U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5148

Prepared in cooperation with the Colorado River Water Conservation District, Eagle County, 

Eagle River Water and Sanitation District, Upper Eagle Regional Water Authority, Colorado 

Department of Transportation, City of Aurora, Town of Eagle, Town of Gypsum, Town of Minturn, 

Town of Vail, Vail Resorts, Colorado Springs Utilities, Denver Water, and the U.S. Department of 

Agriculture Forest Service

Macroinvertebrate-Based Assessment of Biological 

Condition at Selected Sites in the Eagle River Watershed, 

Colorado, 2000–07
Cover: Example of mixed site class used to evaluate the effect of land cover on biological 

condition in the Eagle River watershed, Colorado, 2000–07.
Macroinvertebrate-Based Assessment 

of Biological Condition at Selected Sites 

in the Eagle River Watershed, Colorado, 

2000–07

By Robert E. Zuellig, James F. Bruce, Brian D. Healy, and Cory A. Williams

Prepared in cooperation with the Colorado River Water Conservation District, 

Eagle County, Eagle River Water and Sanitation District, Upper Eagle Regional 

Water Authority, Colorado Department of Transportation, City of Aurora, Town 

of Eagle, Town of Gypsum, Town of Minturn, Town of Vail, Vail Resorts, Colorado 

Springs Utilities, Denver Water, and the U.S. Department of Agriculture Forest 

Service

Scientific Investigations Report 2010–5148

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Zuellig, R.E., Bruce, J.F., Healy, B.D., Williams, C.A., 2010, Macroinvertebrate-based assessment of biological 

condition at selected sites in the Eagle River watershed, Colorado, 2000–07: U.S. Geological Survey Scientific 

Investigations Report 2010–5148, 19 p. 
iii

Contents

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

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................3

Study Area..............................................................................................................................................3

Study Methods ...............................................................................................................................................4

Data Collection .....................................................................................................................................4

Macroinvertebrate Sampling.....................................................................................................4

Macroinvertebrate Data Preparation and Comparability .....................................................4

Assigning Site Classes................................................................................................................7

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

Macroinvertebrate Predictive Model................................................................................................7

Predictive Model Description and Performance....................................................................8

Applying the Predictive Model to the Eagle River Watershed Dataset..............................8

Assessing Biological Condition..........................................................................................................9

Comparing Biological Condition Among Site Classes ....................................................................9

Annual Variability in Observed/Expected .........................................................................................9

Observed and Expected Macroinvertebrate Taxa Occurrences at Urban Sites .....................10

Macroinvertebrate-Based Assessment of Biological Condition.........................................................10

Site Class and Biological Condition.................................................................................................10

Annual Variability of Observed/Expected .......................................................................................14

Observed and Expected Macroinvertebrate Taxa at Urban Sites..............................................15

Summary and Conclusions.........................................................................................................................15

Acknowledgments.......................................................................................................................................17

References Cited..........................................................................................................................................17

Figures

1. Map showing land cover and selected streams and cities in the Eagle River 

watershed, Colorado, 2000–07....................................................................................................2

2. Map showing selected sites and Eagle River watershed subregions, 

Colorado, 2000–07.........................................................................................................................5

3. Representative photographs of the four site classes used to evaluate the 

effect of land cover on biological condition in the Eagle River watershed, 

Colorado, 2000–07.........................................................................................................................8

4. Graph showing summary of observed/expected values calculated from 

124 macroinvertebrate samples collected at 73 sites grouped by different 

subregions of the Eagle River watershed, Colorado, 2000–07 ............................................11

5. Graph showing distribution of macroinvertebrate observed/expected values 

among undeveoped, adjacent road or highway or both, mixed, and 

urban site classes.......................................................................................................................12

6. Map showing site location, site class, biological condition, and Eagle 

River watershed subregions, Colorado...................................................................................13
iv

7. Graph showing distribution of observed/expected values collected from 

14 sites (6 undeveloped sites, 20 samples; 4 Black Gore Creek sites, 

17 samples; 4 Gore Creek sites, 18 samples) representing 3 to 5 years of 

annual variability between 2000 and 2007 ..............................................................................15

8. Graph showing frequencies of observed (Of) and expected (Ef) occcurrence 

of 19 invertebrate taxa that statistically occurred more or less frequently 

than predicted or were impartial to conditions at urban sites in the 

Eagle River watershed, Colorado, 2000–07 ............................................................................16

Tables

1. Description of sites, data source, sample types, and biological assessments 

of selected sites in the Eagle River watershed, Colorado, 2000–07.....................................6

2. Site and data description where 3 to 5 years of annual macroinvertebrate 

samples were collected in the Eagle River watershed, Colorado, 2000–07 .....................14
v

Conversion Factors, Datum, and Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

meters (m) 3.2808 feet (ft)

mile (mi) 1.609 kilometer (km)

Area

acre 0.004047 square kilometer (km2

)

square foot (ft2) 929.0 square centimeter (cm2)

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

 (NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.
vi

Abbreviations

ANOVA Analysis of variance 

ARH Adjacent road or highway or both classified site

CDLG Colorado Department of Local Governments

CDOT Colorado Department of Transportation

CDPHE Colorado Department of Public Health and Environment

ERW Eagle River Watershed

GIS Geographical Information System

NAWQA National Water-Quality Assessment Program

O/E Ratio of observed (O) taxa to expected (E) taxa

OTU Operational taxonomic unit

PRISM Parameter-elevation Regressions on Independent Slopes Model

SD Standard Deviation

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

FS U.S. Department of Agriculture Forest Service
Abstract

The U.S. Geological Survey (USGS), in cooperation 

with the Colorado River Water Conservation District, Eagle 

County, Eagle River Water and Sanitation District, Upper 

Eagle Regional Water Authority, Colorado Department of 

Transportation, City of Aurora, Town of Eagle, Town of 

Gypsum, Town of Minturn, Town of Vail, Vail Resorts, Colorado Springs Utilities, Denver Water, and the U.S. Department 

of Agriculture Forest Service (FS), compiled macroinvertebrate (73 sites, 124 samples) data previously collected in the 

Eagle River watershed from selected USGS and FS studies, 

2000–07. These data were analyzed to assess the biological condition (that is, biologically “degraded” or “good”) at 

selected sites in the Eagle River watershed and determine 

if site class (for example, urban or undeveloped) described 

biological condition. 

