U.S. Department of the Interior

U.S. Geological Survey Water-Resources Investigations Report 98-4236

INTRODUCTION

The Eagle River flows from the Continental Divide 

through the towns of Minturn, Avon, Edwards, Wolcott, Eagle, 

and Gypsum and joins the Colorado River near Dotsero, 

Colorado (fig. 1). Drainage area of the watershed is approximately 950 square miles. Land use in the basin is predominantly forest and rangeland with agriculture and increasing 

urban/recreational land uses along the river corridor. The 

agriculture primarily is hay fields, which require no fertilization. The population of Eagle County, which encompasses the 

entire Eagle River drainage, has increased by approximately 

200 percent between 1970 and 1990 (U.S. Bureau of the 

Census, 1970 and 1990). Influxes of skiers to the county during 

the winter cause temporary population increases. State and 

local agencies are concerned about the effects of increasing 

urban development and the influx of people during winter on 

water quality and stream biota in the Eagle River. One approach 

to determine land-use effects on streams is by monitoring water 

quality over time. A water-quality assessment needs to include 

the evaluation of chemistry, habitat, and biological conditions of 

a stream. The U.S. Geological Survey (USGS) in cooperation 

with the Colorado Department of Public Health and Environment (CDPHE) established five sites in 1997 on the Eagle River 

to characterize baseline water quality and stream biota. This 

baseline information will be useful for future trend assessments 

of water quality and stream biota at these sites. 

 Five sites were selected on the Eagle River to collect 

water-quality and stream-biota data during two low-flow periods (fig. 1). Site 1 represents minimal effects from urban/recreational land use. Sites 2, 3, and 4 are within areas of increasing 

urban/recreational land use. Site 5 is near the mouth of the Eagle 

River and represents the outlet from the study area and the 

Water-Quality and Biological Community Characterization at 

Selected Sites on the Eagle River, Colorado, September 1997 

and February 1998

fig 1

Figure 1. Location of study area, sampling sites, and general land use in the Eagle River Basin.

EXPLANATION

Land Use

Urban/Recreational

Agriculture

Range

Forest

Water

Wetland

Barren

Tundra

Sampling locations

0

0 10 20 KILOMETERS

10 20 MILES

EAGLE RIVER BASIN

COLORADO

1

2

3

4

5

5

Minturn

Vail

Redcliff

Avon

Edwards

Wolcott

Eagle

Dotsero

Grand

Junction

DENVER

Gypsum Eagle

River

COLORADO

RIVER

N
2

cumulative effects of all land uses. Low-flow periods can be

important from water-quality and stream-ecology perspectives

because less water is available to dilute any contaminants

entering the streams.

This study was based on a limited data set from only two

sampling periods. The objectives of this study were to (1) characterize the current water quality and biological community at

selected sites in the Eagle River; (2) assess relations among

nutrient concentrations and stream biota at selected sites; (3)

describe changes in the algal community over time at one site;

and (4) assess differences in summer low-flow conditions and

peak winter recreational-use low-flow conditions. This report

presents results of field measurements and chemical (major

ions and nutrients) and biological (benthic algae and macroinvertebrates) sampling during summer (1997) and winter

(1998) low-flow conditions.

DATA COLLECTION

Water and biological samples were collected in September 1997 and February 1998. Water samples were collected

using the equal-width-increment method and processed using

standard surface-water protocols (Shelton, 1994). Water

samples were analyzed for major ions and nutrients. Field

measurements consisted of temperature, pH, specific conductance, dissolved oxygen, and streamflow. Habitat characteristics were documented at each site using Rapid Bioassessment

Protocols (Barbour and others, 1997). A composite of five each

of algae and macroinvertebrate samples was collected in riffle

areas in the stream and analyzed for species composition and

abundance and biomass for algal samples (Cuffney and others,

1993; Porter and others, 1993).

WATER QUALITY

Hydrology and MajorIon Water Chemistry

Streamflow, major-ion

chemistry, and field measurements represent a starting

point for the basic assessment

of general water quality.

Continuous streamflow data

are available at USGS gaging

stations for sites 2 and 5.

