 2.9 4.1

2 8 3.0 1.3 2.1 2.7 3.7

3 7 3.1 1.3 2.1 3.0 3.9

4 8 2.9 1.4 2.0 2.7 3.6

5 7 3.2 1.2 2.1 3.1 4.1

6 6 3.3 1.1 2.4 3.5 4.1 0.9980

Dissolved ammonia 1 8 0.39 0.63 0.037 0.19 0.35

2 8 0.21 0.22 0.045 0.16 0.24

3 7 0.12 0.12 0.017 0.050 0.21

4 8 0.17 0.20 0.037 0.070 0.25

5 7 0.099 0.097 0.020 0.070 0.15

6 6 0.073 0.085 0.023 0.040 0.073 0.4826

Dissolved nitrite 1 8 0.045 0.026 0.033 0.045 0.063

2 8 0.053 0.035 0.033 0.050 0.073

3 7 0.054 0.032 0.030 0.060 0.080

4 8 0.055 0.036 0.033 0.050 0.090

5 7 0.056 0.029 0.040 0.060 0.075

6 6 0.058 0.037 0.030 0.065 0.085 0.9665

Dissolved nitrate 1 8 1.6 1.6 0.0 1.5 2.6

2 8 1.7 1.5 0.6 1.6 2.5

3 7 1.9 1.4 1.0 2.0 2.9

4 8 1.8 1.5 0.7 1.6 2.6

5 7 1.9 1.4 0.87 2.0 2.9

6 6 2.0 0.95 1.2 2.0 2.6 0.9995

Total organic nitrogen 1 8 0.98 0.34 0.82 0.94 1.1

2 8 0.99 0.24 0.86 1.0 1.1

3 7 0.97 0.31 0.79 0.95 1.0

4 8 0.97 0.33 0.80 0.92 1.0

5 7 1.2 0.43 0.93 0.98 1.2

6 6 1.1 0.43 0.85 0.97 1.4 0.8813

Total phosphorus 1 8 0.18 0.17 0.12 0.15 0.16

2 8 0.12 0.060 0.058 0.15 0.17

3 7 0.090 0.058 0.045 0.060 0.14

4 8 0.11 0.055 0.073 0.11 0.16

5 7 0.11 0.066 0.070 0.090 0.14

6 6 0.080 0.028 0.055 0.080 0.098 0.4536
48 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Constituent Site

(figure 19)

Number of 

analyses Mean Standard deviation

Twenty-fifth 

percentile Median

Seventyfifth 

percentile

p-value

Dissolved orthophosphate 1 8 0.11 0.21 0.018 0.025 0.063

2 8 0.039 0.043 0.010 0.015 0.060

3 7 0.023 0.019 0.010 0.010 0.035

4 8 0.030 0.023 0.010 0.020 0.053

5 7 0.020 0.017 0.010 0.010 0.025

6 6 0.013 0.0082 0.010 0.010 0.010 0.8800

Total organic phosphorus 1 8 0.074 0.072 0.038 0.080 0.14

2 8 0.083 0.059 0.030 0.070 0.11

3 7 0.067 0.051 0.035 0.050 0.090

4 8 0.084 0.057 0.040 0.065 0.11

5 7 0.094 0.064 0.060 0.080 0.095

6 6 0.067 0.027 0.045 0.065 0.078 0.9702

Table 20. Statistical summary of nutrient concentrations at each site and results of Kruskal-Wallis test of site concentration 

differences in North Sterling Reservoir, March–September 1995—Continued

[All concentrations are in milligrams per liter; p-value represents the probability that site concentrations have identical distributions; differences not significant at p-values greater than 0.05]

Table 21. Statistical summary of nutrient concentrations in euphotic and bottom samples and results of Wilcoxon Rank-Sum 

test of euphotic and bottom concentration differences in North Sterling Reservoir, March–September 1995

[e, euphotic; b, bottom; all concentrations are in milligrams per liter; p-value represents the probability that e and b concentrations have identical distributions; differences not significant at p-values greater than 0.05]

Constituent Location Number of 

analyses Mean Standard deviation

Twentyfifth 

percentile

Median

Seventyfifth 

percentile

p-value

Total nitrogen e 23 3.1 1.2 2.1 3.1 4.1

b 21 3.1 1.3 2.0 3.2 4.3 0.9437

Dissolved ammonia e 23 0.073 0.079 0.025 0.050 0.080

b 21 0.31 0.41 0.040 0.22 0.34 0.0031

Dissolved nitrite e 23 0.050 0.030 0.020 0.060 0.070

b 21 0.057 0.031 0.040 0.060 0.090 0.3921

Dissolved nitrate e 23 1.7 1.3 0.94 1.8 2.5

b 21 1.9 1.4 0.86 2.0 3.3 0.9155

Total organic nitrogen e 23 1.2 0.36 0.94 1.1 1.5

b 21 0.86 0.20 0.76 0.82 0.98 0.0017

Total phosphorus e 23 0.11 0.059 0.055 0.12 0.15

b 21 0.13 0.11 0.060 0.10 0.15 0.9906

Dissolved orthophosphate e 23 0.020 0.019 0.010 0.010 0.015

b 21 0.062 0.13 0.010 0.030 0.050 0.0151

Total organic phosphorus e 23 0.092 0.055 0.045 0.080 0.13

b 21 0.064 0.053 0.030 0.060 0.080 0.1138
NORTH STERLING RESERVOIR 49

EXPLANATION

Outlier data value greater than 1.5

times the interquartile range

outside the quartile

Data value less than or equal

to 1.5 times the interquartile range

outside the quartile

75th percentile

25th percentile

Median

0

1

2

3

4

5

6

7

CONCENTRATION, IN MILLIGRAMS PER LITER 

AS NITROGEN

March 10 June 1 July 27 September 14

NH3 NO2 NO3 TN TOrgN NH3 NO2 TN TOrgN NH NO3 3 NO2 NO3 TN TOrgN NH3 NO2 TN TOrgN NO3

0

1

2

3

4

5

6

7

CONCENTRATION, IN MILLIGRAMS PER 

LITER AS NITROGEN

NH3 NO2 NO3 TN TOrgN NH3 NO2 TN TOrgN NH NO3 3 NO2 NO3 TN TOrgN NH3 NO2 TN TOrgN NO3

EUPHOTIC SAMPLES

BOTTOM SAMPLES

NH3—Dissolved ammonia

NO2—Dissolved nitrite

NO3—Dissolved nitrate

TN—Total nitrogen

TOrgN—Total organic nitrogen

Figure 22. Nitrogen and phosphorus concentrations in euphotic and bottom samples in 

North Sterling Reservoir, March–September 1995.
50 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