An independently developed predictive model was 

applied to calculate a site-specific measure of taxonomic 

completeness for macroinvertebrate communities, where taxonomic completeness was expressed as the ratio of observed 

(O) taxa to those expected (E) to occur at each site. Macroinvertebrate communities were considered degraded at sites 

were O/E values were less than 0.80, indicating that at least 

20 percent of expected taxa were not observed. Sites were 

classified into one of four classes (undeveloped, adjacent road 

or highway or both, mixed, urban) using a combination of 

riparian land-cover characteristics, examination of topographic 

maps and aerial imagery, screening for exceedances in waterquality standards, and best professional judgment. Analysis 

of variance was used to determine if site class accounted for 

variability in mean macroinvertebrate O/E values. Finally, 

macroinvertebrate taxa observed more or less frequently than 

expected at urban sites were identified. 

This study represents the first standardized assessment 

of biological condition of selected sites distributed across the 

Eagle River watershed. Of the 73 sites evaluated, just over 

half (55 percent) were considered in good biological condition (O/E greater than 0.80). The remaining sites were either 

consistently biologically degraded (30 percent; O/E less than 

0.80) or varied annually between good and degraded condition 

(15 percent; O/E is less than or greater than 0.80). Sites primarily affected by urbanization were among the most severely 

degraded (lowest O/E values) when compared to other site 

classes. Although most urban sites were among the most 

severely degraded (lowest O/E values), a few sites had nearly 

intact macroinvertebrate communities (O/E near 1.0). Similar 

observations were noted among sites classified as mixed. 

Thirteen macroinvertebrate taxa were identified that 

occurred more or less frequently than expected at urban sites. 

Additionally, six other taxa were impartial (tolerant) to the 

same conditions. Combined, these 19 taxa provide an opportunity to enhance the interpretation of future studies in the 

Eagle River watershed, but will require better insight into the 

responses of these taxa to specific stressors. Understanding the 

sources of variability affecting biological condition along with 

why some sites expected to be degraded, but showed otherwise, will have clear implications for mitigation efforts. Integrating results of this study with field and laboratory investigations will greatly enhance the ability to identify causal factors 

affecting biological condition at degraded sites, the logical 

next step. Information generated from such integrative studies 

will be imperative for well targeted mitigation efforts in the 

Eagle River watershed. 

Introduction

Eagle River, a tributary of the Colorado River, drains 

an area of approximately 970 square miles (mi2) in Colorado 

flowing from the Continental Divide through Red Cliff, 

Minturn, Avon, Edwards, Wolcott, Eagle, and Gypsum (fig. 1), 

and continues downstream approximately 6 miles to the confluence with the Colorado River at Dotsero. Several environmental factors that affect the biological condition of streams 

have been modified to some degree in the Eagle River watershed (for example, surface-water hydrology, water quality, 

land cover, and riparian area disturbance), where changes 

are expected to intensify in the future (Bledsoe and others, 

2005). State and local agencies are concerned about the effects 

Macroinvertebrate-Based Assessment of Biological 

Condition at Selected Sites in the Eagle River Watershed, 

Colorado, 2000–07

By Robert E. Zuellig, James F. Bruce, Brian D. Healy, and Cory A. Williams
2 Macroinvertebrate-Based Assessment of Biological Condition at Selected Sites in Eagle River Watershed, CO, 2000–07

Figure 1. Land cover and selected streams and cities in the Eagle River watershed, Colorado, 2000–07.

EAGLE RIVER

City Streams

Land cover

COLORADO

Dotsero

DENVER

Colorado

 River

Eagle River

Continental Divide

Base from U.S. Geological Survey 

digital data, 1983, 1:400,000 Universal 

Transverse Mercator projection Zone 13

0

0 5 10 20 KILOMETERS

5 10 20 MILES

EXPLANATION

Agricultural land

Barren land

Forest land

Rangeland

Tundra

Urban or built-up land

Water

Wetland

106°15'W

106°45'W

39°45'N

39°30'N

Vail

Avon

Edwards

Eagle

Gypsum

Red Cliff

Minturn

Wolcott

Gore Creek

Black Gore Creek

Eagle River

Homestake Creek

Cross Creek

Creek

Lake

Brush Creek

Gypsum Creek
Introduction 3

of increasing urban development and human population on 

water quality and the biological condition of regional streams 

(Deacon and Spahr, 1998). Current (2010) and expected future 

development along the Eagle River and Gore Creek corridors 

could adversely affect water quality and stream biota (Bledsoe 

and others, 2005; Wynn and others, 2001).

Several local studies have noted site-specific differences 

in community structure of fish, macroinvertebrate, and diatom 

communities in the upper part of the Eagle River watershed 

(Wynn and others, 2001; Hydrosphere, 2007; Healy, 2008), 

but relatively few have made clear connections between 

observed patterns and specific stressors (Vieira and others, 

2005). Even less is known about factors affecting the biological condition downstream from Avon to the Colorado River. 

Local entities are interested in better understanding how 

specific land-management activities affect the biological and 

water quality of the Eagle River watershed in order to guide 

watershed-wide water-resource management. In response to 

these needs, the U.S. Geological Survey (USGS), in cooperation with the Colorado River Water Conservation District, 

Eagle County, Eagle River Water and Sanitation District, 

Upper Eagle Regional Water Authority, Colorado Department 

of Transportation, City of Aurora, Town of Eagle, Town 

of Gypsum, Town of Minturn, Town of Vail, Vail Resorts, 

Colorado Springs Utilities, Denver Water, and the U.S. 

Department of Agriculture Forest Service (FS), compiled previously collected macroinvertebrate data from the Eagle River 

watershed. These data were analyzed to assess the biological condition (that is, biologically “degraded” or “good”) of 

selected sites. This assessment of biological condition herein 

differs from what has been done locally in the past because 

regionally derived biological assessment tools are now available, which facilitates quantitative predictions of the loss of 

biological diversity in Eagle River watershed streams based on 

regional reference conditions. This study represents the first 

standardized assessment of biological condition of selected 

sites within the Eagle River watershed.

Purpose and Scope

This report presents a macroinvertebrate-based assessment of biological condition using a compilation of macroinvertebrate data collected by USGS and FS from up to 73 

sites in the Eagle River watershed between 2000 and 2007. 

Data were compiled from independent studies; therefore, 

all data types were not collected in all years at all sites. The 

objectives of this report were to (1) assess the biological 

condition of selected sites in the Eagle River watershed using 

macroinvertebrate communities and (2) evaluate whether the 

dominant types of land cover were associated with biological 

condition. Additionally, short-term annual variability (3 to 5 

years) in observed taxa to those expected (O/E) values was 

described and macroinvertebrate taxa that occurred more or 

less frequently than predicted at urban sites were identified. 

