INTRODUCTION

Two main-stem reservoirs on the

lower Arkansas River, Pueblo Reservoir

and John Martin Reservoir, are used to regulate and manage streamflow for irrigation

and domestic water supplies, flood control,

and recreation. Water storage and release

operations for these reservoirs can affect

streamflow and water quality of the downstream river. One water-quality property

that can be affected is specific conductance.

In 1996, the U.S. Geological Survey completed a study of the effects of Pueblo

Reservoir and John Martin Reservoir operations on specific conductance at five sites

located on the Arkansas River between

Pueblo Reservoir and Lamar (fig. 1).

This fact sheet presents an overview of

the study findings. A detailed discussion of

the study results is presented in Lewis and

Brendle (1998).

Specific Conductance

and Dissolved Solids

 Specific conductance, in microsiemens per centimeter at 25 degrees Celsius

(μS/cm), is a property of water that is

attributable to salinity, or dissolved solids,

which is a measure of salts such as sodium

chloride or calcium bicarbonate. Pure water

has a very low specific conductance—a few

hundredths of a microsiemen per centimeter. As the dissolved-solids concentration

of water increases, the specific conductance

increases (fig. 2).

Specific Conductance

Can Affect Water

Suitability

High specific conductance indicates

high dissolved-solids concentration;

dissolved solids can affect the suitability

of water for domestic, industrial, and agricultural uses. The secondary maximum

contaminant level for dissolved solids in

drinking water is 500 milligrams per liter

(mg/L) (U.S. Environmental Protection

Agency, 1986). In the lower Arkansas

River, 500 mg/L of dissolved solids is

equivalent to a specific conductance of

about 700 to 800 μS/cm (Cain, 1987).

At higher levels, drinking water may have

an unpleasant taste or odor or may even

cause gastrointestinal distress. Additionally,

high dissolved-solids concentration can

cause deterioration of plumbing fixtures

and appliances. Relatively expensive

water-treatment processes, such as reverse

osmosis, are needed to remove excessive

dissolved solids from water.

Agriculture also can be adversely

affected by high-specific-conductance

water. Depending on the crop, agricultural

losses might occur when salinity reaches

700 to 850 mg/L (U.S. Department of the

Interior, 1994), which is equivalent to

a specific conductance of about 950 to

1,200 μS/cm in the Arkansas River (Cain,

1987). The salinity hazard of irrigation

water is a relation developed by the

U.S. Salinity Laboratory (Richards,

1954) that describes the qualitative effect

of saline water on irrigated crops. The

hazard is based on the specific conductance

of the water and is divided into four classes

of salinity hazard ranging from low (less

than 250 μS/cm) to very high (greater than

2,250 μS/cm). As specific conductance

increases, special agricultural management

practices may be needed, and crops having

a substantial salinity tolerance may need

to be grown. Generally, crops with a

higher salinity tolerance have a lower

market value than more salt-sensitive

crops (Miles, 1977).

Purgatoire River

River

River

Apishapa

Huerfano

St. Charles River

Big

Sandy

Creek

Fountain Creek NEW MEXICO OKLAHOMA

KANSAS

EXPLANATION

WATER-QUALITY

 SAMPLING SITE

Study area

C O L O R A D O

0 25 50 75 MILES

0 25 50 75 KILOMETERS

Colorado

Springs

Lamar

Las Animas

La Junta

Nepesta

Trinidad

Salida

Buena

Vista

Leadville

Granite

Canon

City

Pueblo

Portland Avondale

Pueblo

John Martin Reservoir Pueblo

Reservoir

~

37°

38°

39°

106°

105°

104°

103°

102°

DENVER

RIVER

ARKANSAS

Figure 1. Location of study area.