Hydrographs for these sites are

shown in figure 2. Streamflow

during the September 1997

and February 1998 sampling

periods (fig. 2) represents the

low-flow conditions for the

Eagle River. The September

sampling period represents the

lower part of the recession of

the annual snowmelt runoff

peak, and the February sampling period represents the winter

base-flow condition. Streamflow generally increased downstream, and the flows were greater during the September

sampling period as compared to the February sampling period.

Based on water years 1993–97, the mean daily flows for

September at sites 2 and 5 are 158 and 298 ft3/s (cubic feet per

second). Flows during the September sampling at these two

sites (204 and 331 ft3/s) were slightly greater than the longer

term mean flows. Flows measured during the February sampling at sites 2 and 5 were 60 and 207 ft3/s. These values are

similar to the February mean daily flows (based on water years

1993–97) of 64 and 176 ft3/s for sites 2 and 5, respectively.

Major-ion chemistry (ions that represent the majority of

the dissolved-solids content [Hem, 1992]) and field measurements commonly change as the sources of water generating the

streamflow change. Concentrations of the major ions from the

samples collected during the two periods are listed in table 1.

The water chemistry changed in a downstream direction as the

water flowed from the higher mountainous areas down into the

lower Eagle River Basin where sedimentary rocks predominate. The Eagle Valley Evaporite and the Eagle Valley Formation are common in the lower part of the study area at sites 4, 5,

and to some extent site 3 (Tweto and others, 1978). The sites in

the lower part of the study area, especially site 4, had greater

percentages of sodium and chloride than sites upstream.

Concentrations of the major ions increased downstream and

were greater in the February samples than in the September

samples. Water chemistry was different between the two

sampling periods possibly because the percentage of flow

contributed by or interacting with ground water increased. For

example, the percentage of sulfate was generally greater in the

February than the September samples.

photo 1

Low-flow conditions at site 3 in September Photograph by Cory Stephens
3

Data from field measurements for the two sampling

periods are shown in figure 3. The specific conductance of the

water (an indication of the dissolved ionic concentration)

increased rapidly as the water moved between sites 3 and 4,

probably as a result of point and nonpoint influences and interactions with the sedimentary geology of the lower Eagle River

Basin. Dissolved oxygen was at or near saturation at all sites

during both data-collection periods. The greater dissolvedoxygen concentrations for February are a result of the colder

water temperatures (water temperatures during the September

sampling period ranged from 13 to 18.5 degrees Celsius and

during the February sampling period ranged from 2 to

4.5 degrees Celsius). The pH ranged from 8.1 to 8.8, and the

maximum value was measured at site 4 during both periods.

Nutrients

Nutrients (nitrogen and phosphorus) are essential for

plant growth. Algae and aquatic vegetation depend on nitrogen

and phosphorus compounds for nutrient supply (Hem, 1992).

Excessive concentrations of nutrients may lead to an overabundance of aquatic plants. Selected results from the sampling for nutrients are shown in figure 4. Ranges of concentrations for all samples collected by other programs of the USGS

during water years 1993 to 1997 for sites 2 and 5 also are

shown in figure 4. The ranges of 1993 to 1997 data are shown

to provide comparison of data collected for this study to longer

term data and concentrations that represent other seasons. Data

are from the U.S. Geological Survey National Water Information System (NWIS).

STREAMFLOW, IN CUBIC FEET PER SECOND

DATE

Eagle River at Gypsum, site 5

Eagle River at Avon, site 2

Sample for this study

6,000

5,000

4,000

3,000

2,000

1,000

0

09/01/96 12/01/96 03/01/97 06/01/97 09/01/97 12/01/97 03/01/98

Figure 2. Annual hydrograph and streamflow during the sampling period.

Table 1. Major-ion concentrations [concentrations in milligrams per liter; <, less than]

Site

number Site

Site name

abbreviation

Cations

Calcium Magnesium Sodium Potassium

Sept.

1997

Feb.

1998

Sept.

1997

Feb.

1998

Sept.

1997

Feb.

1998

Sept.

 1997

Feb.