All macroinvertebrate data evaluated in this report, as well as 

some accompanying diatom data not presented in this report, 

are documented in Zuellig and Bruce, (2010), available online 

at http://pubs.usgs.gov/ds/5021.

Study Area

Several previous studies provide a detailed description of 

the Eagle River watershed study area (for example, Bledsoe 

and others, 2005). In brief, the Eagle River watershed (ERW) 

drains approximately 950 mi2 where the river flows north from 

its headwaters to the confluence of Gore Creek near Avon, 

then generally west to its terminus where it joins the Colorado 

River at Dotsero. Major tributaries include Gore Creek, 

Homestake Creek, Cross Creek, Lake Creek, and Brush Creek 

(fig. 1). Gore Creek, the largest tributary in the upper part of 

the watershed, flows through Vail draining approximately 

102 mi2 before it reaches its confluence with the Eagle River 

near Minturn. Land-surface elevations range from approximately 14,005 to 6,130 ft. 

The geology of the ERW is highly variable, which differentially affects water-quality conditions in discrete parts 

of the watershed. For example, a few tributaries near Wolcott 

drain highly erosive soils (for example, soils developed within 

Pierre Shale) that contribute fine sediments to the Eagle River 

during snowmelt or summertime rainfall events (Bledsoe and 

others, 2005). Additionally, the southern part of the watershed 

(roughly the area upstream from Minturn, fig. 1) is part of 

the Colorado Mineral Belt (Wilson and Sims, 2003), which 

has contributed to a multibillion dollar mining industry in 

Colorado (Tweto, 1979). 

Temperature, precipitation, and streamflow patterns are 

typical of regional river watersheds in Colorado. Monthly 

average temperatures typically range from 14 °F in January 

to 56 °F in July in the Vail area and from 18 °F to 67 °F near 

Eagle. Precipitation typically ranges from 22 inches per year 

in the higher elevations to around 11 inches in the lower 

valleys (National Oceanic and Atmospheric Administration, 

2009). Much of the precipitation falls in the form of snow 

throughout the winter months resulting in a snowmelt-driven 

hydrograph (Bledsoe and others, 2005). Typically, streamflow 

increases in early spring as snow melts at lower elevations, 

peaks around mid June, and slowly recedes throughout the 

summer months. During late spring through late summer, 

streamflow is supplemented by a combination of localized 

high-intensity convective thunderstorms. Base flow is sustained by tributary groundwater when thunderstorms diminish 

in late summer until the snow begins to melt the following 

spring. Streamflow is intensively managed throughout the 

watershed by storing and diverting water for municipal and 

agricultural use, maintaining minimum instream flow requirements, and snow making for local ski areas. 

In 2001, approximately 5 percent of the ERW was considered developed (urban or agricultural) and 95 percent was 

undeveloped (primarily forested) (U.S. Geological Survey, 

2007). Most developed lands in the watershed generally occur 
4 Macroinvertebrate-Based Assessment of Biological Condition at Selected Sites in Eagle River Watershed, CO, 2000–07

along stream corridors and can account for as much as 35 

percent of land cover of stream-side areas (Bledsoe and others, 

2005). Ski tourism and second home development currently 

are the largest contributors to increases in developed lands in 

the ERW. Human population in the area is projected to nearly 

double by 2030 (Colorado Department of Local Government, 

2003), which is expected to increase developed lands along 

stream corridors (Bledsoe and others, 2005).

Study Methods

One-hundred twenty-four macroinvertebrate samples collected from 73 sites (fig. 2) were compiled from various USGS 

and FS projects in the ERW between 2000 and 2007 (table 1). 

Data were compiled from various projects and agencies; therefore, not all data types were collected at all sites with the same 

frequency. For example, some sites were visited only once; 

whereas, others were visited annually during a 5-year period. 

Therefore by necessity, various combinations of samples and 

sites were used to address each objective. Site assessments of 

biological condition were made by applying an independently 

developed indicator and establishing a threshold value defining biological degradation. 

Data Collection 

Macroinvertebrate Sampling 

The USGS collected semiquantitative-macroinvertebrate 

samples during 2000 and 2001 following National WaterQuality Assessment (NAWQA) Program protocols described 

by Cuffney and others (1993). In general, five discrete 

collections were made from riffle areas at each site with a 

slack sampler equipped with 500-micron mesh and a 2.69 ft2

(0.25 m2) sampling grid along a pre-defined stream reach (20 

times the stream width, Fitzpatrick and others, 1998). The 

contents of the five collections were composited, elutriated, 

and poured through a 500-micron mesh sieve, and preserved 

with 10-percent buffered formalin in the field. All samples 

were transported to Chadwick and Associates in Littleton, 

Colo., where they were processed using methods described 

by Klemm and others (1990). All taxa were identified to the 

lowest practical resolution (genus or species) and enumerated. 

All USGS macroinvertebrate samples were collected during 

base-flow conditions during September; some USGS sites 

were sampled in both 2000 and 2001 (table 1).

The FS collected macroinvertebrate samples between 

2003 and 2007 using methods described by Hawkins and 

others (2001) (table 1). In general, eight macroinvertebrate 

samples were collected and composited from four riffle locations within an 8-ft stream reach (length of stream encompassing four distinct riffle areas) using a surber sampler equipped 

with 500-micron mesh and a 1-ft2 sampling grid. Macroinvertebrate samples from each site were composited and preserved 

in the field and sent to Aquatics Associates, Inc., Fort Collins, 

Colo., for sample processing following protocols developed 

by Klemm and others (1990). Additionally, the FS sampled 

macroinvertebrate communities from a series of sites along the 

Eagle River in the upper part of the watershed (table 1, fig. 2, 

sites 5, 9, 11-12, 17, 22, and 24). At these sites, five replicate 

macroinvertebrate samples were collected from a single riffle 

using a Hess sampler (each replicate represents 0.923 ft2 of 

streambed) equipped with 500-micron mesh and preserved 

separately in the field and sent to Aquatics Associates Inc., Ft. 

Collins, Colo. for taxa identification and sample processing 

(Klemm and others, 1990). All taxa in each sample were identified to the lowest practical resolution (for example, genus or 

species) and enumerated. All FS macroinvertebrate samples 

were collected during base-flow conditions during September. 