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 166–97

July 1998

Relations of Main-Stem Reservoir Operations and Specific Conductance

in the Lower Arkansas River, Southeastern Colorado
Specific Conductance

in the Arkansas River

The specific conductance of water

in the lower Arkansas River markedly

increases between Pueblo Reservoir

and Lamar, about 160 miles (mi) downstream (fig. 3). Low-specific-conductance

snowmelt runoff (generally less than

200 μS/cm) from the upper Arkansas River

Basin is the primary source of streamflow

in the lower Arkansas River. Downstream

from Pueblo Reservoir, the large increase

in specific conductance (fig. 3) is attributable to natural and human effects, but the

primary cause is the continual use and reuse

of water for agricultural irrigation (Cain,

1987). When water is used for irrigation,

part of the water evaporates or is consumed

by plants, concentrating the original amount

of dissolved solids in less water; thus, the

dissolved-solids concentration and the specific conductance in the remaining water is

increased. The remaining higher specificconductance water reenters the river as

irrigation-return flow. The rate of increase

in specific conductance in the lower

Arkansas River tends to increase downstream because high-specific-conductance

irrigation-return flows compose an increasingly large percentage of streamflow (Cain,

1987).

Specific conductance also varies

during the year as a result of the temporal

variability of streamflow. Specific conductance generally is lowest in May–August,

when streamflow generally is largest,

and increases with decreasing streamflow

in the fall, winter, and spring (fig. 4).

PUEBLO RESERVOIR

OPERATIONS

Storage began in Pueblo Reservoir in

1974, and the dam was completed in 1975

on the main stem of the Arkansas River near

Pueblo. The operation of Pueblo Reservoir,

in particular the Winter Water Storage

Program (WWSP), has a notable effect on

streamflow in the lower Arkansas River.

The WWSP allows participating irrigation

canal companies, which have historically

diverted winter flows from the Arkansas

River, to store water during the winter in

Pueblo Reservoir, in John Martin Reservoir,

and in several small off-channel reservoirs.

Prior to the WWSP, farmers diverted

Arkansas River streamflow onto fallow

fields for winter irrigation. This practice

was followed to increase soil moisture for

later use by crops during the growing season. Winter irrigation is inefficient, however, and the WWSP replaced this practice.

Water stored during the winter as part of

the WWSP can be released to the river

and used by participating canal companies

throughout the year at times when natural

streamflow is insufficient to meet irrigationwater needs. The WWSP began in 1975

and has been in operation every year since

then, except for the 1977–78 winter-storage

period. During 1975–94, the median annual

volume of WWSP water stored in Pueblo

Reservoir was about 42,200 acre-feet

(acre-ft) (Thomas C. Simpson, Southeastern

Colorado Water Conservancy District,

written commun., 1997).

PUEBLO RESERVOIR

OPERATIONS HAVE

AFFECTED SPECIFIC

CONDUCTANCE IN

THE ARKANSAS RIVER

Specific-conductance data collected at three Arkansas River sites located

between Pueblo Reservoir and John Martin

Reservoir (fig. 1) were analyzed for

changes that might have occurred after

Pueblo Reservoir was constructed. These

three sites are Pueblo, Avondale, and

300 700 400 500 600

SPECIFIC CONDUCTANCE, IN MICROSIEMENS PER 

CENTIMETER AT 25 DEGREES CELSIUS

150

500

200

250

300

350

400

450

DISSOLVED-SOLIDS CONCENTRATION, 

IN MILLIGRAMS PER LITER

0 170 50 100 150

MILES DOWNSTREAM FROM PUEBLO RESERVOIR

0

4,000

1,000

2,000

3,000

MEDIAN SPECIFIC CONDUCTANCE, IN

MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

Figure 2. Relation of dissolved-solids concentration

to specific conductance in water samples collected

in the Arkansas River at Pueblo, 1990–93.

Figure 3. Median specific conductance in the

Arkansas River, 1990–93.
Las Animas. Data collected at each site

were separated into two seasons: growing

season (March 16–November 14) and

winter-storage season (November 15–

March 15). Data collected prior to 1975

were compared to data collected from

1975 through 1994. At individual stations,

the record for specific-conductance data

generally was shorter than the record for

streamflow. Therefore, the streamflow

record for individual stations was shortened

to match the specific-conductance record

for that station. This matching of records

facilitated the direct comparison of streamflow and specific-conductance trends at

individual stations.