1998

1 Eagle River below Minturn Minturn 17 33 6.4 12 2.0 12 0.6 1.1

2 Eagle River at Avon Avon 32 48 7.7 13 4.2 12 0.9 1.3

3 Eagle River at Edwards Edwards 37 54 8.7 14 5.0 11 1.0 1.5

4 Eagle River near Wolcott Wolcott 44 72 9.9 17 39 79 1.6 3.0

5 Eagle River at Gypsum Gypsum 75 100 14 22 44 64 2.0 2.8

Site

number Site

Site name

abbreviation

Anions

Alkalinity Chloride Fluoride Sulfate

Sept.

1997

Feb.

1998

Sept.

1997

Feb.

1998

Sept.

1997

Feb.

1998

Sept.

1997

Feb.

1998

1 Eagle River below Minturn Minturn 53 70 0.6 2.3 <0.1 <0.1 19 81

2 Eagle River at Avon Avon 71 90 3.2 7.9 <0.1 0.1 43 90

3 Eagle River at Edwards Edwards 78 96 4.0 8.9 <0.1 0.1 52 98

4 Eagle River near Wolcott Wolcott 112 111 58 120 <0.1 0.1 65 140

5 Eagle River at Gypsum Gypsum 113 135 49 93 <0.1 0.1 140 210
4

Nutrient concentrations generally were low but increased

in a downstream direction, most often reaching maximum

concentrations at site 4. The increase in concentrations reflected

inputs from point and nonpoint sources. Nutrient concentrations

of the February period were nearly always greater than those of

the September period. Nitrate concentrations were less than the

10 mg/L (milligram per liter) stream standard and ammonia was

less than the State of Colorado aquatic life standards that vary

according to temperature and pH. The ammonia and nitrite plus

nitrate data collected during the September and February samplings are similar to the intermediate range of the 1993–97 data.

The September phosphorus value is near the minimum observed

1993–97 data at sites 2 and 5 (fig. 4). Total phosphorus concentrations are not shown because the dissolved concentrations are

within 0.02 mg/L of the total concentrations; thus, most of the

phosphorus was detected in the dissolved state during these

sampling periods.

The loads for dissolved inorganic nitrogen (DIN) (ammonia plus nitrite plus nitrate) and dissolved phosphorus are

shown in figure 5. The loads generally were greater during the

February period and exhibited similar downstream changes for

both sampling periods. Loads increased substantially between

sites 2 and 4 (as a result of point and nonpoint sources), reaching peak values at site 4. Dissolved inorganic nitrogen loads

then decreased slightly at site 5, and phosphorus loads

decreased substantially at site 5.

Figure 3. Field mesurements for September 1997 and February 1998

sampling.

Figure 4. Nutrient concentrations for September 1997 and

February 1998 sampling and ranges of concentrations for samples

collected in 1993–97.

RIVER MILES UPSTREAM FROM MOUTH OF EAGLE RIVER

STREAMFLOW, IN CUBIC 

FEET PER SECOND pH, IN STANDARD UNITS

DISSOLVED OXYGEN, IN

MILLIGRAMS PER LITER

SPECIFIC CONDUCTANCE, 

IN MICROSIEMENS PER 

CENTIMETER AT 

25 DEGREES CELSIUS

SITE NAME AND NUMBER

Minturn

1 Avon2 Edwards3 Wolcott4 Gypsum5

0

45 35 25 15 5

100

200

300

400

8.0

8.1

8.2

8.3

8.4

8.5

8.6

8.7

8.8

8.9

7

8

9

12

13

11

10

100

300

500

700

900

1,100

September 1997

February 1998

SITE NAME AND NUMBER

Minturn

1

Avon

2

Edwards

3

Wolcott

4

September 1997

February 1998

Number of samples

Maximum

Mean

Minimum

1993-97 Data

0.4

0.6

0.8

1.0

1.2

1.4

0

0.05

0.10

0.15

0.20

0.25

DISSOLVED NITRITE PLUS 

NITRATE AS NITROGEN, IN 

MILLIGRAMS PER LITER

DISSOLVED AMMONIA 

NITROGEN, IN 

MILLIGRAMS PER LITER

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.2

0

DISSOLVED PHOSPHORUS, IN

MILLIGRAMS PER LITER

0

0.02

45 35 25 15

RIVER MILES UPSTREAM FROM MOUTH OF EAGLE RIVER

5

20

19

18

21

20

20

20

Gypsum

5

EXPLANATION
5

Habitat

One important aspect of relating biological communities

to water quality is the assessment of available habitat for

aquatic organisms. Habitat is the physical environment in

which aquatic organisms live. Biological communities may be

affected by habitat quality as well as water quality. Rapid Bioassessment Protocols (RBPs) (Barbour and others, 1997) are