Macroinvertebrate Data Preparation and 

Comparability 

Three adjustments were made to the compiled ERW 

data in preparation for data analysis. First, taxonomic names 

used by the different laboratories were harmonized. Second, 

the laboratory results were composited from the five FS Hess 

samples (Cuffney and others, 1993; Hawkins and others, 

2001). Third, the raw data counts were standardized for each 

sample to 300 by randomly resampling individuals without 

replacement using a personal computer. Such adjustments 

to raw data counts can help alleviate the effect of varying 

levels of sampling effort (for example, area sampled in the 

field or subsample target counts) on estimates of taxa richness 

(Peterson and Zumberge, 2006), even when different sampling 

devices are used (Cao and others, 2005). Fixed-count adjustments are necessary even if samples are collected using the 

same method because standardized laboratory subsampling 

procedures result in a wide range of individuals extracted from 

each sample, which differentially affects taxa richness across 

samples (Vinson and Hawkins, 1996). 

Although three different protocols were used to collect 

the macroinvertebrate data compiled for this study, the differences were considered negligible after the adjustments were 

made as described in the previous paragraph. For example, all 

methods targeted riffle habitats, collected multiple samples 

along a stream reach, scrubbed the overlying substrate within 

a defined area, disturbed the underlying substrate, and used a 

similar mesh size (425 or 500 microns). The primary difference among field methods was the area of stream bottom 

sampled (FS Hess sampler, 4.6 ft2; FS, 8 ft2; USGS, 13.5 ft2). 

Carlisle and Hawkins (2008) recently aggregated data collected with different sampled areas of stream bottoms (ranged 

from 7.8 to 13.5 ft2) and found that the ratio of observed (O) 

taxa to those expected (E) (O/E) values for samples collected 

from smaller areas were negatively biased by 0.06 units (a 

relatively small amount). Considering the results of Carlisle 

and Hawkins (2008), the lower end of any bias in O/E values 

evaluated in this study likely is around 0.06 units, although 
Study Methods 5

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Lower Eagle Gore

Middle Eagle Upper Eagle

Site location Streams

Number corresponds to site 

identification in table 1

Basin subregions

Base from U.S. Geological Survey 

digital data, 1983, Universal 

Transverse Mercator projection 

Zone 13

0

0 5 10 20 KILOMETERS

5 10 20 MILES

EXPLANATION

Scale 1:400,000

Figure 2. Selected sites and Eagle River watershed subregions, Colorado, 2000–07.
Table 1. Description of sites, data source, sample types, and biological assessments of selected sites in the Eagle River watershed, Colorado, 2000–07. [ID, identification; latitude and longitude in degrees, minutes, and seconds; NAVD 88, North American Vertical Datum of 1988; ft, feet; mi2, square miles; FS, U.S. Forest ServGeological Survey; O/E, ratio of observed to expected macroinvertebrate taxa; --, not applicable] Site ID (fig. 2) Station name Station ID Latitude Longitude Elevatio n NAVD 88 (ft) Drainag e area (mi2) Basin group Site class Data source Number of years macroinvertebrate samples collected 2000-07 Most recent O/E value 

1 East Fork Eagle River above Colorado Trail Bridge EFER-R1-1.06 39° 25' 11" 106° 18' 07" 9,326 18.7 Upper Eagle Mixed FS 2005 0.94 2 South Fork Eagle River above East Fork SFER-R1-0.08 39° 25' 15" 106° 18' 48" 9,304 19.0 Upper Eagle Mixed FS 2005 0.65 3 Pearl Creek approximately 1.4 miles from mouth PERL-R1-2.37 39° 27' 04" 106° 16' 32" 10,238 2.44 Upper Eagle Undeveloped FS 2005 1.05 4 Resolution Creek above Pearl Creek RESL-R1-1.680 39° 27' 06" 106° 18' 37" 9,546 11.5 Upper Eagle Adjacent road or highway, or both FS 2006 0.87 5 Eagle River Below Resolution Creek EAGL-R1-95.27 39° 26' 57" 106° 19' 42" 9,206 58.7 Upper Eagle Mixed FS 2005 1.06 6 Eagle River at Red Cliff 09063000 39° 30' 30" 106° 21' 58" 8,654 70.0 Upper Eagle Mixed USGS 2000, 2001 1.06 7 East Fork Homestake Creek above County Line EFHS-R2-2.883 39° 21' 14" 106° 27' 15" 10,415 5.3 Upper Eagle Undeveloped FS 2006 1.02 8 Homestake Creek near Red Cliff 09064500 39° 28' 24" 106° 22' 02" 8,783 58.2 Upper Eagle Adjacent road or highway, or both USGS 2000 0.91 9 Eagle River above Homestake Creek EAGL-85.162 39° 30' 26" 106° 22' 43" 8,585 100.2 Upper Eagle Mixed FS 2007 0.59 