Arkansas River

Above Pueblo

After construction of Pueblo

Reservoir, streamflow in the Arkansas

River upstream from Pueblo increased

during the growing season and decreased

during the winter-storage season, largely

due to the effects of the WWSP. Because

specific conductance tends to be inversely

proportional to streamflow, specific conductance at this site would be expected to

increase during the winter-storage season

and decrease during the growing season.

The opposite occurred. The median specific

conductance increased slightly from 460 to

465 μS/cm between 1966–74 and 1975–94

during the growing season and decreased

substantially from 720 to 575 μS/cm during

the winter-storage season (fig. 5).

The changes in specific conductance after construction of Pueblo

Reservoir are not a direct function of

changes in streamflow, but are more

directly related to the storage and subsequent mixing of seasonally low-specificconductance water and seasonally highspecific-conductance water in Pueblo

Reservoir. Low-specific-conductance

water generally flowed into the reservoir

during May–August and mixed with highspecific-conductance water that flowed

into the reservoir during the fall, winter,

and spring. The result was a narrowing

of the range in specific-conductance

values after 1974.

Arkansas River

Near Avondale

After construction of Pueblo

Reservoir, streamflow near Avondale

increased during the growing season

and decreased during the winter-storage

season. The decreased winter streamflow

was caused by storage of WWSP water in

Pueblo Reservoir. The increase in streamflow during the growing season partly is

attributable to the release of WWSP water

from Pueblo Reservoir. Increased inflow

from Fountain Creek (fig. 1) also accounts

for some of the increased streamflow.

Population growth in the Colorado Springs

area and the associated increase in unit

runoff and wastewater discharge are

partly responsible for the increase in the

median annual streamflow of Fountain

Creek at Pueblo, which increased

from about 37,000 acre-ft per year in

1969–74 to about 67,000 acre-ft per

year in 1975–94.

Specific conductance near Avondale

changed appreciably after 1974 (fig. 6),

but the changes were different from the

changes observed at the site upstream

from Pueblo (fig. 5). Between 1969–74 and

1975–94, the median specific conductance

increased from 580 to 700 μS/cm during

the growing season and increased from 900

to 1,060 μS/cm during the winter-storage

season (fig 6).

 The combined effect of reservoir

operations, as observed at the site upstream

from Pueblo, and an increase in the inflow

of dissolved solids from Fountain Creek

apparently caused the changes in specific

conductance near Avondale. Although

specific conductance in Fountain Creek

decreased after 1974, it remained substantially higher than in the Arkansas

River near Avondale. The increase in

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

1987

3,500

0

500

1,000

1,500

2,000

2,500

3,000

MONTHLY MEAN STREAMFLOW, 

IN CUBIC FEET PER SECOND

400

1,100

500

600

700

800

900

1,000

EXPLANATION

STREAMFLOW SPECIFIC CONDUCTANCE

MONTHLY MEAN SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

SEASON

GROWING WINTERSTORAGE

SEASON

400

800

500

600

700

1966-74

1975-94

MEDIAN SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

Figure 4. Relation of streamflow to specific

conductance for the Arkansas River

near Avondale, 1987.

Figure 5. Specific conductance for the

Arkansas River above Pueblo, 1966–94.
streamflow combined with the high specific

conductance in Fountain Creek resulted

in an estimated 58-percent increase in the

dissolved-solids load contributed to the

Arkansas River between 1969–74 and

1975–94. Pueblo Reservoir operations

decreased streamflow in the Arkansas

River during the winter-storage season.

As a result, the increased dissolved-solids

load from Fountain Creek was subject to

less dilution at the site near Avondale;

therefore, specific conductance increased

during the growing season between

1969–74 and 1975–94 (fig. 6). During

the growing season, specific conductance

in the reservoir outflow increased and the

dissolved-solids load from Fountain Creek

increased, which combined to increase

specific conductance near Avondale.

