What happens to nutrients in offstream reservoirs in 

the lower South Platte River Basin?

 The practice of storing South Platte River water in offstream reservoirs reduces nutrient concentrations but 

also contributes to the growth of algae, which may adversely affect the recreational use of the reservoirs. 

Results of a study of five offstream reservoirs in the lower South Platte River Basin during the 1995 irrigation season showed that the reservoirs trapped 20 to 88 percent of incoming nitrogen and phosphorus, except 

for phosphorus in one reservoir. Total nitrogen concentrations in the reservoirs were highest in March and 

decreased through September, largely as a result of uptake by algae and other aquatic life for growth. Total 

phosphorus concentrations in the reservoirs were more variable because of the recycling of phosphorus by 

aquatic life. Chlorophyll-a concentrations indicated that the amount of algae in all reservoirs increased during 

the summer and that all reservoirs were eutrophic. This study was done by the U.S. Geological Survey as part of 

the National Water-Quality Assessment (NAWQA) Program.

By L.A. Sprague, R.A. Kimbrough, and A.J. Ranalli

Jackson Reservoir during summer. Offstream reservoirs in the 

lower South Platte River Basin provide habitat for a variety of 

birds.

Why can nutrients be a problem in 

reservoirs? 

 Nutrients such as nitrogen and phosphorus are 

essential for all plant and animal life. In excess 

amounts, however, nutrients can accelerate the 

growth of aquatic plants and cause algal blooms that 

deprive deeper water of the sunlight needed by other 

aquatic plants and animals. This process is known as 

eutrophication. The subsequent decay of algae and 

other aquatic plants consumes dissolved oxygen, which 

can lead to fishkills. Algal blooms and excess aquatic 

plant growth also can make a water body unsuitable 

for recreation. Sources of nutrients in the South Platte 

River are both natural and human derived and include 

precipitation, discharges from wastewater-treatment 

plants, and fertilizers and manure used on croplands in 

agricultural areas and on lawns in urban areas. 

 In 1995, as part of the National Water-Quality 

Assessment (NAWQA) South Platte River Basin Study, 

the U.S. Geological Survey did a study to determine 

how the practice of storing South Platte River water 

in offstream reservoirs affects the concentration of 

nutrients in the stored water 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, 

boating, and swimming and are habitats for birds, 

including bald eagles, white pelicans, and a variety of 

ducks and geese. 

NATIONAL WATER–QUALITY ASSESSMENT PROGRAM

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 044–02

September 2002
Figure 1. Offstream reservoirs and river sites sampled in the South Platte River Basin in 1995. These reservoirs are valuable 

for irrigation water, recreation, and wildlife habitat.

Figure 2. Seasonal variation in storage in North Sterling and 

Prewitt Reservoirs was substantial. Storage patterns for the 

other three reservoirs showed similar increases through the 

spring and decreases through the summer as water was 

released for irrigation.

 

 High nutrient concentrations have been measured 

in the South Platte River, the source of water to the 

reservoirs. Nutrient concentrations in the South Platte 

River just downstream from Denver, Colo., were 

among the highest measured from 1992 through 1995 

in the 20 river basins sampled throughout the United 

States by the NAWQA Program (Dennehy and others, 

1998). Concentrations in the South Platte River from 

Kersey to Julesburg commonly exceeded 0.15 mg/L 

(milligrams per liter) of total nitrogen and 0.01 mg/L 

of total phosphorus, the generally accepted inflow 

limits for the control of algal blooms in reservoirs 

(U.S. Environmental Protection Agency, 2000).

 

 The reservoirs are filled by diversions from the 

South Platte River, and the amount of water diverted 

from the river in any given year can be substantial. 

The combined storage capacity of the five reservoirs—about 235,000 acre-feet—equals nearly onefourth of the annual streamflow in the South Platte 

River at Kersey, Colo. Water from the reservoirs is 

released through irrigation canals and delivered to 

agricultural fields, where some of the water infiltrates 

down into ground water and ultimately returns to the 

river.