EXPLANATION

Outlier data value greater than 1.5

times the interquartile range

outside the quartile

Data value less than or equal

to 1.5 times the interquartile range

outside the quartile

75th percentile

25th percentile

Median

0

0.2

0.4

0.6

0.8

CONCENTRATION, IN MILLIGRAMS PER 

LITER AS PHOSPHORUS

TP OP TOrgP OP TOrgP TP OP TOrgP TP OP TOrgP TP

CONCENTRATION, IN MILLIGRAMS PER 

LITER AS PHOSPHORUS

0

0.2

0.4

0.6

0.8 March 10 June 1 July 27 September 14

TP OP TOrgP OP TOrgP TP OP TOrgP TP OP TOrgP TP

EUPHOTIC SAMPLES

BOTTOM SAMPLES

TOrgP—Total organic phosphorus

OP—Dissolved orthophosphate

TP—Total phosphorus

Figure 22. Nitrogen and phosphorus concentrations in euphotic and bottom samples in 

North Sterling Reservoir, March–September 1995—Continued
NORTH STERLING RESERVOIR 51

5

4

3

2

1

0

1

5

4

3

2

1

0

1

DISSOLVED ORTHOPHOSPHATE CONCENTRATION, IN MILLIGRAMS PER LITER

0 15 20 25 10 5 10 0 15 20 5

0 15 20 25 20 5 10 0 30 40 10

DISSOLVED INORGANIC NITROGEN CONCENTRATION, IN MILLIGRAMS PER LITER

August 16

March 21 August 16

March 21

Sediment/Water interface

Site 1 measured concentrations 

Site 2 measured concentrations 

EXPLANATION

DISTANCE ABOVE AND BELOW SEDIMENT/WATER

INTERFACE, IN CENTIMETERS

potential source of nutrients to the reservoir. On March 

21, concentrations of dissolved inorganic nitrogen 

measured in the first centimeter of the sediment were 

lower than in the overlying water column at sites 1 and 

2 (fig. 23). The estimated flux rates for inorganic 

nitrogen were –17 mg/m2/d at site 1 and –8.8 mg/m2/d 

at site 2 (both into the sediment). Concentrations of 

dissolved orthophosphate measured in the first centimeter of the sediment were similar to those in the 

overlying water column at site 1 and higher than in the 

overlying water column at site 2 on March 21. The 

estimated flux rate for orthophosphate was 0 mg/m2

/d 

at site 1 and 1.9 mg/m2/d at site 2. On August 16, 

concentrations of dissolved inorganic nitrogen 

measured in the first centimeter of the sediment were 

higher than in the overlying water column at sites 1 

and 2. The estimated flux rates for inorganic nitrogen 

were 80 mg/m2/d at site 1 and 53 mg/m2/d at site 2. 

Concentrations of dissolved orthophosphate measured 

in the first centimeter of the sediment also were higher 

than in the overlying water column at sites 1 and 2 on 

August 16. The estimated flux rates for orthophosphate were 28 mg/m2/d at site 1 and 23 mg/m2/d at site 

2. The bottom dissolved oxygen concentrations at site 

1 and site 2 on March 10 were close to 13 mg/L; 

concentrations on August 17 fell somewhere between 

0 mg/L, the minimum measured on July 27, and 

3 mg/L, measured on September 14. Under the 

oxygenated conditions in March, the flux of dissolved 

inorganic nitrogen was into the sediments and the rate 

Figure 23. Concentrations of dissolved inorganic nitrogen and orthophosphate in bottom-sediment pore water and the overlying water column in North Sterling Reservoir, March 21 and August 16, 1995.
52 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

of dissolved orthophosphate flux into the overlying 

water column was small. Under the anoxic conditions 

in mid-August, the flux of both constituents was into 

the overlying water column, and the rates had 

increased.

Mass Balance

Mass-balance calculations for nitrogen indicate 

that about 59 percent of the total mass of nitrogen—

the mass that had been in storage in March plus the 

mass that had entered in inflows through September —

was trapped in the reservoir (represented by the “unaccounted for” term), while 37 percent was released 

through the outflow (table 22). Only 4 percent of the 

mass remained in the water column by September. 

Mass-balance calculations for phosphorus indicate that 

about 42 percent of the total mass of phosphorus was 

trapped in the reservoir, while 47 percent was released 

through the outflow. Only about 12 percent of the 

mass remained in the water column by September. 

Much of the nitrogen was trapped in the reservoir 

through biological uptake and deposition of particulate 

organic nitrogen onto bottom sediments. Much of the 

phosphorus was trapped through sedimentation of 

orthophosphate adsorbed to inorganic particles, deposition of particulate organic phosphorus onto bottom 

sediment, and biological uptake. From July through 

September, the reservoir had short residence times, 

and some of the nitrogen and recycled phosphorus still 

in the water column was flushed out. Overall, a 

substantial portion of the total nutrient mass was 

trapped in the reservoir during the study period, and 

the reservoir acted as a sink for both nitrogen and 

phosphorus. 

Algal Growth

In June, July, and September, dissolved orthophosphate concentrations were below the algal 

growth-limiting concentration of 0.010 mg/L at most 

sites (table 23), indicating that phosphorus was potentially the limiting nutrient in those months. In 

September, concentrations of dissolved inorganic 

nitrogen were below the detection limit at some sites 

as well, suggesting that both phosphorus and nitrogen 

were limiting by the end of the study period. Values of 

N:P indicate that phosphorus was the limiting nutrient 

throughout the entire study period. In March, 

N:P values ranged from 37 to 90; in June, N:P values 

ranged from 26 to 80; in July, N:P values ranged from 

30 to 46; and in September, N:P values ranged from 

Table 22. Mass-balance calculations for total nitrogen and total phosphorus in North Sterling Reservoir, March–September 

1995

[From equation 1: Unaccounted for mass = Final storage mass – Initial storage mass – Inflow mass + Outflow mass]

Date

Total nitrogen mass, in kilograms

Initial storage Inflow Outflow Final storage Unaccounted 

for

March 10 424,228

March 11–May 11 62,824 18,401

May 12–July 11 0 54,029

July 12–August 16 0 69,301

August 17–September 14 0 40,230

September 14 18,540

Total 424,228 62,824 181,962 18,540 –286,549

Date

Total phosphorus mass, in kilograms

Initial storage Inflow Outflow Final storage Unaccounted 

for

March 10 12,683

March 11–May 11 5,711 345

May 12–July 11 0 1,455

July 12–August 16 0 3,898

August 17–September 14 0 2,931

September 14 2,121

Total 12,683 5,711 8,628 2,121 –7,645
JULESBURG RESERVOIR 53

9.0 to 19. Because blue-green algae are less common 

when N:P values exceed 29, green algae likely 

predominated over blue-green algae until September 

(Ryding and Rast, 1989).