Arkansas River

at Las Animas

Streamflow at Las Animas is

substantially smaller than at the upstream

sites above Pueblo and near Avondale.

The downstream decrease in streamflow

is attributable to irrigation diversions

upstream from Las Animas. Much of

the streamflow at Las Animas is from

irrigation-return flow (Cain, 1987). After

1974, streamflow at Las Animas increased

substantially in every month. The increase

in streamflow at Las Animas probably was

caused by a combination of factors, including the WWSP and associated changes

in seasonal irrigation practices and the

increased importation of Colorado River

Basin water for irrigation. Prior to the

construction of Pueblo Reservoir and the

beginning of the WWSP, most winter

streamflow was diverted and consumed

upstream from Las Animas. With the

adoption and full implementation of the

WWSP and the increased use of transmountain water for irrigation, streamflow

at Las Animas increased significantly.

Prior to 1978, about 14 percent of the

annual streamflow at the upstream end

of the lower basin flowed past Las Animas.

After 1978, that percentage increased to

about 26 percent.

The increase in streamflow at

Las Animas increased the dilution potential

and resulted in substantial decreases in

specific conductance between 1961–74 and

1975–94 (fig. 7). The median specific conductance decreased during the growing

season from 2,820 μS/cm in 1961–74 to

2,250 μS/cm in 1975–94 (fig. 7). Similarly,

the median specific conductance decreased

during the winter-storage season from

3,300 μS/cm in 1961–74 to 2,730 μS/cm

in 1975–94 (fig. 7). The increase in streamflow and dilution potential at Las Animas

apparently was large enough to offset the

increasing specific-conductance trend in the

Arkansas River measured farther upstream

near Avondale.

JOHN MARTIN

RESERVOIR

OPERATIONS

John Martin Reservoir was completed

in 1948 on the main stem of the Arkansas

River between Las Animas and Lamar.

Storage of irrigation water in John Martin

Reservoir is by agreement between the

States of Colorado and Kansas, under the

Arkansas River Compact, which was signed

in 1948. The Compact ensures that both

States receive their percentage shares of

Arkansas River flows (Abbott, 1985).

The Compact dictates a winter- and a

summer-storage period. During the winterstorage period, November 1 to March 31,

most of the reservoir inflow is stored.

Prior to 1980, provisions were made for the

release of stored water, without reference to

the volume of stored water assigned to each

State. To ensure that each State received

its share of stored water, release demands

by each State were made concurrently.

Although the Compact was to ensure that

Colorado and Kansas irrigators received

their legal shares of Arkansas River water,

it did not “result in the most efficient utilization possible of the water in its control”

(Arkansas River Compact Administration,

1980, p. 40). Prior to 1980, after the winterstorage period, reservoir storage usually

was drawn down to empty or almost empty

very early in the irrigation season, often by

the middle of April (fig. 8). From 1955

through 1979, reservoir storage was completely depleted by April 30 in 15 of the

25 years.

To help alleviate the problems

associated with historical reservoir operations, a new reservoir operating plan was

adopted in 1980 (Arkansas River Compact

Administration, 1980). Under the new

plan, water not immediately called for

and released to downstream irrigators

SEASON

GROWING WINTERSTORAGE

SEASON

0

1,500

500

1,000

1969-74

1975-94

MEDIAN SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

SEASON

GROWING WINTERSTORAGE

SEASON

2,200

3,400

2,400

2,600

2,800

3,000

3,200

1961-74

1975-94

MEDIAN SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

Figure 6. Specific conductance for the

Arkansas River near Avondale, 1969–94.

Figure 7. Specific conductance for the

Arkansas River at Las Animas, 1961–94.
was stored in separate storage accounts

for Colorado and Kansas. One State can call

for the release of its stored water independently of the other State. This change in

operating practices generally resulted in

downstream irrigators waiting until later in

the irrigation season to release their stored

water to the river; therefore, the 1980 operating plan has probably contributed to

increased storage in John Martin Reservoir

(fig 8). Storage of water in John Martin

Reservoir also has benefited from increased

inflow, as noted in the “Arkansas River

at Las Animas” section. At Las Animas,

located about 3 mi upstream from

the reservoir, median annual streamflow of the Arkansas River increased

142 percent from 76,400 acre-ft in

1955–79 to 185,200 acre-ft in

1980–94.