Oct Nov Dec Jan Feb Mar Apr May June July Aug Sept

0

10

20

30

20

40

60

80

0

STORAGE, IN THOUSANDS OF ACRE-FEET

OCTOBER 1994 - SEPTEMBER 1995

North Sterling

Prewitt

0 50 100 MILES

0 50 100 KILOMETERS

South Platte River

Kiowa

Lodgepole

North Fork

Bijou Creek

Creek

Creek

Bear Creek

Elder 

Creek

South Platte River

Cache

Thompson

Creek

Riverside

Reservoir

Big Dry Creek

Clear Creek

Big

River

LaPoudre River

Creek

Box

St.

Vrain

Creek

North Fork Cache

Cherry

Plum

Creek

Beaver Creek

39°

41°

Tarryall

Lonesome Creek

Crow Creek

105°

103°

101°

Julesburg

Reservoir

Jackson

Reservoir

Prewitt

Reservoir

Study area

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

digital data, 1974 to 1993

North

Sterling

Reservoir

Julesburg

Balzac

Weldona Kersey

Boulder

Loveland

Fort Collins

Greeley

Fort Morgan

Cheyenne

Henderson

Sterling

North Platte

DENVER

COLORADO

WYOMING NEBRASKA

WYOMING

NEBRASKA

COLORADO
Figure 3. Concentrations of nitrogen and phosphorus in the South Platte River closely 

corresponded to concentrations in nearby reservoirs, indicating that the river was the 

predominant source of nutrients to the reservoirs.

 The five reservoirs sampled during this study and 

their maximum storage capacities are, in downstream 

order, Riverside (65,000 acre-feet), Jackson 

(35,629 acre-feet), Prewitt (28,840 acre-feet), North 

Sterling (74,010 acre-feet), and Julesburg 

(31,800 acre-feet) (fig. 1). Most of the reservoirs are 

near the flood plain of the South Platte River on fairly 

flat land. As a result, the reservoirs are shallow, with 

mean depths around 20 feet. An exception is North 

Sterling Reservoir, which has a mean depth of about 

30 feet and a maximum depth of about 55 feet. The 

reservoirs typically begin to fill in October and reach 

maximum storage from March through May. The 

volume of water in the reservoirs decreases over the 

summer as water is released for irrigation. An example 

of the seasonal variation in storage for two of the five 

reservoirs is shown in figure 2. 

How did nutrient concentrations in the 

reservoirs change during the irrigation 

season?

 Nutrient concentrations in the South Platte River 

decrease downstream from Denver because of dilution 

of discharge from the basin’s largest wastewater-treatment plants and replacement of diverted river water 

with more dilute ground-water inflow (Litke, 1996). 

As in the South Platte River, reservoir nutrient concentrations generally decreased in the downstream 

direction during March 1995. Nutrient concentrations 

measured in the reservoirs were similar to concentrations at nearby locations on the South Platte River, an 

indication that the river was the predominant source not 

only of water to the reservoirs but also of nitrogen and 

phosphorus (fig. 3). 

 

 Concentrations of total nitrogen, computed as the 

sum of ammonia, nitrate, nitrite, and organic nitrogen, generally were highest in the reservoirs in March 

and decreased through September (fig. 4). In March, 

most of the nitrogen was in the form of nitrate. By late 

summer, concentrations of nitrate, nitrite, and 

ammonia—the forms of nitrogen most readily used 

by algae for growth—generally were below detectable levels in all reservoirs, and only organic nitrogen 

was left in measurable quantities. Early decreases in 

nitrogen concentrations resulted from the inflow of 

more dilute river water and the uptake of nitrogen, 

particularly nitrate, by algae for growth. Once inflows 

from the river were cut off, continued algae growth led 

to further decreases in nitrate concentrations. In eutrophic lakes and reservoirs, low concentrations of nitrate 

resulting from summer algal uptake are not unusual 

(Horne and Goldman, 1994). 