10 Eagle River below Homestake Creek near Red Cliff 393030106224700 39° 30' 30" 106° 22' 47" 8,820 164.1 Upper Eagle Mixed USGS 2000, 2001 1.19 11 Eagle River upstream of Belden EAGL-82.442 39° 31' 27" 106° 23' 45" 8,398 168.9 Upper Eagle Mixed FS 2007 0.53 12 Eagle River upstream of Fall Creek EAGL-81.615 39° 31' 37" 106° 23' 59" 8,344 169.4 Upper Eagle Mixed FS 2007 0.39 13 Eagle River below Bishop Gulch EAGL-79.449 39° 32' 41" 106° 24' 16" 8,156 184.9 Upper Eagle Mixed FS 2007 0.46 14 Eagle River near Minturn 09064600 39° 33' 14" 106° 24' 07" 8,078 186.0 Upper Eagle Mixed USGS 2000 0.98 15 Unnamed tributary to Two Elk Creek EARL-R1-0.191 39° 34' 49" 106° 20' 48" 9,505 1.5 Upper Eagle Mixed FS 2003 0.81 16 Two Elk Creek below Tea Cup Bowl TOEL-R1-5.197 39° 34' 58" 106° 21' 12" 9,220 7.7 Upper Eagle Mixed FS 2004, 2005 1.08 17 Eagle River above Two Elk Creek EAGL-76.850 39° 33' 51" 106° 24' 09" 8,014 186.7 Upper Eagle Mixed FS 2007 0.52 18 Cross Creek near Reeds Meadow CROS-R3-15.51 39° 29' 05" 106° 30' 10" 9,870 16.13 Upper Eagle Undeveloped FS 2005, 2006, 2007 1.02 19 Cross Creek approximately 3.2 miles above wilderness boundary CROS-R2-8.40 39° 31' 16" 106° 27' 14" 9,147 28.42 Upper Eagle Undeveloped FS 2005 0.87 20 Cross Creek above wilderness boundary CROSS_01R-4.9955 39° 32' 35" 106° 26' 03" 8,682 32.57 Upper Eagle Undeveloped FS 2005 1.09 21 Cross Creek near Minturn 09065100 39° 34' 05" 106° 24' 43" 7,992 34.2 Upper Eagle Mixed USGS 2000 1.05 22 Eagle River below Cross Creek EAGL-74.978 39° 34' 39" 106° 24' 47" 7,917 238.4 Upper Eagle Mixed FS 2007 0.69 23 West Grouse Creek below pack trail crossing WGRO-2.794 39° 34' 30" 106° 27' 51" 9,391 4.7 Upper Eagle Mixed FS 2007 0.95 24 Eagle River below Game Creek EAGL-71.778 39° 35' 54" 106° 26' 03" 7,804 258.6 Upper Eagle Mixed FS 2007 0.49 25 Eagle River above Gore Creek near Minturn 393627106264000 39° 36' 27" 106° 26' 40" 7,730 259.9 Upper Eagle Mixed USGS 2000 1.04 26 Gore Creek near Red Buffalo Pass GORE-R9-27.102 39° 37' 21" 106° 11' 20" 10,636 1.39 Gore Undeveloped FS 2005 1.02 27 Gore Creek approximately 3.5 miles above wilderness boundary GORE-R8-25.38 39° 37' 28" 106° 12' 20" 10,172 5.46 Gore Undeveloped FS 2004, 2005, 2007 1.09 28 Gore Creek approximately 2 miles above wilderness boundary GORE-R7-23.05 39° 37' 33" 106° 13' 49" 9,779 8.20 Gore Undeveloped FS 2004, 2005, 2007 0.95 29 Gore Creek at Upper Station near Minturn 09065500 39° 37' 33" 106° 16' 39" 8,600 14.4 Gore Undeveloped FS, USGS 2000, 2001, 2004, 2005, 2007 1.09 30 Black Gore Creek below Black Lake No. 2 BGOR-R4-10.64 39° 33' 16" 106° 13' 40" 10,339 2.1 Gore Adjacent road or highway, or both FS 2004, 2005, 2006, 2007 0.92 31 Black Gore Creek above Polk Creek BGOR-R3-5.73 39° 35' 26" 106° 14' 45" 9,482 4.6 Gore Adjacent road or highway, or both FS 2004, 2005, 2006, 2007 0.88 32 Polk Creek approximately 1.25 miles above wilderness boundary POLK-R3-2.90 39° 34' 26" 106° 13' 42" 10,277 1.48 Gore Undeveloped FS 2004, 2005, 2006 1.05 33 Polk Creek above wilderness boundary POLK-R2-0.97 39° 35' 19" 106° 14' 12" 9,911 2.46 Gore Undeveloped FS 2004, 2005, 2006, 2007 0.95 34 Black Gore Creek near Minturn 09066000 39° 35' 47" 106° 15' 52" 9,150 12.6 Gore Adjacent road or highway, or both FS, USGS 2000, 2004, 2005, 2006, 2007 0.87 35 Timber Creek at mouth TIMBER-R1-01R-0.4444 39° 35' 58" 106° 16' 11" 9,119 3.4 Gore Adjacent road or highway, or both FS 2005 0.98 36 Black Gore Creek at I-70 mile post 183 BGOR-R1-1.83 39° 36' 37" 106° 16' 34" 8,921 18.6 Gore Adjacent road or highway, or both FS 2004, 2005, 2006, 2007 1.01 37 Black Gore Creek near Vail 09066050 39° 37' 24" 106° 16' 47" 8,570 19.6 Gore Adjacent road or highway, or both USGS 2000, 2001 0.94 38 Gore Creek at Bighorn Park GORE-R4-17.37 39° 37' 48" 106° 17' 15" 8,516 35.4 Gore Urban FS 2005, 2006, 2007 0.56 39 Gore Creek above Katsos 393836106182500 39° 38' 36" 106° 18' 25" 8,355 47.6 Gore Urban FS, USGS 2000, 2004, 2005, 2006, 2007 0.56 40 Booth Creek approximately 1.0 mile above wilderness boundary booth_R3_02R-2.5407 39° 39' 53" 106° 18' 47" 9,317 4.10 Gore Undeveloped FS 2005 1.02 41 Gore Creek downstream of Pulis Bridge 393825106213400 39° 38' 25" 106° 21' 34" 8,236 58.7 Gore Urban FS, USGS 2000, 2004, 2005, 2006, 2007 0.63 42 Unnamed tributary to Mill Creek draining Northeast Bowl NEBL-R1-0.374 39° 37' 17" 106° 21' 01" 9,548 1.1 Gore Mixed FS 2003, 2005 0.72 43 Unnamed tributary to Mill Creek near Mid Vail PTAR-R1-0.066 39° 37' 36" 106° 21' 59" 8,878 1.1 Gore Mixed FS 2005 0.54 44 Mill Creek near Vail 393824106221700 39° 38' 24" 106° 22' 17" 8,210 7.5 Gore Mixed USGS 2000, 2001 0.36 45 Middle Creek approximately 0.5 miles above wilderness boundary MIDL-R4-1.211 39° 39' 08" 106° 22' 44" 8,644 5.82 Gore Undeveloped FS 2005 1.02 46 Middle Creek above wilderness boundary MIDL-R3-0.675 39° 38' 52" 106° 22' 50" 8,419 5.90 Gore Undeveloped FS 2005 1.09 47 Gore Creek at Lower Station at Vail 09066310 39° 38' 28" 106° 23' 37" 8,060 77.1 Gore Urban USGS 2000 1.02 48 Red Sandstone Creek above South Fork Red Sandstone RDSS-R3-7.588 39° 41' 57" 106° 23' 35" 9,629 2.4 Gore Adjacent road or highway, or both FS 2006 0.98 49 Red Sandstone Creek below Sandstone Creek Road RDSS-R2-5.338 39° 40' 55" 106° 24' 07" 9,206 7.4 Gore Adjacent road or highway, or both FS 2006 1.02 50 Red Sandstone Creek at Town of Vail city limit RDSS-R1-0.843 39° 38' 53" 106° 23' 47" 8,238 13.8 Gore Adjacent road or highway, or both FS 2006 1.03 51 Gore Creek below wastewater treatment plant 393826106235300 39° 38' 26" 106° 23' 53" 8,050 91.1 Gore Urban USGS 2000 0.66 52 Gore Creek at Stephens Park 393715106253600 39° 37' 15" 106° 25' 36" 7,825 100.1 Gore Urban FS, USGS 2000, 2004, 2005, 2006, 2007 0.51 53 Gore Creek at mouth near Minturn 09066510 39° 36' 34" 106° 26' 50" 7,730 102.0 Gore Urban USGS 2000, 2001 0.66 54 Beaver Creek below wilderness boundary BVRC-R1-7.075 39° 34' 24" 106° 31' 58" 9,322 7.1 Middle Eagle Undeveloped FS 2005 1.05 55 Beaver Creek above Avon 393501106313200 39° 35' 01" 106° 31' 32" 8,840 8.1 Middle Eagle Mixed USGS 2000, 2001 1.09 56 Beaver Creek at Avon 09067000 39° 37' 47" 106° 31' 20" 7,453 14.8 Middle Eagle Urban USGS 2000 0.72 57 Eagle River at Avon 09067005 39° 37' 54" 106° 31' 19" 7,410 395.0 Middle Eagle Urban USGS 2000, 2001 0.90 58 June Creek above June Creek Ditch JUNE-R1-3.430 39° 40' 07" 106° 33' 11" 7,996 5.2 Middle Eagle Adjacent road or highway, or both FS 2006 0.64 59 Eagle River at Edwards 393845106353000 39° 38' 45" 106° 35' 30" 7,180 423.8 Middle Eagle Urban USGS 2000, 2001 0.66 60 Berry Creek above USFS road crossing BERY-R1-2.841 39° 39' 59" 106° 34' 40" 7,719 5.0 Middle Eagle Mixed FS 2006 0.85 61 West Lake Creek below wilderness boundary WLAK-R1-11.324 39° 32' 16" 106° 36' 56" 9,592 4.34 Middle Eagle Undeveloped FS 2006 0.86 62 West Lake Creek near Edwards 393523106364700 39° 35' 23" 106° 36' 47" 8,300 11.0 Middle Eagle Adjacent road or highway, or both USGS 2000 1.08 63 Lake Creek near Edwards 09067200 39° 38' 51" 106° 36' 31" 7,160 49.0 Middle Eagle Mixed USGS 2000, 2001 0.63 64 Squaw Creek below wilderness boundary SQAW-R3-12.28 39° 33' 52" 106° 38' 17" 9,357 2.1 Middle Eagle Undeveloped FS 2005 1.08 65 Squaw Creek above Poison Gulch SQAW-R1-8.75 39° 35' 31" 106° 38' 57" 8,512 4.12 Middle Eagle Undeveloped FS 2005, 2006 1.06 66 Squaw Creek at mouth near Edwards 393930106382001 39° 39' 30" 106° 38' 20" 7,120 17.4 Middle Eagle Mixed USGS 2000 0.57 67 Eagle River at Eagle Springs Golf Course near Wolcott 394129106393300 39° 41' 29" 106° 39' 33" 6,980 517.0 Middle Eagle Urban USGS 2000 0.72 68 Eagle River below Milk Creek near Wolcott 394220106431500 39° 42' 18" 106° 43' 33" 6,820 600.0 Middle Eagle Adjacent road or highway, or both USGS 2000, 2001 0.82 69 Eagle River above Brush Creek at Eagle 393852106503200 39° 38' 52" 106° 50' 32" 6,500 646.2 Lower Eagle Urban USGS 2000 0.82 70 East Brush Creek above confluence 393221106450700 39° 32' 21" 106° 45' 07" 7,630 32.6 Lower Eagle Adjacent road or highway, or both USGS 2000 0.84 71 Brush Creek at mouth near Eagle 393851106503400 39° 38' 51" 106° 50' 34" 6,500 147.3 Lower Eagle Urban USGS 2000, 2001 0.59 72 Eagle River at Gypsum 09069000 39° 39' 00" 106° 57' 06" 6,276 842.0 Lower Eagle Mixed USGS 2000, 2001 0.47 73 Gypsum Creek at mouth 393858106570900 39° 38' 58" 106° 57' 09" 6,280 102.8 Lower Eagle Urban USGS 2000, 2001 0.60 
Data Analysis 7