Although chlorophyll-a measurements cannot 

provide information on the composition of the 

phytoplankton community, they can provide a measure 

of the quantity of algae present. Chlorophyll-a concentrations measured in the euphotic zone in June ranged 

from 16 μg/L at site 2 to 26 μg/L at sites 1 and 3 

(table 24). In late July, chlorophyll-a concentrations 

ranged from 12 μg/L at site 1 to 21 μg/L at site 2. By 

September, algal biomass had increased substantially; 

chlorophyll-a concentrations ranged from 32 μg/L at 

site 1 to 96 μg/L at site 3. Based on mean chlorophylla concentrations, North Sterling Reservoir is classified 

as eutrophic. Carlson’s chlorophyll TSI values ranged 

from 67 to 72 in June, from 64 to 69 in July, and from 

74 to 84 in September (table 24), indicating that the 

algal biomass approximately doubled from late spring 

to late summer. The chlorophyll, Secchi-disk, and 

phosphorus TSI values were similar, particularly in the 

summer, supporting the conclusion that phosphorus 

was the limiting nutrient during this period.

JULESBURG RESERVOIR

Julesburg Reservoir is in Logan and Sedgwick 

Counties, Colo., about 20 mi southwest of Julesburg 

(fig. 1). It is owned by Julesburg Irrigation District and 

also is operated as a State Wildlife Area by the Colorado Division of Wildlife. The reservoir has an estimated maximum storage capacity of 31,800 acre-ft 

and receives most of its surface-water inflow from 

Harmony Ditch (fig. 24), which originates in the South 

Platte River. Water released from the reservoir through 

Highline Canal is used primarily for irrigation.

During the study period, reservoir storage 

ranged from 22,666 acre-ft in March to 6,168 acre-ft 

in September (fig. 25 and table 25) and maximum 

depth ranged from about 18 ft in March to about 3 ft in 

September. The reservoir normally is filled during the 

fall and winter, reaching maximum volumes in the 

early spring. In 1995, the reservoir reached maximum 

storage in March and water was released for irrigation 

beginning in May. There were inflows during March 

and the end of September and outflows from May 

through September. Correspondingly, storage volumes 

during the study period decreased through the beginning of September then increased slightly through the 

end of September. Measurable residence times in the 

reservoir ranged from 180 days in June to 15 days in 

August as outflow increased and storage decreased 

(table 25).

Water-quality samples were collected at five 

sites throughout the reservoir (fig. 24) on March 23, 

June 2, July 28, and September 15, 1995. Waterquality samples at sites 1 and 5 and depth profiles of 

water temperature, dissolved oxygen, pH, and specific 

conductance at all five sites were not obtained on 

September 15 because the reservoir was extremely 

Table 23. Concentrations of biologically available nitrogen 

and phosphorus and N:P values for North Sterling Reservoir, March–September 1995

[<, less than; --, missing data; DIN, dissolved inorganic nitrogen; DOP, 

dissolved orthophosphate; N:P, mass ratio of nitrogen to phosphorus; 

values of N:P calculated from total nitrogen and total phosphorus if DIN

or DOP concentrations below detection limits]

Site

(figure 19)

Concentration,

in milligrams per liter N:P

DIN DOP

March 10

1 3.9 0.060 65

2 3.8 0.060 64

3 3.8 0.050 76

4 3.9 0.060 65

5 3.6 0.040 90

6 2.8 < 0.010 37

June 1

1 2.2 < 0.010 26

2 2.3 < 0.010 80

3 2.1 < 0.010 77

4 2.2 < 0.010 62

5 2.2 < 0.010 44

6 1.9 < 0.010 41

July 27

1 1.2 < 0.010 33

2 1.1 < 0.010 37

3 1.3 < 0.010 42

4 1.3 < 0.010 46

5 1.1 < 0.010 30

6 1.1 < 0.010 42

September 14

1 0.38 0.020 19

2 0.26 < 0.010 9.2

3 < 0.065 < 0.010 9.7

4 0.10 < 0.010 9.2

5 < 0.065 < 0.010 9.0

6 -- -- --
54 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

shallow. Samples for chlorophyll-a were collected on 

June 2 and July 28 from sites 1, 2, and 3; samples on 

September 15 were collected from sites 2, 3, and 4. 

Additional water-quality samples were collected from 

Harmony Ditch and Highline Canal (when flowing) on 

March 23, June 2, July 10, August 15, and September 

15, 1995. Bottom-sediment samples were collected 

from sites 1 and 2 in the early spring, on March 22 and 

April 4, and in the summer, on August 15.

Water Temperature and Dissolved Oxygen

The temperature profile for March 23 indicates 

that the reservoir was well mixed at all sites in late 

winter; temperatures were approximately 9oC from 

surface to bottom (fig. 26). As spring and summer 

progressed, the water column remained nearly 

isothermal and no stratification was observed. Stratification from warmer inflow water entering as overflow 

did not occur, as there was no inflow during this 

period. Additionally, because the reservoir is relatively 

shallow and the surrounding terrain is flat, frequent 

strong winds helped keep the water column well 

mixed during the study period. On March 23 and 

June 2, oxygen was uniformly distributed with depth 

at all five sites (fig. 26). Slightly reduced dissolved 

oxygen concentrations with depth first were observed 

on June 2 at sites 1, 2, and 4 due to biological decomposition of sinking organic matter. A zone of low 

dissolved oxygen may have appeared first at the main 

sedimentation zone near the outlet at site 4 and begun 

to spread upreservoir to sites 1 and 2 by June 2 (Cole 

and Hannan, 1990). The lack of inflow during June 

and July promoted the upreservoir progression of low 

dissolved oxygen conditions; by July 28, a stronger 

vertical gradient of dissolved oxygen existed at all 

sites. Though the reservoir was well mixed at this 

time, rates of consumptive metabolism near the bottom 

were high enough to decrease bottom dissolved 

oxygen concentrations.

pH and Specific Conductance

In Julesburg Reservoir, pH ranged seasonally 

between 8.0 and 8.6 (fig. 26). On all sampling dates, 

pH was constant from surface to bottom at all five 

sites. The lack of a pH difference between zones of 

photosynthesis and decomposition indicates that the 

system was well buffered by bicarbonate and 

carbonate, preventing large changes in pH, and that it 

likely was well mixed by wind. On all sampling dates, 

specific conductance also was constant from surface to 

bottom at all five sites (fig. 26). The maximum specific 

conductance measured during the study period was 

approximately 2,030 μS/cm on March 23. At this time, 

specific conductance was influenced primarily by 

inflow water; specific conductance was measured at 

2,020 μS/cm in Harmony Ditch on March 23. With 

little additional inflow after that date, specific conductance decreased slightly to near 1,600 μS/cm by the 

end of July. Because of the high salinity, use of this 

Table 24. Chlorophyll-a concentrations, Secchi-disk depths, total phosphorus concentrations, and associated trophic-state 

indices for North Sterling Reservoir, March–September 1995

[μg/L, micrograms per liter; mg/L; milligrams per liter; TSI, Trophic State Index, Chl, chlorophyll-a; SD, Secchi disk; TP, total phosphorus]

Site

(figure 

19)

Chlorophyll-a

concentration

(μg/L)

TSI(Chl)

Secchi-disk 

depth

(feet)