 

 Concentrations of total phosphorus, computed as the 

sum of dissolved orthophosphorus and total organic 

phosphorus, did not decrease in the same way as nitrogen throughout the summer (fig. 4). This difference 

likely was due to the recycling of phosphorus within 

the reservoir by processes such as excretion from fish 

and zooplankton, decomposition of dead plants and 

animals, and the release of phosphorus from reservoir 

bottom sediment during periods of anoxia (the absence 

0

9

3

6

EXPLANATION

Reservoirs, March 1995

South Platte River sites,

 October 1994–March 1995

Kersey

Riverside

Jackson

Weldona

North Sterling

Balzac

Prewitt

Julesburg

Julesburg

0

1.0

0.2

0.4

0.6

0.8

MEDIAN CONCENTRATION, IN

MILLIGRAMS PER LITER

TOTAL NITROGEN

TOTAL PHOSPHORUS
Figure 4. Nitrogen concentrations generally decreased from March through September, largely as a result 

of uptake by algae for growth. Phosphorus concentrations were more variable because of the recycling of 

phosphorus within the reservoir.

of oxygen) (Horne and Goldman, 1994). Concentrations of orthophosphorus—the form of phosphorus most 

readily used by algae for growth—fluctuated during the 

summer, evidence that phosphorus was recycled back 

into the water after biological uptake.

 Unlike nutrient concentrations in the other four reservoirs, concentrations of both nitrogen and phosphorus 

in Julesburg Reservoir increased substantially in 

September. Julesburg Reservoir was the only one of the 

five reservoirs that received late summer inflow from 

the South Platte River.

Did the reservoirs remove nutrients from 

the stored water before it was released for 

irrigation?

 With the exception of phosphorus in Jackson 

Reservoir, the mass of nutrients leaving the reservoirs 

in irrigation water was less than the mass that came into 

the reservoir during the study period. The percentage of 

the total nutrient mass trapped in the reservoir through 

processes such as biological uptake and sedimentation

of nutrients bound to sinking particles ranged from 

Table 1. The percentage of nutrient mass trapped in the reservoir from 

March through September 1995. Reservoirs with long water residence 

times generally trapped a higher percentage of nutrients through 

processes like biological uptake and sedimentation

Organic nitrogen

Ammonia

Nitrite

Nitrate

NITROGEN

EXPLANATION

Organic phosphorus

Orthophosphorus

PHOSPHORUS

0

0

0.4

0.2

0.4

0.2

0

6

3

0

6

3

0

0.4

0.2

0

0.4

0.2

0

0.8

0.2

0.4

0.6

March June July Sept March June July Sept

1995 1995

March June July Sept

1995

March June July Sept

1995

0

6

3

MEDIAN CONCENTRATION, IN MILLIGRAMS PER LITER

AS NITROGEN OR PHOSPHORUS

0

6

3

0

6

3

JACKSON PREWITT

RIVERSIDE

NORTH STERLING JULESBURG

Measurements Riverside Jackson Prewitt

North

Sterling Julesburg

Nitrogen,

 percent trapped

68 63 88

86

59

42

49

Phosphorus,

 percent trapped

25 0 20

Residence time

 (days)

149 469 57 32 82
49 to 88 percent for nitrogen and from 0 to 86 percent 

for phosphorus (table 1). Generally, reservoirs with 

shorter water residence times—the time necessary for 

the volume of water in a reservoir to be removed by 

outflowing water—trapped a smaller percentage in the 

reservoir because some of the nutrient mass was flushed 

out before it could be used by algae.

 There were several exceptions to this general conclusion. Jackson Reservoir gained nearly as much phosphorus as it trapped within the reservoir during the 

irrigation season, resulting in no net loss. Outflow from 

the reservoir was minimal until August; therefore, the 

long period of storage allowed phosphorus to be regenerated in the reservoir without being removed in the 

outflow. In North Sterling Reservoir, large outflows from 

July through September flushed out a large part of the 

recycled phosphorus before it could be trapped within 

the reservoir again.

Table 2. Concentrations of chlorophyll-a indicated that the 

amount of algae increased during the summer in all reservoirs

(μg/L, micrograms per liter)

How were the changes in nutrient 

concentrations related to the growth of 

algae in the reservoirs?

 The amount of algae (indicated by the concentration 

of chlorophyll-a, a green pigment in algae) increased 

during the summer in all reservoirs (table 2). Because 

algae used nitrogen for growth, the increase in the 

amount of algae generally corresponded to the decrease 

in nitrogen concentrations during the summer. In Jackson Reservoir, excessive algal growth near the water 

surface during July may have blocked the light necessary to sustain growth from reaching underlying water, 

leading to a drop in chlorophyll-a concentrations by 

September.