JOHN MARTIN RESERVOIR

OPERATIONS HAVE

AFFECTED SPECIFIC

CONDUCTANCE IN THE

ARKANSAS RIVER

Specific-conductance data from

two Arkansas River sites located downstream from John Martin Reservoir

(fig. 1) were analyzed for changes that

might have occurred after the adoption

of the 1980 operating plan. These two

sites are downstream from John Martin

Reservoir and at Lamar. Data collected at

each site were separated into two seasons:

growing season (April 1–October 31)

and winter-storage season (November 1–

March 31). Data collected prior to 1980

were compared to data collected from

1980 through 1994.

Arkansas River Below

John Martin Reservoir

After the adoption of the 1980

operating plan, streamflow at the site

downstream from John Martin Reservoir

increased substantially during the growing

season and decreased slightly during the

winter-storage season. The largest increase

in the daily mean streamflow for any month

was in July; the daily mean streamflow

increased from 490 cubic feet per second

(ft3/s) in 1955–79 to 878 ft3/s in 1980–94.

The overall increase in streamflow for

1980–94 was attributable to increased

inflow into the reservoir and subsequent

reservoir releases. In addition to increased

inflow, a large increase in streamflow during the middle of the growing season probably was attributable to the new operating

plan, which allows downstream irrigators to

release the stored water later in the growing

season rather than at the very beginning of

the growing season.

Specific conductance changed

appreciably after the implementation of

the 1980 operating plan (fig. 9). During the

growing season, the median specific conductance decreased from 2,180 μS/cm in

1955–79 to 2,050 μS/cm in 1980–94. During the winter-storage season, the median

specific conductance decreased substantially from 3,650 μS/cm in 1955–79 to

2,640 μS/cm in 1980–94. The overall

decrease in specific conductance was a

result of two main factors. The first was

decreased specific conductance in the reservoir inflow, as indicated at the Las Animas

site. At Las Animas, the median specific

conductance for the growing season

decreased from 2,655 μS/cm in 1961–79

to 1,855 μS/cm in 1980–94, and the median

specific conductance for the winter-storage

season decreased from 3,500 μS/cm in

1964–79 to 2,710 μS/cm in 1980–94.

The second factor was increased reservoir

inflow and reservoir storage and a corresponding increase in mixing of seasonally

low-specific-conductance water and seasonally high-specific-conductance water in the

reservoir; thus, the range in specific conductance narrowed after 1980.

Arkansas River

at Lamar

Changes in streamflow and specific

conductance measured at the Lamar site

were very similar to changes at the site

downstream from John Martin Reservoir.

Streamflow at Lamar increased substantially during the growing season; the largest

increases occurred during July. However,

streamflow also increased slightly during

the winter-storage season.

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC 0

140,000

20,000

40,000

60,000

80,000

100,000

120,000

RESERVOIR CONTENTS, IN ACRE-FEET

1955-79

1980-94

SEASON

GROWING WINTERSTORAGE

SEASON

2,000

4,000

2,500

3,000

3,500

MEDIAN SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

1955-79

1980-94

Figure 8. Median end-of-month contents of

John Martin Reservoir, 1955–79 and 1980–94.

Figure 9. Specific conductance for

the Arkansas River below John Martin

Reservoir, 1955–94.
During the growing season, the

median specific conductance decreased

from 3,400 μS/cm in 1964–79 to

2,995 μS/cm in 1980–94 (fig. 10). During

the winter-storage season, the median

specific conductance decreased from

4,900 μS/cm in 1964–79 to 4,375 μS/cm in

1980–94 (fig. 10). In addition to increased

winter streamflow downstream from the

reservoir and an associated increase in the

dilution potential of the river, the same

factors that affected specific conductance

at the site downstream from John Martin

Reservoir probably caused the changes in

specific conductance at Lamar.