TSI(SD)

Total phosphorus concentration

(mg/L)

TSI(TP)

June 1

1 26 72 3.5 59 0.15 76

2 16 67 4.0 57 0.040 57

3 26 72 3.5 59 0.040 57

July 27

1 12 64 3.0 61 0.060 63

2 21 69 3.0 61 0.060 63

3 14 65 3.0 61 0.050 61

September 14

1 32 74 1.0 77 0.16 77

2 65 81 1.0 77 0.18 79

3 96 84 0.50 87 0.17 78
JULESBURG RESERVOIR 55

Reservoir sampling site and number

Inflow/outflow sampling site

EXPLANATION 0

0 1 2 KILOMETERS

1 2 MILES Base from U.S. Geological Survey and

U.S. Census Bureau digital data,

1974 to 1993

2

3

1

5

4

Highline Canal

Harmony Ditch No. 1

South Platte River

Julesburg Reservoir

South Platte Study Area

Julesburg

Reservoir

3

N

Figure 24. Location of sampling sites in Julesburg Reservoir.
56 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

0

100

200

300

400

500

0

5,000

10,000

15,000

20,000

25,000

RESERVOIR VOLUME, IN ACRE-FEET

March May July September

Inflow 

Outflow 

End-of-month volume

April June August

1995

EXPLANATION

RESERVOIR INFLOW OR OUTFLOW, IN 

ACRE-FEET PER DAY

water for irrigation could have been detrimental to 

crop quality and yield (Aryes and Westcot, 1976). 

Nutrient Dynamics

Water-Column Concentrations

A statistical summary of nutrient concentrations 

at the five sites sampled in Julesburg Reservoir, 

including both euphotic and bottom samples, is 

provided in table 26. Median concentrations of total 

nitrogen during the study period ranged from 1.6 mg/L 

at sites 3, 4, and 5 to 1.8 mg/L at site 2, and median 

concentrations of total phosphorus during the study 

period ranged from 0.020 mg/L at site 4 to 0.040 mg/L 

at sites 1 and 5. A statistical analysis of nutrient 

concentrations using the Kruskal-Wallis test indicates 

that the differences in nutrient concentrations between 

the sites were not significant (α=0.05). Because inflow 

Figure 25. Inflow, outflow, and end-of-month storage volume in Julesburg Reservoir, March–September 1995.

Table 25. End-of-month storage, monthly median outflow, 

and residence time in Julesburg Reservoir, March–

September 1995

[acre-ft, acre-feet; acre-ft/d, acre-feet per day; --, could not be calculated]

Month

End-ofmonth 

storage

(acre-ft)

Monthly 

median 

outflow

(acre-ft/d)

Residence 

time

(days)

March 22,666 0 --

April 22,521 0 --

May 20,930 0 --

June 19,655 109 180

July 14,961 288 52

August 5,372 363 15

September 6,168 0 --
JULESBURG RESERVOIR 57

0 5 10

0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14

0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14

15 20 25 30 10 0 15 20 25 30 10 5 0 15 20 25 30 5

WATER TEMPERATURE, IN DEGREES CELSIUS

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

March 23 June 2 July 28

20

15

10

5

0

20

15

10

5

0

20

15

10

5

0

20

15

10

5

0

DEPTH, IN FEET

Median euphotic 

 zone depth

Site 1 

Site 2 

Site 3 

Site 4 

Site 5 

EXPLANATION

0 500 1,000 1,500 2,000 0 500 1,000 1,500 2,000 0 500 1,000 1,500 2,000

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER CENTIMETER

pH, IN STANDARD UNITS

Figure 26. Depth profiles of water temperature, dissolved oxygen, pH, and specific conductance in Julesburg Reservoir, 

March–September 1995.
58 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Table 26. Statistical summary of nutrient concentrations at each site and results of Kruskal-Wallis test of site concentration 

differences in Julesburg Reservoir, March–September 1995

[All concentrations are in milligrams per liter; p-value represents the probability that site concentrations have identical distributions; differences not significant at p-values greater than 0.05]

Constituent

Site

(figure 

24)

Number of 

analyses Mean Standard deviation

Twentyfifth 

percentile

Median

Seventyfifth 

percentile

p-value

Total nitrogen 1 5 1.9 0.89 1.2 1.7 2.2

2 6 1.7 0.63 1.1 1.8 2.2

3 5 1.7 0.96 0.92 1.6 2.1

4 5 1.6 0.75 0.98 1.6 2.3

5 3 1.6 0.67 1.2 1.6 1.9 0.9631

Dissolved ammonia 1 5 0.10 0.064 0.030 0.14 0.15

2 6 0.074 0.061 0.020 0.075 0.13

3 5 0.070 0.073 0.015 0.020 0.15

4 5 0.071 0.072 0.020 0.020 0.14

5 3 0.12 0.058 0.10 0.15 0.15 0.3861

Dissolved nitrite 1 5 0.022 0.011 0.010 0.030 0.030

2 6 0.028 0.016 0.015 0.030 0.037

3 5 0.048 0.052 0.020 0.030 0.030

4 5 0.036 0.029 0.010 0.040 0.040

5 3 0.023 0.012 0.020 0.030 0.030 0.5679

Dissolved nitrate 1 5 0.69 0.66 0.050 0.61 1.4

2 6 0.64 0.67 0.095 0.39 1.2

3 5 0.70 0.55 0.20 0.62 1.2

4 5 0.50 0.53 0.070 0.42 0.60

5 3 0.67 0.66 0.33 0.59 0.98 0.8781

Total organic nitrogen 1 5 1.0 0.37 0.87 0.95 1.1

2 6 0.94 0.58 0.59 0.77 0.88

3 5 0.93 0.54 0.68 0.69 0.85

4 5 1.0 0.53 0.78 0.84 0.88

5 3 0.75 0.10 0.70 0.75 0.80 0.5185

Total phosphorus 1 5 0.062 0.055 0.020 0.040 0.080

2 6 0.042 0.050 0.013 0.025 0.037

3 5 0.070 0.080 0.030 0.030 0.060

4 5 0.056 0.070 0.020 0.020 0.040

5 3 0.037 0.025 0.025 0.040 0.050 0.7743

Dissolved orthophosphate 1 5 0.011 0.0045 0.010 0.010 0.010

2 6 0.012 0.0047 0.010 0.010 0.010

3 5 0.012 0.0045 0.010 0.010 0.010

4 5 0.011 0.0043 0.010 0.010 0.010

5 3 0.017 0.012 0.010 0.010 0.020 0.4380

Total organic phosphorus 1 5 0.052 0.055 0.010 0.030 0.070

2 6 0.032 0.050 0.0025 0.015 0.027

3 5 0.058 0.080 0.020 0.020 0.040

4 5 0.046 0.070 0.010 0.010 0.030

5 3 0.020 0.026 0.0050 0.010 0.030 0.6590
JULESBURG RESERVOIR 59

volumes were small and outflow volumes were 

substantial throughout much of the study period, residence times were short, resulting in a minimal longitudinal gradient.