 

 Chlorophyll-a concentrations also provided an 

indication of the trophic state of each reservoir, a 

relative classification that reflects the nutrient enrichment in a water body. Higher average concentrations 

of chlorophyll-a in the range from 2.7 to 150 μg/L are 

characteristic of eutrophic reservoirs with high nutrient 

concentrations, low light penetration, high amounts of 

algae, and frequent algal blooms (Organization for 

Economic Cooperation and Development, 1982). 

Average concentrations of chlorophyll-a measured 

during the summer of 1995 indicate that all five 

reservoirs were eutrophic (table 2). Eutrophication may 

result in deterioration of water quality and aquatic life 

in a reservoir. 

SUMMARY

 The results of this study indicate that the practice of 

storing South Platte River water in offstream reservoirs substantially decreases nitrogen concentrations 

during the irrigation season. Nitrogen concentrations 

in the reservoirs were highest in March and decreased 

through September, largely as a result of nitrate uptake 

by algae. Phosphorus concentrations in the reservoirs 

did not show the same consistent decrease from March 

through September, likely because phosphorus was 

continuously recycled back to algae through processes 

such as excretion from fish, decay of aquatic plants and 

animals, and release of orthophosphorus from bottom 

sediment during periods of low oxygen. Overall, with 

the exception of phosphorus in Jackson Reservoir, 

the reservoirs acted as a trap for both nitrogen and 

phosphorus. Associated with the loss of nitrogen and 

phosphorus, however, was an increase in the amount of 

algae, which may have adversely affected the recreational use of the reservoirs.

May/June 1.8 2.5 1.6 26 1.0

July 7.0 150 23 14 14

September

Average

33 72

75

37 65 150

14 21 35 55

Riverside

(μg/L)

Date Jackson

(μg/L)

Prewitt

(μg/L)

North

Sterling

(μg/L) 

Julesburg

(μg/L)

High nutrient concentrations can contribute to excessive algal growth, 

as seen here in Julesburg Reservoir.
Printed on recycled paper

REFERENCES

Dennehy, K.F., Litke, D.W., Tate, C.M., Qi, S.L., 

 McMahon, P.B., Bruce, B.W., Kimbrough, R.A., 

 and Heiny, J.S., 1998, Water quality in the South 

 Platte River Basin, Colorado, Nebraska, and Wyoming,

 1992–95: U.S. Geological Survey Circular 1167, 37 p.

Horne, A.J., and Goldman, C.R., 1994, Limnology: 

 New York, McGraw-Hill, 576 p.

Litke, D.W., 1996, Sources and loads of nutrients in the

 South Platte River, Colorado and Nebraska, 1994–95: 

 U.S. Geological Survey Water–Resources

 Investigations Report 96–4029, 31 p.

FOR MORE INFORMATION

This fact sheet is based on the following report:

Sprague, Lori A., 2002, Nutrient dynamics in five offstream reservoirs in the Lower South Platte River Basin, 

March - September 1995: U.S. Geological Survey Water-Resources Investigations Report 02–4142, 

72 pages.

For more information on the South Platte NAWQA study, please contact:

Project Chief, South Platte River Basin NAWQA

U.S. Geological Survey

P.O. Box 25046, MS 415

Lakewood, CO 80225

Or visit our Website at:

http://co.water.usgs.gov/nawqa/splt/

Additional information on NAWQA activities nationwide can be found at:

http://water.usgs.gov/nawqa/

For more information on USGS activities in Colorado, please contact:

District Chief

Water Resources Discipline

U.S. Geological Survey

P.O. Box 25046, MS 415

Lakewood, CO 80225

Or visit the Colorado District Website at:

http://co.water.usgs.gov/

Organization for Economic Cooperation and

 Development (OECD), 1982, Eutrophication of

 waters—Monitoring, assessment, and control: 

 Paris, OECD Cooperative Programme on

 Monitoring of Inland Waters, 154 p.

U.S. Environmental Protection Agency, 2000, Nutrient 

 criteria technical guidance manual—Lakes and

 reservoirs: U.S. Environmental Protection Agency

 Report 822–B00–001 [variously paged].