qualitative and evaluate instream habitat, channel morphology,

bank features, and riparian vegetation. This protocol involves

rating 10 habitat parameters as optimal, suboptimal, marginal,

or poor. Habitat scores for the sampling sites in this study were

in the optimal and suboptimal ranges, which indicated suitable

habitat was present for aquatic communities at all of the study

sites. The RBPs are used as a screening tool and may not

account for subtle differences in habitat that can affect

biological communities.

Algae

Benthic algae are important primary producers in streams

and are a food source for macroinvertebrates. Algal communities and algal biomass can be indicators of water-quality conditions. Algae integrate water-quality conditions from several

weeks to months. Relative abundance of the different taxa in an

algal community can be useful in interpretation of water-chemistry constituents, especially nutrients. Although nitrogen and

phosphorus are essential for aquatic life, an overabundance of

nutrients in surface water can cause nuisance growth of algae.

The overabundance of algae can lead to water-quality

concerns, including large shifts in dissolved oxygen. Aquatic

life, such as fish, can be adversely affected by these shifts if

concentrations of dissolved oxygen become too low.

 Chlorophyll-a values were used to estimate the algal

biomass at the sampling locations (fig. 6). The chlorophyll-a

values reported in this study are indicative of unenriched to

moderately enriched streams. Median chlorophyll-a values for

unenriched and moderately enriched streams are 1.7 and

21 milligrams per square meter, respectively (Biggs, 1996).

Algal biomass generally increased as nutrient concentrations

increased except at site 4.

Distributions of the major algal divisions are shown in

figure 7. Diatoms comprised more than 97 percent of the algal

biomass at the study sites. Because diatoms represent most of

the algal biomass, diatom tolerance groups for nutrients were

used to assess changes in the algal community (Bahls, 1992).

The tolerance groups are based on the species and categorized

as “sensitive” (oligotrophic), “less sensitive” (mesotrophic),

and “most tolerant” (eutrophic). All five sampling sites contained predominantly “sensitive” and “less sensitive” species.

The percentages of the “most tolerant” species among sites are

shown in figure 8. Sites 3 and 4 had the largest percentages of

the “most tolerant” species. For both sampling periods, the

percentage of the “most tolerant” species generally followed

patterns similar to the DIN concentrations (fig. 8). Several factors may contribute to these changes in the algal community in

September and February; however, the most likely reason for

the changes is nutrients from point and nonpoint sources.

Figure 5. Dissolved inorganic nitrogen and dissolved phophorus

loads.

Figure 6. Values of chlorophyll-a samples.

0

10

20

30

40

50

60

0

45 35 25 15 5

100

200

300

400

500

RIVER MILES UPSTREAM FROM MOUTH OF EAGLE RIVER

DISSOLVED PHOSPHORUS LOAD,

IN KILOGRAMS PER DAY

DISSOLVED INORGANIC NITROGEN

LOAD, IN KILOGRAMS PER DAY

SITE NAME AND NUMBER

Minturn

1

Avon

2

Edwards

3

Wolcott

4

Gypsum

5

September 1997

February 1998

20

18

16

14

12

10

8

6

4

2

0 Minturn

1

Avon

2

Edwards

3

SITE NAME AND NUMBER

Wolcott

4

Gypsum

5

September 1997

February 1998

CHLOROPHYLL , IN MILLIGRAMS PER 

SQUARE METER

-a
6

Site 4 was sampled for algal biomass and community

composition once each month from September through

February to assess changes in the algal community over time.

The median chlorophyll-a value was 0.9 milligram per square

meter, indicating that the algal biomass generally was low during the sampling period. The median value for the percentage

of “most tolerant” species was 8 percent. This percentage

remained fairly consistent and resembled the September and

February results shown in figure 8.