Samples collected near the reservoir surface and 

near the reservoir bottom showed little difference 

between euphotic and bottom concentrations of 

nitrogen and phosphorus during most of the study 

period. Median concentrations of total phosphorus, 

orthophosphate, and dissolved inorganic nitrogen for 

all sites combined were slightly higher near the bottom 

during July, likely due to biological uptake in the 

euphotic zone. Overall, however, results of a Wilcoxon 

Rank-Sum test indicate that there were no statistically 

significant differences (α=0.05) between euphotic and 

bottom nutrient concentrations during the study period 

(table 27). The lack of variation with depth likely was 

because the reservoir never developed strong stratification to limit mixing between surface water and bottom 

water.

Although there was little longitudinal and 

vertical variability in nutrient concentrations, results of 

a Kruskal-Wallis test indicate that there was significant (α=0.05) temporal variability in total nitrogen 

and total phosphorus concentrations from March 

through September (fig. 27). Median concentrations of 

total nitrogen for all data combined decreased from 

2.2 mg/L in March to 0.97 mg/L in July. The decrease 

in total nitrogen was driven largely by a decrease in 

nitrate; median concentrations of nitrate dropped from 

1.4 mg/L in March to 0.070 mg/L in July. After reservoir inflows ceased at the end of March, continued 

biological activity resulted in assimilation of most of 

the nitrate by the end of July, and most of the 

Table 27. Statistical summary of nutrient concentrations in euphotic and bottom samples and results of Wilcoxon Rank-Sum 

test of euphotic and bottom concentration differences in Julesburg Reservoir, March–September 1995

[e, euphotic; b, bottom; all concentrations are in milligrams per liter; p-value represents the probability that e and b concentrations have identical distributions; differences not significant at p-values greater than 0.05]

Constituent Location Number of 

analyses Mean Standard deviation

Twentyfifth 

percentile

Median

Seventyfifth 

percentile

p-value

Total nitrogen e 18 1.7 0.67 1.1 1.7 2.2

b 6 1.6 0.93 0.98 1.1 1.9 0.6634

Dissolved ammonia e 18 0.091 0.064 0.020 0.13 0.15

b 6 0.062 0.061 0.020 0.025 0.11 0.5649

Dissolved nitrite e 18 0.037 0.031 0.023 0.030 0.040

b 6 0.018 0.0098 0.010 0.015 0.027 0.0709

Dissolved nitrate e 18 0.67 0.54 0.17 0.60 1.3

b 6 0.54 0.69 0.070 0.13 1.1 0.6879

Total organic nitrogen e 18 0.96 0.49 0.67 0.81 0.88

b 6 0.95 0.38 0.73 0.88 1.0 0.5480

Total phosphorus e 18 0.052 0.060 0.020 0.025 0.040

b 6 0.062 0.050 0.033 0.050 0.075 0.3257

Dissolved orthophosphate e 18 0.011 0.0047 0.010 0.010 0.010

b 6 0.012 0.0041 0.010 0.010 0.010 0.4870

Total organic phosphorus e 18 0.041 0.060 0.010 0.010 0.030

b 6 0.050 0.050 0.023 0.035 0.063 0.2912
60 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

EXPLANATION

Outlier data value greater than 1.5

times the interquartile range

outside the quartile

Data value less than or equal

to 1.5 times the interquartile range

outside the quartile

75th percentile

25th percentile

Median

0

1

2

3

4

5

6

7

CONCENTRATION, IN MILLIGRAMS PER 

LITER AS NITROGEN

NH3 NO2 NO3 TN TOrgN NH3 NO2 TN TOrgN NH NO3 3 NO2 NO3 TN TOrgN NH3 NO2 TN TOrgN NO3

March 23 June 2 July 28 September 15

0

0.2

0.4

0.6

0.8

TP OP TOrgP OP TOrgP TP OP TOrgP TP OP TOrgP TP

CONCENTRATION, IN MILLIGRAMS PER 

LITER AS PHOSPHORUS

NH3—Dissolved ammonia

NO2—Dissolved nitrite

NO3—Dissolved nitrate

TN—Total nitrogen

TOrgN—Total organic nitrogen

TOrgP—Total organic phosphorus

OP—Dissolved orthophosphate

TP—Total phosphorus

Figure 27. Nitrogen and phosphorus concentrations, euphotic and bottom samples combined in 

Julesburg Reservoir, March–September 1995.
JULESBURG RESERVOIR 61

5

4

3

2

1

0

1

5

4

3

2

1

0

1

DISSOLVED ORTHOPHOSPHATE CONCENTRATION, IN MILLIGRAMS PER LITER

DISSOLVED INORGANIC NITROGEN CONCENTRATION, IN MILLIGRAMS PER LITER

0

0 1 2 3 4 5 0 2 4 6 8 10

0.2 0.4 0.6 0.8 1.0 0 1 2 3 4 5

August 15

March 22 and April 4 

March 22 and April 4 

August 15

Sediment/Water interface

EXPLANATION

DISTANCE ABOVE AND BELOW SEDIMENT/WATER

INTERFACE, IN CENTIMETERS

Site 1 measured concentrations 

Site 2 measured concentrations 

Data missing

remaining nitrogen was in the form of total organic 

nitrogen. By September, the median concentrations of 

total nitrogen and nitrate had increased to 2.5 and 

0.42 mg/L, respectively, as inflows from the river 

began again. During March and September, reservoir 

water quality was influenced by inflowing water from 

the South Platte River. The influence of river water 

quality likely was greater in September, when inflow 

volumes were larger relative to the volume in storage. 

Concentrations of total phosphorus did not have the 

same consistent decrease during the study period as 

concentrations of total nitrogen. Median concentrations of total phosphorus for all data combined were 

lowest in March and June, at 0.010 mg/L, and highest 

in September, at 0.18 mg/L (fig. 27). The large 

increase in September was due to the increased inflow 

in the latter part of that month. Before September, 

internal recycling kept phosphorus levels relatively 

constant.

Bottom-Sediment Flux

Estimates of nutrient flux from the bottom sediment indicate that the sediment was a potential source 

of nutrients to the reservoir. In early spring, concentrations of dissolved inorganic nitrogen measured in 

the first centimeter of the sediment were lower than in 

the overlying water column at site 1 but higher than 

in the overlying water column at site 2 (fig. 28). 