the upper limit of indicator-value bias is unknown. Ideally, 

data comparability using different methods and laboratories 

would be evaluated from side-by-side comparisons of samples 

collected from the same sites at the same time (Taylor and 

others, 2001). Such data for side-by-side comparisons were 

not available for this study so based on previous studies, it was 

assumed that the above adjustments improved the comparability of these data. Preferably, results from studies such as this 

one would be generated from data collected using identical 

field and laboratory methods, which need to be considered for 

future monitoring efforts in the ERW. 

Assigning Site Classes

All 73 sites were classified into one of four classes 

(table 1; undeveloped, adjacent road or highway or both, 

mixed, and urban) using a combination of riparian land-cover 

characteristics, examination of topographic maps and aerial 

imagery, screening for exceedances in water-quality standards, 

and best professional judgment. Riparian land-cover characteristics were calculated using a geographic information system 

(GIS) that included watershed-level topographic and landcover data. Stream networks were obtained from the National 

Hydrography Dataset (U.S. Geological Survey, 2007). Topographic information and digitized vector polygon watershed 

boundaries were derived from 30-m resolution USGS National 

Elevation Data (U.S. Geological Survey, 2007). Land-cover 

data were derived from an enhanced version of the National 

Land Cover Dataset (Multi-Resolution Land Characteristics 

Consortium, 2001), a 16-class, 30-m resolution raster dataset 

derived from multiseason Landsat 5 and Landsat 7 imagery 

centered on a nominal collection year of 2001. Riparian corridors were delineated with a 100-m buffer around all stream 

segments upstream from each sampling site and land-cover 

attributes were calculated. Urban sites initially were classified based on the presence of greater than 1.0 percent urban 

land cover and no agriculture land cover within the riparian 

corridor; whereas, sites with both urban and agriculture land 

cover initially were classified as mixed. Sites with no urban 

or agriculture land cover initially were classified as undeveloped. Undeveloped sites also were screened for known 

exceedances of State and Federal water-quality standards and 

adjusted accordingly (that is, sites that exceeded water-quality 

standards were removed from the undeveloped site class). 