 Macroinvertebrates

Macroinvertebrates can integrate water-quality conditions

from several months to a year. The relative percentage of the

macroinvertebrate groups indicates changes in the macroinvertebrate community in a downstream direction (fig. 9). Diversity

of organisms commonly is used as an indicator of water-quality

conditions. As diversity in a macroinvertebrate community

decreases, water quality generally degrades. The ShannonWeaver index is calculated on the basis of species and number

of organisms. The Shannon-Weaver diversity index indicates a

decrease in diversity when comparing site 1 to downstream

sites (fig. 10). The percentage of Trichoptera (caddisflies)

increased in a downstream direction, with maximum percentages at site 4. Site 4 also had the highest number of

macroinvertebrates. The community at site 4 was dominated by

one organism, the caddisfly Brachycentrus occidentalis. This

caddisfly accounted for 73 and 55 percent of the entire macroinvertebrate community in September and February at site 4,

respectively. Although these organisms prefer to filter feed

from the water column, they are capable of grazing on benthic

algae when limited drifting food resources are available

(Gallepp, 1977). Grazing on algae by these organisms may be

one factor for reduced algal biomass at site 4 (fig. 6); however,

more data are needed to explain the anomaly at site 4.

100

90

80

70

60

50

100

90

80

70

60

50 Minturn

1

Avon

2

Edwards

3

Wolcott

4

Gypsum

5

FEBRUARY 1998

SEPTEMBER 1997

Chrysophyta (Diatoms)

Chlorophyta (Green algae)

Cyanophyta (Blue-green algae)

Euglenophyta (Euglenoid algae)

EXPLANATION

SITE NAME AND NUMBER

PERCENTAGE OF MAJOR ALGAL DIVISIONS

0

2

4

6

8

12

10

0

0.2

0.4

0.6

0.6

0.5

0.4

0.3

0.2

0.1

0

0.8

1.2

1.4

1.6

1

0

2

4

6

8

12

14

10

Minturn

1

Avon

2

Percent Tolerant

Dissolved Inorganic Nitrogen

Edwards

3

Wolcott

4

Gypsum

5

SITE NAME AND NUMBER

FEBRUARY 1998

SEPTEMBER 1997

EXPLANATION

PERCENTAGE OF "MOST TOLERANT" 

ALGAL SPECIES PERCENTAGE OF "MOST TOLERANT" 

ALGAL SPECIES

DISSOLVED INORGANIC NITROGEN, IN

MILLIGRAMS PER LITER

DISSOLVED INORGANIC NITROGEN, IN

MILLIGRAMS PER LITER

Figure 7. Relative percentages of major algal divisions. Figure 8. Comparison between the percentage of “most tolerant” taxa

for algae and dissolved inorganic nitrogen (DIN).
7

 The differences in the macroinvertebrate community at

sites 4 and 5 between the sampling periods may reflect the

larger increase in algal biomass at site 5 in February, compared

to September. In February, the larger sized organisms

(Trichoptera) were not as abundant at site 5. The smaller sized

organisms, Diptera (midges), comprised a larger percentage of

the macroinvertebrates at site 5 in February. The smaller sized

organisms may require less food resources compared with the

larger caddisflies found at site 5 during September.

Figure 9. Relative percentages and densities of dominant macroinvertebrate groups (numbers in parenthesis are densities of organisms per

square meter and are rounded according to Britton and Greeson, 1987).

Figure 10. Diversity index for macroinvertebrate samples.

Low-flow conditions at site 4 in February Photograph by Norm Spahr

September 1997

(2000) (5200) (4700) (8400) (3900)

0

20

40

60

80

100

Ephemeroptera

Plecoptera

Trichoptera

Coleoptera

Diptera

Non-Insects

February 1998

(2300) (2900) (3100) (4100) (3200)

0

20

40

60

80

100

Minturn

1

Avon

2

Edwards

3

Wolcott

4

Gypsum

5

EXPLANATION

SITE NAME AND NUMBER

PERCENTAGE OF DOMINANT MACROINVERTEBRATE GROUPS

DIVERSITY INDEX (BASE 2)

SITE NAME AND NUMBER

Minturn 1Avon

2

Edwards 3Wolcott 4Gypsum 5

September 1997

February 1998

4.5

5.0

0

3.5

3.0

4.0

2.5

2.0

1.5

1.0

0.5
8

Printed on recycled paper

SUMMARY

State and local agencies are concerned about the effects

of increasing urban development and the influx of people during winter on water quality and stream biota in the Eagle River.