The estimated flux rates for inorganic nitrogen were 

–4.4 mg/m2/d (into the sediment) at site 1 and 

1.1 mg/m2/d at site 2. Concentrations of dissolved 

Figure 28. Concentrations of dissolved inorganic nitrogen and orthophosphate in bottom-sediment pore water and the overlying water column in Julesburg Reservoir, March 22, April 4, and August 15, 1995.
62 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

orthophosphate measured in the first centimeter of the 

sediment were lower than in the overlying water 

column at site 1 in early spring (data from –0.5 cm 

depth were unavailable for site 2). The estimated flux 

rate for orthophosphate was –0.39 mg/m2/d (into the 

sediment) at site 1. In the summer, concentrations of 

dissolved inorganic nitrogen measured in the first 

centimeter of the sediment were higher than in the 

overlying water column at sites 1 and 2. The estimated 

flux rates for inorganic nitrogen were 41 mg/m2

/d at 

site 1 and 30 mg/m2/d at site 2. Concentrations of 

dissolved orthophosphate measured in the first centimeter of the sediment also were higher than in the 

overlying water column at sites 1 and 2 in the summer. 

The estimated flux rates for orthophosphate were 

3.3 mg/m2/d at site 1 and 3.4 mg/m2/d at site 2. The 

bottom dissolved oxygen concentrations at site 1 and 

site 2 in early spring were close to 10 mg/L, the 

minimum measured on March 22; concentrations on 

August 15 were close to 8 mg/L, the minimum 

measured on July 28. Because bottom sediment 

release is redox-dependent, movement from the sediment into the overlying water column is restricted if 

the sediment/water interface is well oxygenated 

(Horne and Goldman, 1994). Therefore, these flux 

calculations may overestimate the movement of nutrients between bottom sediment and the water column.

Mass Balance

Mass-balance calculations for nitrogen indicate 

that about 49 percent of the total mass of nitrogen—

the mass that had been in storage in March plus the 

mass that had entered in inflows through September—

was trapped in the reservoir (represented by the “unaccounted for” term), while 39 percent was released 

through the outflow and 13 percent remained in the 

water column by September (table 28). Mass-balance 

calculations for phosphorus indicate that about 

20 percent of the total mass of phosphorus was trapped 

in the reservoir, while 56 percent was released through 

the outflow and 24 percent remained in the water 

Table 28. Mass-balance calculations for total nitrogen and total phosphorus in Julesburg Reservoir, March–September 1995

[From equation 1: Unaccounted for mass = Final storage mass – Initial storage mass – Inflow mass + Outflow mass]

Date

Total nitrogen mass, in kilograms

Initial storage Inflow Outflow Final storage Unaccounted 

for

March 8 61,254

March 9–March 23 14,849 0

March 24–June 2 11,046 3,916

June 3–July 10 15,077 7,539

July 11–August 15 10,858 20,087

August 16– September 15 35,736 25,979

September 15 19,028

Total 61,254 87,566 57,520 19,028 –72,272

Date

Total phosphorus mass, in kilograms

Initial storage Inflow Outflow Final storage Unaccounted 

for

March 8 559

March 9–March 23 144 0

March 24–June 2 1,052 24

June 3–July 10 1,713 411

July 11–August 15 543 1,086

August 16– September 15 1,642 1,636

September 15 1,370

Total 559 5,094 3,157 1,370 –1,126
JULESBURG RESERVOIR 63

column by September. Much of the nitrogen likely was 

trapped in the reservoir through biological uptake and 

deposition of particulate organic nitrogen onto bottom 

sediments (Wetzel, 1983). Much of the phosphorus 

likely was internally recycled due to processes such as 

excretion from zooplankton and fish, decay of aquatic 

plants and animals, and release from bottom sediment. 

In July and August, the reservoir had relatively short 

residence times and some of the recycled phosphorus 

still in the water column was flushed out; much of the 

nitrogen, however, had already been utilized by algae 

and was not available to be flushed out. As a result, a 

greater percentage of the total mass of phosphorus was 

released through the outflow. Overall, a substantial 

portion of the total nutrient mass was trapped in the 

reservoir during the study period and the reservoir 

acted as a sink for both nitrogen and phosphorus. 

Algal Growth

Throughout the study period, dissolved orthophosphate concentrations generally were below the 

algal-growth-limiting concentration of 0.010 mg/L, 

indicating that phosphorus consistently was the 

limiting nutrient (table 29). Values of N:P also indicate 

that phosphorus was the limiting nutrient throughout 

the entire study period. In March, N:P values ranged 

from 70 to 220; in June, N:P values ranged from 27 to 

140; in July, N:P values ranged from 4.0 to 43; and in 

September, N:P values ranged from 14 to 16. Because 

blue-green algae are less common when N:P values 

exceed 29, green algae likely predominated over bluegreen algae until September.

Although chlorophyll-a measurements cannot 

provide information on the composition of the 

phytoplankton community, they can provide a measure 

of the quantity of algae present. Chlorophyll-a concentrations measured in the euphotic zone in June ranged 

from 0.50 μg/L at site 2 to 1.0 μg/L at sites 1 and 3 

(table 30). In late July, chlorophyll-a concentrations 

ranged from 9.7 μg/L at site 3 to 17 μg/L at site 1. By 

September, algal biomass had increased substantially 

due to increased nutrient loads in inflowing water; 

chlorophyll-a concentrations ranged from 93 μg/L at 

site 2 to 180 μg/L at site 4. Based on mean chlorophyll-a concentrations, Julesburg Reservoir is classified as eutrophic. Carlson’s TSI values ranged from 33 

to 40 in June, from 62 to 67 in July, and from 84 to 91 

in September (table 30), indicating that the algal 

Table 29. Concentrations of biologically available nitrogen 

and phosphorus and N:P values for Julesburg Reservoir, 

March–September 1995

[<, less than; --, missing data; DIN, dissolved inorganic nitrogen; DOP, 

dissolved orthophosphate; N:P, mass ratio of nitrogen to phosphorus; 

values of N:P calculated from total nitrogen and total phosphorus if DIN or 

DOP concentrations below detection limits]

Site

(figure 24)

Concentration, 

in milligrams per liter N:P

DIN DOP

March 23

1 1.5 < 0.010 110

2 1.6 < 0.010 110

3 1.5 < 0.010 70

4 1.5 < 0.010 115

5 1.5 < 0.010 220

June 2

1 0.8 < 0.010 85

2 0.8 < 0.010 140

3 0.8 < 0.010 80

4 0.8 < 0.010 80

5 0.8 < 0.010 27

July 28

1 0.090 < 0.010 24

2 0.10 < 0.010 25

3 0.21 < 0.010 30

4 0.080 < 0.010 43

5 0.12 0.03 4.0

September 15

1 -- -- --

2 0.23 < 0.010 16

3 1.31 < 0.010 15

4 0.51 < 0.010 14

5 -- -- --

biomass doubled approximately five times during the 

spring and summer. The chlorophyll, Secchi-disk, and 

phosphorus TSI values were similar, supporting the 

conclusion that phosphorus was the limiting nutrient 

during this period.
64 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

SUMMARY AND CONCLUSIONS

In 1995, the USGS conducted a study to characterize nutrient concentrations in five off-stream reservoirs—Riverside, Jackson, Prewitt, North Sterling, 

and Julesburg—during the irrigation season from 

March through September. Water-quality data were 

obtained from the reservoir water column, the inflow 

and outflow canals, and the reservoir bottom sediment 

from March through September. Additional analyses 

of nutrient mass balances, water residence times, and 

chlorophyll-a concentrations were used to help explain 

nutrient dynamics in each reservoir.