Initial classifications were verified and adjusted by examination of USGS 7.5-minute topographic maps and aerial imagery 

for other types of disturbance within two stream segments 

upstream from each site. Stream segments were defined by 

the confluence of upstream tributaries. Other types of disturbance included adjacent roads or highways or both (including 

native-surfaced roads), unimproved road-stream crossings, 

hydrologic alteration from reservoirs or diversions, logging, 

mining, or ski-area development activities. After initial 

urban and undeveloped site classifications were adjusted, all 

sites with more than one obvious nearby (that is, within two 

stream segments) upstream disturbance (for example, mining, 

channelization, wastewater effluent) were classified as mixed. 

Final adjustments were made using best professional judgment that was based on historical knowledge of each site and 

FS field notes. The above classification scheme resulted in 18 

undeveloped sites, 15 adjacent road or highway sites or both, 

26 mixed sites, and 14 urban sites (table 1). See figure 3 for 

representative photographs of each site class. 

Data Analysis

An independently developed predictive model (Paul and 

others, 2005) was applied to calculate a measure of taxonomic 

completeness for 124 macroinvertebrate samples collected 

from 73 sites in the ERW. The model calculated taxonomic 

completeness as the ratio of observed (O) taxa collected at 

each site to the taxa that were expected (E) to occur at each 

site in the absence of environmental degradation (Hawkins, 

2006). Biological condition (that is, biologically “degraded” 

or “good”) was assessed by setting an O/E threshold value to 

determine biological degradation. Initial assessments were 

made and summarized across the four subwatersheds of the 

ERW: Gore Creek, Upper Eagle, Middle Eagle, and Lower 

Eagle. Biological condition also was summarized across the 

four land-cover classes (undeveloped, adjacent road or highway or both, mixed, and urban) and analysis of variance was 

used to determine whether mean macroinvertebrate O/E values 

were associated with different land-cover classes. Additionally, it statistically was determined which macroinvertebrate 

taxa occurred more or less frequently at urban sites than predicted using the z-statistic. 

Macroinvertebrate Predictive Model 

An independently developed predictive model constructed by the Western Center for Monitoring and Assessment 

of Freshwater Ecosystems (Utah State University, Logan) 

was applied to the macroinvertebrate dataset. The predictive 

model is detailed by Charles P. Hawkins (Western Center for 

Monitoring and Assessment of Freshwater Ecosystems, Utah 

State University, Logan) in Paul and others (2005). The model 

can be accessed by requesting a user name and password 

from the Western Center for Monitoring and Assessment of 

Freshwater Ecosystems at http://129.123.10.240/WMCPortal/

DesktopDefault.aspx?tabindex=0&tabid=1. 

The model measures taxonomic completeness represented 

as the ratio of observed (O) taxa to the taxa that were expected 

(E) to occur at a site (O/E) in the absence of environmental 

degradation (Hawkins 2006). Values of O/E near 1.0 imply 

that the observed taxa (O) found at a site closely resemble the 

taxa that were predicted (E) to occur; whereas, O/E values 

<1.0 implies some degree of biological degradation. The 

description of constructing and evaluating predictive models 

are well documented and detailed elsewhere (Moss and others, 

1987; Hawkins and Carlisle, 2001; Clarke and others, 2003; 
8 Macroinvertebrate-Based Assessment of Biological Condition at Selected Sites in Eagle River Watershed, CO, 2000–07

Figure 3. Representative photographs of the four site classes used to evaluate the effect of land cover on biological 

condition in the Eagle River watershed, Colorado, 2000–07.

Ostermiller and Hawkins, 2004; Van Sickle and others, 2005), 

and are not detailed herein. However, a brief summary of the 

model’s performance is provided as detailed in Paul and others 

(2005), as well as how the model was applied to the ERW 

macroinvertebrate data evaluated in this report. 

Predictive Model Description and Performance

The predictive model (Paul and others, 2005) was constructed from reference site data collected in Colorado (97 

sites) using a variety of sampling methods associated with different programs (for example, U.S. Environmental Protection 

Agency Environmental Monitoring and Assessment Program, 

Utah State University Western Center for Monitoring and 

Assessment of Freshwater Ecosystems Science to Achieve 

Results program, USGS NAWQA, and CDPHE). The model 

uses three predictor variables (longitude in decimal degrees, 

mean annual air temperature in °C 10, and log10 watershed 

area in km2) to generate estimates of E in which O was compared. In general, the model was both accurate and precise, 

and there was little evidence that the model was biased (under 

or over predicted E) towards any major river watershed or 

ecoregion in Colorado (Paul and others, 2005). Overall, model 

performance was comparable or better than most macroinvertebrate predictive models in use in the United States or 

other countries (Paul and others, 2005).

Applying the Predictive Model to the Eagle River 

Watershed Dataset

Two steps were necessary before the model was applied 

to the ERW data. First, the macroinvertebrate identifications 

in the ERW dataset were harmonized with the operational 

taxonomic units (OTU) used to develop the model. Second, 

climatological data were added to the GIS dataset described 

earlier that were obtained from PRISM (Parameter-elevation 

Regressions on Independent Slopes Model) datasets, which 

represent 30-year (1961–90) climate averages (Spatial Climate 

Analysis Service, 2004). These data were used to calculate the 

climate-related predictor variable used as part of the model 

(that is, mean annual air temperature in °C 10). 
Data Analysis 9

The appropriateness of applying the model to the ERW 

data was evaluated in two ways. First, mean O/E and standard 

deviation (SD) values calculated from ERW reference sites 

(undeveloped sites, table 1) were compared to the range of 

values from the reference sites used to develop the predictive 

model. Because theoretical O/E values from reference sites 

should equal 1.0, it was determined if ERW reference site 

O/E values statistically were different from 1.0 using a onesample t-test (SPSS, 2005) with a p value of less than 0.05. 

This statistical test determined whether there was significant 

bias in predictions of E at ERW sites. A second statistical test 

(chi-squared test as described in Clarke and others, 2003) was 

used to determine whether the combined predictor variable 

distributions for each ERW site were within the experience of 

the model, which provided assurance that the predictive model 

was appropriate for assessing the biological condition of ERW 

sites.