This study provides baseline information to begin to address

this issue. Water and biological samples were collected at five

sites during low-flow conditions in the lower Eagle River Basin

to characterize water quality and stream biota and provide a

preliminary assessment of changes in nutrients and effects on

biological communities. Nutrient concentrations generally

were low but increased at sites downstream. Algal biomass

generally increased as nutrient concentrations increased except

at site 4. The algal community at sites 3 and 4 contained a

larger percentage of the “most tolerant” type species; nutrient

concentrations also were highest at these two sites. The high

number of organisms and the dominance of the Brachycentrus

caddisfly at site 4 with its tendency to graze on the algae may

be a factor for the lower algal biomass values at site 4 compared to other sites. The composition of the dominant macroinvertebrate groups changed in a downstream direction. Diversity in the macroinvertebrate communities generally decreased

at downstream sites.

Although changes occurred in the biological communities

at sites located downstream, State standards for nutrients were

not exceeded. The algae did not appear to be at nuisance levels

during the sampling period. However, continued water-quality

and stream-biota monitoring of the Eagle River will be

important to assess the effects of continued population growth

in Eagle County.

REFERENCES

Bahls, L.L., 1992, Periphyton bioassessment methods for

Montana streams: Helena, Mont., Department of Health

and Environmental Sciences, Water Quality Bureau, 18 p.,

plus appendices.

Barbour, M.T., Gerritsen, J., Snyder, B.D., and Stribling, J.B.,

1997, Revision to rapid bioassessment protocols for use in

streams and rivers—Periphyton, benthic macroinvertebrates and fish: U.S. Environmental Protection Agency,

Office of Water, Washington, D.C., EPA 841–D–97–002,

variously paged.

Biggs, B.J.F., 1996, Patterns in benthic algae of streams,

chap. 2, in Stevenson, R.J., Bothwell, M.L., and Lowe,

R.L., eds., Algal ecology-Freshwater benthic ecosystems:

San Diego, Academic Press, Harcourt Brace and Company, p. 31–56.

Britton, L.J., and Greeson, P.E., 1987, Methods for collection

and analysis of aquatic biological and microbiological

samples: Techniques of Water-Resources Investigations of

the U.S. Geological Survey, book 5, chap. A4, 363 p.

Cuffney, T.F., Gurtz, M.E., and Meador, M.R., 1993, Methods

for collecting benthic invertebrate samples as part of the

National Water-Quality Assessment Program: U.S. Geological Survey Open-File Report 93–406, 66 p.

Gallepp, G.W., 1977, Responses of caddisfly larvae (Brachycentrus spp.) to temperature, food availability and current

velocity: American Midland Naturalist, v. 98, p. 59–84.

Hem, J.D., 1992, Study and interpretation of the chemical

characteristics of natural water: U.S. Geological Survey

Water-Supply Paper 2254, 263 p.

Porter, S.D., Cuffney, T.F., Gurtz, M.E., and Meador, M.R.,

1993, Methods for collecting algal samples as part of the

National Water-Quality Assessment Program:

U.S. Geological Survey Open-File Report 93–409, 39 p.

Shelton, L.R., 1994, Field guide for collecting and processing

stream-water samples for the National Water-Quality

Assessment Program: U.S. Geological Survey Open-File

Report 94–455, 42 p.

Tweto, Ogden, Moench, R.H., and Reed, J.C., Jr., 1978, Geologic map of the Leadville 1ο X 2o Quadrangle, northwestern Colorado: U.S. Geological Survey Miscellaneous

Investigations Series Map I–999.

U.S. Bureau of the Census, 1970, 1990, 1970–1990 Census of

population and housing: Washington D.C., data on

CD-ROM.

–Jeffrey R. Deacon and Norman E. Spahr

Technical assistance: Robert W. Boulger, Scott V. Mize,

and Verlin C. Stephens

Editing, Manuscript, and Layout: Mary A. Kidd

and Nancy L. Bruce

Information on technical reports and hydrologic data can

be obtained from:

U.S. Geological Survey,

Water Resources Division

Bldg. 53, Denver Federal Center

Mail Stop 415, Box 25046

Denver, CO 80225–0046

email: jrdeacon@usgs.gov

nspahr@usgs.gov