The five reservoirs studied are in similar 

geographic, climatic, and land-use areas, and as a 

result, have a number of similarities in their internal 

nutrient dynamics. Dissolved nitrogen concentrations 

in the reservoirs were highest in March and decreased 

through September as a result of dilution from river 

inflows and biological activity. From March through 

June, decreases in nitrogen concentrations in the river 

contributed to corresponding decreases in reservoir 

concentrations. From July through September, inflows 

from the river were cut off, and biological activity in 

the reservoirs likely led to further decreases in nitrate 

concentrations, which fell to near or below detectable 

levels. Phosphorus concentrations in the reservoirs did 

not show the same consistent decrease from March 

through September. Phosphorus likely was recycled 

continuously back to algae during the study period 

through processes such as excretion from fish, decay 

of aquatic plants and animals, and release of orthophosphate from bottom sediment during periods of 

low oxygen (Wetzel, 1983). With the exception of 

phosphorus in Jackson Reservoir, the reservoirs acted 

as a sink for nitrogen and phosphorus; the percentage 

of the total mass (initial storage plus inflows) trapped 

in the reservoirs during the study period ranged from 

49 to 88 percent for nitrogen and from 20 to 86 percent 

for phosphorus. 

The nutrient loading, morphology, and operation 

of the five reservoirs differ, however, leading to several 

important differences in nutrient dynamics among the 

reservoirs. Approximately 95 percent of the total 

wastewater discharge to the South Platte River occurs 

upstream from Kersey, which leads to high nutrient 

loading into Riverside Reservoir, the closest reservoir 

downstream from Kersey. Mean nutrient concentrations in the reservoirs decreased in a downstream 

direction from Riverside Reservoir to Julesburg Reservoir as distance from the wastewater loading increased 

(table 31). North Sterling was an exception to this 

pattern. It was the deepest reservoir and therefore the 

most resistant to wind-induced mixing. The strong 

stratification and resulting anoxia that developed likely 

allowed for substantial internal nutrient loading from 

bottom-sediment releases that offset the decrease in 

external nutrient loading from the river. Also, North 

Table 30. Chlorophyll-a concentrations, Secchi-disk depths, total phosphorus concentrations, and associated trophic-state 

indices for Julesburg Reservoir, March–September 1995

[μg/L, micrograms per liter; mg/L; milligrams per liter; TSI, Trophic State Index; Chl, chlorophyll-a; SD, Secchi disk; TP, total phosphorus]

Site

(figure 24)

Chlorophyll-a

concentration

(μg/L)

TSI(Chl) Secchi-disk depth

(feet) TSI(SD)

Total

phosphorus 

concentration

(mg/L)

TSI(TP)

June 2

1 1.0 40 7.5 48 0.020 47

2 0.50 33 7.5 48 0.010 37

3 1.0 40 6.0 51 0.020 47

June 28

1 17 67 4.0 57 0.040 57

2 14 65 4.0 57 0.040 57

3 9.7 62 3.0 61 0.030 53

September 15

2 93 84 0.75 81 0.14 75

3 150 89 0.75 81 0.21 81

4 180 91 1.0 77 0.18 79
SUMMARY AND CONCLUSIONS 65

Sterling Reservoir has the longest inlet canal of the 

five reservoirs, and agricultural runoff along its length 

could have contributed to increased nutrient loading in 

the canal between the river and the reservoir.

The variations in nutrient loading contributed to 

differences in the nutrient limiting algal growth in the 

reservoirs (table 31). In all of the reservoirs, nitrogen 

concentrations decreased substantially due to biological activity once inflows had ended. However, more 

phosphorus was internally recycled throughout the 

study period in Riverside and Jackson Reservoirs due 

to the higher initial loading of phosphorus that 

occurred during periods of inflow. As a result, relatively high concentrations of phosphorus were available throughout the study period in Riverside and 

Jackson Reservoirs, and nitrogen became the limiting 

nutrient by midsummer as biological activity depleted 

the available supply of nitrogen. Prewitt, North Sterling, and Julesburg Reservoirs, with lower initial loadings of phosphorus, were phosphorus-limited 

throughout the study period, with additional nitrogen 

limitation occurring as biological uptake reduced 

nitrogen concentrations to near or below laboratory 

detection limits. 

The timing of minimum nitrogen levels in each 

reservoir was related to the timing of river inflows and 

the degree of stratification in the reservoir. In all five 

reservoirs, dissolved nitrogen concentrations 

decreased during periods of inflow as a result of dilution from river inflows and biological activity. After 

inflows were ended, biological activity decreased 

nitrate concentrations to near or below detectable 

levels by the next reservoir sampling date in all reservoirs except North Sterling. In North Sterling Reservoir, nutrient flux from the bottom sediment during 

periods of anoxia in the summer likely maintained 

nutrient supplies once inflows ended; after the reservoir became well mixed and well oxygenated in 

September, continued biological activity decreased 

nitrate concentrations to below detectable levels. In 

Julesburg Reservoir, nutrient concentrations increased 

once inflows began again in September, offsetting 

losses due to biological uptake. 

The percentage of the total nitrogen and phosphorus mass (the mass that had been in storage in 

March plus the mass that had entered in inflows 

through September) lost through outflow and trapped 

within the reservoir due to processes such as biological 

uptake and sedimentation varied among the reservoirs 

(table 31). Generally, reservoirs with short residence 

times such as North Sterling and Julesburg had a 

higher percentage of the total mass lost through the 

outflow, whereas reservoirs with longer residence 

times like Jackson and Prewitt trapped more of the 

total mass within the reservoir. Riverside Reservoir, 

with the largest external loading of phosphorus and the 

greatest total outflow volume of all the reservoirs, lost 

the highest percentage of phosphorus in outflows. 

Julesburg Reservoir, which essentially acted as a flowthrough reservoir (inflow = outflow) during part of the 

Table 31. Comparison of morphological, operational, and nutrient characteristics of Riverside, Jackson, Prewitt, North Sterling, and Julesburg Reservoirs, March–September 1995

[mg/L, milligrams per liter; TN, total nitrogen; TP, total phosphorus; N, nitrogen; P, phosphorus; TSI, Trophic State Index; <, less than; Chl, chlorophyll-a]

Riverside Jackson Prewitt North Sterling Julesburg

Mean residence time (days) 149 469 857 32 82

Maximum depth (feet) 24 18 22 54 18

Degree of summer stratification Moderate Weak Weak Strong Weak

Date of inflow cut-off August 8 May 31 July 15 May 29 June 4 1

Mean TN concentration (mg/L) 3.60 2.78 1.95 3.07 1.71

Mean TP concentration (mg/L) 0.53 0.18 0.074 0.12 0.054

Limiting nutrient N and P N and P P P P

Percent TN mass trapped in reservoir 68 63 88 59 49

Percent TN mass lost in outflow 28 12 <1 37 39

Percent TP mass trapped in reservoir 25 +5 (gained) 86 42 20

Percent TP mass lost in outflow 65 36 <1 47 56

Median TSI(Chl) 59 72 63 72 63

1 Inflow to Julesburg Reservoir was restarted on September 15.
66 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

study period, lost the highest percentage of nitrogen 

and the second-highest percentage of phosphorus in 

outflows. This was probably because warmer 

inflowing water formed a density overflow that 

progressed toward the surface-level outlet with 

minimal mixing during periods when inflow equaled 

outflow.