CONCLUSIONS

An analysis of historical specificconductance data indicates that specific

conductance in the Arkansas River changed

as the result of main-stem reservoir operations. Specific conductance upstream from

Pueblo and at Las Animas tended to

decrease following the construction of

Pueblo Reservoir. Likewise, specific conductance downstream from John Martin

Reservoir and at Lamar decreased after

implementation of the 1980 John Martin

Reservoir operating plan. The decrease in

specific conductance at the site upstream

from Pueblo is beneficial from a municipal

drinking-water-supply perspective because

streamflow in this area provides drinking

water to the greater Pueblo area. Although

specific conductance increased at the site

near Avondale, the increase after 1974 was

not large enough to change the salinity hazard for irrigated agriculture. Although specific conductance decreased at Las Animas,

downstream from John Martin Reservoir,

and at Lamar, the decrease was not large

enough to change the salinity hazard for

irrigated agriculture. The salinity hazard

at all three sites remained high to very

high (greater than 2,250 μS/cm).

REFERENCES

Abbott, P.O., 1985, Description of watersystems operations in the Arkansas

River Basin, Colorado: U.S. Geological Survey Water-Resources Investigations Report 85–4092, 67 p.

Arkansas River Compact Administration,

1980, Thirty-second annual report:

Lamar, Colorado, 50 p.

Cain, Doug, 1987, Relations of specific

conductance to streamflow and

selected water-quality characteristics

of the Arkansas River Basin,

Colorado: U.S. Geological Survey

Water-Resources Investigations

Report 87–4041, 93 p.

Lewis, M.E., and Brendle, D.L., 1998,

Relations of streamflow and specificconductance trends to reservoir operations in the lower Arkansas River,

southeastern Colorado: U.S. Geological Survey Water-Resources Investigations Report 97–4239, 48 p.

Miles, D.L., 1977, Salinity in the Arkansas Valley of Colorado: Denver,

U.S. Environmental Protection

Agency and Colorado State

University, Interagency Agreement, EPA–IAG–D4–0544, 80 p.

Richards, L.A., ed., 1954, Diagnosis and

improvement of saline and alkali

soils: Washington, D.C., U.S. Department of Agriculture Handbook 60,

160 p.

U.S. Department of the Interior, 1994,

Salinity update: Denver, Bureau of

Reclamation, 17 p.

U.S. Environmental Protection Agency,

1986, Secondary maximum contaminant levels (section 143.3 of part 143,

National secondary drinking-water

regulations): U.S. Code of Federal

Regulations, Title 40, Parts 100–149,

p. 587–590.

This study was conducted in cooperation with the Colorado Springs Utilities;

Pueblo Board of Water Works; Southeastern Colorado Water Conservancy

District; Pueblo County, Department of

Planning and Development; city of Aurora,

Department of Utilities; St. Charles Mesa

Water District; Upper Arkansas Area

Council of Governments; Upper Arkansas

Water Conservancy District; city of Pueblo,

Department of Utilities; Pueblo West

Metropolitan District; Fremont Sanitation

District; and the cities of Rocky Ford,

Las Animas, and Lamar.

—Michael E. Lewis,

Water Resources Division,

Pueblo, CO

GROWING

SEASON

WINTERSTORAGE

SEASON

5,000

2,500

3,000

3,500

4,000

4,500

1964-79

1980-94

MEDIAN SPECIFIC CONDUCTANCE,

IN MICROSIEMENS PER CENTIMETER

AT 25 DEGREES CELSIUS

Figure 10. Specific conductance for the

Arkansas River at Lamar, 1964–94.

Information on technical reports and hydrologic

data related to this study can be obtained from:

Subdistrict Chief

U.S. Geological Survey

201 West 8th Street, Suite 200

Pueblo, CO 81003

(719) 544–7155, ext. 130