Algal biomass generally increased throughout 

the study period in all five reservoirs as temperatures 

and light intensity increased during the summer. In 

Julesburg Reservoir, the late-summer influx of nutrients in inflowing river water led to a fivefold increase 

in chlorophyll-a concentrations. In Jackson Reservoir, 

the only reservoir without any significant inflow 

during the summer to replenish nutrient supplies, chlorophyll-a concentrations decreased sharply by 

September. Based on measured chlorophyll-a concentrations and Carlson’s TSI values, all five reservoirs 

could be considered eutrophic by the end of the study 

period.

The results of this study indicate that the practice of storing South Platte River water in off-stream 

reservoirs substantially decreases dissolved nitrogen 

concentrations during the irrigation season. Associated 

with the decreased nitrogen concentrations, however, 

is an increase in algal biomass, which could adversely 

affect the recreational use of the reservoirs. 

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APPENDIX
APPENDIX 71

Appendix. Nutrient concentrations in inflow and outflow canals for Riverside, Jackson, Prewitt, North Sterling, and Julesburg 

Reservoirs, March–September 1995

[--, canal not flowing and could not be sampled; in months with no data shown, neither the inflow nor the outflow was flowing; Inflow1 for Prewitt Reservoir is Prewitt Inlet Canal; Inflow2 for Prewitt Reservoir is Lower Platte and Beaver Ditch; <, less than]

Date Location

Concentration, 

in milligrams per liter

Total organic 

nitrogen

Dissolved 

ammonia

Dissolved 

nitrite

Dissolved 

nitrate

Dissolved 

orthophosphate

Total organic 

phosphorus

Riverside Reservoir

May 9 Inflow 0.87 0.23 0.050 3.9 0.45 0.21

Outflow -- -- -- -- -- --

June 7 Inflow 0.66 0.040 0.020 0.98 0.17 0.14

Outflow 0.91 0.29 0.15 3.1 0.49 0.020

July 12 Inflow 0.58 0.020 0.020 1.2 0.16 0.11

Outflow 1.1 0.51 0.17 1.3 0.51 0.090

August 30 Inflow -- -- -- -- -- --

Outflow 1.19 0.015 0.010 0.040 0.40 0.15

September 28 Inflow -- -- -- -- -- --

Outflow 1.1 0.015 0.020 0.030 0.060 0.22

Jackson Reservoir

March 8 Inflow 0.25 0.15 0.030 4.3 0.28 0.050

Outflow -- -- -- -- -- --

July 12 Inflow -- -- -- -- -- --

Outflow 1.1 0.93 0.070 0.39 0.080 0.13

August 30 Inflow -- -- -- -- -- --

Outflow 1.5 0.13 0.010 0.040 0.12 0.18

September 28 Inflow -- -- -- -- -- --

Outflow 0.25 0.15 0.030 4.3 0.28 0.050

Prewitt Reservoir

March 9 Inflow1 0.38 0.020 0.030 4.0 0.17 0.030

Inflow2 -- -- -- -- -- --

Outflow -- -- -- -- -- --

May 10 Inflow1 0.79 0.015 0.020 4.0 0.19 0.12

Inflow2 1.2 0.020 0.050 4.7 0.18 0.19

Outflow -- -- -- -- -- --

June 6 Inflow1 0.46 0.040 0.020 1.4 0.18 0.020

Inflow2 -- -- -- -- -- --

Outflow -- -- -- -- -- --

July 11 Inflow1 0.48 0.020 0.010 1.6 0.15 0.030

Inflow2 0.17 0.030 0.020 2.0 0.14 0.57

Outflow -- -- -- -- -- --

August 17 Inflow1 -- -- -- -- -- --

Inflow2 1.2 0.13 0.030 0.59 0.10 0.26

Outflow 1.1 0.11 0.010 0.040 0.010 0.070

September 13 Inflow1 -- -- -- -- -- --

Inflow2 1.6 0.015 0.020 3.1 0.020 0.070
72 Nutrient Dynamics in Five Off-Stream Reservoirs in the Lower South Platte River Basin, March–September 1995

Date Location

Concentration, 

in milligrams per liter

Total organic 

nitrogen

Dissolved 

ammonia

Dissolved 

nitrite

Dissolved 

nitrate

Dissolved 

orthophosphate

Total organic 

phosphorus

North Sterling Reservoir

March 10 Inflow 0.88 0.020 0.050 6.1 0.25 0.24

Outflow -- -- -- -- -- --

May 11 Inflow 1.4 0.015 0.020 4.1 0.22 0.28

Outflow 0.81 0.090 0.050 2.3 0.010 0.050

July 11 Inflow -- -- -- -- -- --

Outflow 0.75 0.45 0.090 1.3 0.050 0.020

August 16 Inflow -- -- -- -- -- --

Outflow 1.1 0.30 0.060 0.17 0.040 0.050

September 14 Inflow -- -- -- -- -- --

Outflow 1.1 0.36 0.010 0.040 0.010 0.10

Julesburg Reservoir

March 23 Inflow 0.39 0.015 0.030 2.7 0.030 < 0.010

Outflow -- -- -- -- -- --

June 2 Inflow 0.96 0.14 0.030 0.97 0.16 0.040

Outflow 0.84 0.16 0.040 0.61 0.010 < 0.010

July 10 Inflow 0.71 0.090 0.030 1.4 0.15 0.10

Outflow 0.85 0.050 0.030 0.13 0.010 0.050

August 15 Inflow 0.56 0.14 0.030 0.29 0.010 0.040

Outflow 1.6 0.20 0.010 0.040 0.010 0.090

September 15 Inflow 0.99 0.015 0.020 2.7 0.040 0.13

Outflow 2.3 0.015 0.070 0.30 0.010 0.16

Appendix. Nutrient concentrations in inflow and outflow canals for Riverside, Jackson, Prewitt, North Sterling, and Julesburg 

Reservoirs, March–September 1995—Continued

[--, canal not flowing and could not be sampled; in months with no data shown, neither the inflow nor the outflow was flowing; Inflow1 for Prewitt Reservoir is Prewitt Inlet Canal; Inflow2 for Prewitt Reservoir is Lower Platte and Beaver Ditch; <, less than]