The model produced unbiased estimates of E when 

applied to the ERW data, as mean (1.02) and SD (0.07) of O/E 

values from ERW reference sites were well within the range of 

mean (1.00) and SD (0.17) values of the reference sites used to 

develop the predictive model. Additionally, mean O/E values 

of ERW reference sites were not statistically distinguishable 

from 1.0 (p = 0.24) and mean O/E reference site values from 

the model also were equal to 1.0 (Paul and others, 2005). Furthermore, of the 73 sites included in this study (table 1), none 

of the combined predictor variable distributions (longitude 

in decimal degrees, mean annual air temperature in °C 10, 

and log10 watershed area in km2) were outside the range of the 

model; therefore, all sites were considered for further analysis. 

Assessing Biological Condition

The intent of this investigation was to assess the biological condition of selected ERW sites using macroinvertebrate 

O/E values; therefore, a threshold was set to define two 

levels of biological condition: sites where O/E was greater 

than > 0.80 were considered in “good” biological condition, 

and sites where O/E was less than < 0.80 were considered biologically “degraded.” Several methods for setting thresholds 

to define degraded biological condition have been suggested 

(Barbour and others, 1999; Hemsley-Flint, 2000; Clarke and 

others, 2003; Van Sickle and others, 2005). Herein, it was 

assumed that a 20-percent loss of expected taxa (O/E < 0.80) 

was ecologically meaningful, corresponding with the early 

stages of environmental degradation described by Davies and 

Jackson (2006). From a statistical standpoint, this threshold 

(0.80) also was near the 10th percentile of model reference 

site O/E values (0.79); therefore, sites with O/E values below 

the 10th-percentile value would be statistically distinguishable from reference sites (p > 0.01). Using the 10th percentile 

value as a threshold for assessing biological condition in this 

study, reasonably balances between type I and type II statistical errors (that is, balances between calling a reference site 

degraded when it is not, and calling a degraded site reference 

when it is not) while indicating an ecologically meaningful 

loss of taxa (O/E < 0.80). It is emphasized that site assessments of biological condition (“good” or “degraded”) made 

herein are unique to this study and are not directly comparable to assessments made based on other approaches, where 

degraded biological conditions are defined using different 

indicators, thresholds, or methods. 

Biological assessments were summarized by grouping 

sites into different subregions of the ERW (fig. 2, table 1) as 

defined hereafter as Gore Creek, Upper Eagle, Middle Eagle, 

and Lower Eagle. The Gore Creek subregion was inclusive of 

the entire Gore Creek watershed. The Upper Eagle included 

the Eagle River and all tributaries upstream from the Gore 

Creek confluence. The Middle Eagle was defined by the Eagle 

River and all tributaries between the Gore Creek confluence 

downstream to Wolcott. Finally, the Lower Eagle included the 

Eagle River and all tributaries from Wolcott downstream to the 

confluence with the Colorado River. 

Comparing Biological Condition Among Site 

Classes 

Test statistics (F, p) were calculated from analysis of variance (ANOVA) to determine if site class (undeveloped, adjacent road or highway or both, mixed, and urban) accounted for 

variability in mean O/E values using the most recent sample 

collected at each site (table. 1). Additionally, post-hoc tests 

of pairwise differences were conducted between site classes 

based on Bonferroni adjustments of experimental error (SPSS 

ver. 13.0, SPSS, 2005). ANOVA and post-hoc tests were considered significant if p < 0.05. 

Annual Variability in Observed/Expected

The FS initiated a macroinvertebrate monitoring program in 2004 at selected sites in the Gore Creek watershed in 

response to stakeholder concerns related to potential effects 

of excessive sediment deposition along Black Gore and Gore 

Creeks (Healy, 2008). As a result, 3 to 4 macroinvertebrate 

samples were collected at 13 sites annually within the Gore 

Creek watershed between 2004 and 2007. Four of these sites 

were previously sampled by the USGS in 2000. Additionally, an undeveloped site on Cross Creek was sampled during 

2004, 2005, and 2007 by FS. The combination of these annual 

samples provided the opportunity to examine annual variability in O/E values over a 3–5 year period among 14 sites 

(undeveloped sites 18, 27, 28, 29, 32, 33, adjacent road and /or 

highway (Black Gore) sites 30, 31, 34, 36, urban (Gore) sites 

38, 39, 41, 52). The coefficient of variation (CV = [100 standard deviation/mean]) of O/E values was used as a measure of 

the magnitude of annual variability. 
10 Macroinvertebrate-Based Assessment of Biological Condition at Selected Sites in Eagle River Watershed, CO, 2000–07

Observed and Expected Macroinvertebrate Taxa 

Occurrences at Urban Sites

Macroinvertebrate taxa that were observed more or less 

frequently than predicted by the model at urban sites were 

determined following the methods of Carlisle and others 

(2008) and Carlisle and Hawkins (2008), by recording the 

occurrence frequencies (Of) of all observed taxa among urban 

sites, and by calculating expected frequencies (Ef) of occurrence by summing the predicted probabilities of capture 

for each taxon among all urban sites. If the null hypothesis 

was true for a given taxon (Of = Ef), then it was observed as 

frequently as it was predicted; however, Of > or < Ef indicates 

which taxa are observed more or less frequently than predicted among sites. Analysis was limited to the most recent 

sample collected at each of the 14 urban sites (table 1). The 

null hypothesis was tested for each taxon using the z-statistic, 

where z = (Of – Ef) / Variance( ) Of (Carlisle and Hawkins, 

2008). Taxa with z >1.96 were considered to be tolerant 

increasers if Of>Ef or intolerant decreasers if Of<Ef. Additionally, taxa that were observed as often as predicted (± one site) 

were considered to be tolerant of the environmental conditions 

found at urban classified sites. Reported results were restricted 

to only those taxa that were predicted or observed at one-half 

or more of the 14 urban sites. See Carlisle and Hawkins (2008) 

for details to calculate Variance (Of). 

Macroinvertebrate-Based Assessment 

of Biological Condition

The number of sites available for assessment varied by 

subregion as did sampling intensity. The Gore Creek subregion 

was the most heavily sampled (28 sites, 65 samples), followed 

by the Upper Eagle (25 sites, 30 samples), Middle Eagle (15 

sites, 21 samples), and the Lower Eagle (5 sites, 8 samples) 

(fig. 4). Overall, 55 percent of sites (40 sites) were considered 

in good biological condition; whereas, 30 percent (22 sites) 

were biologically degraded. The additional 15 percent of sites 

(11 sites) had O/E values that ranged both above and below 

the threshold (O/E < 0.80) depending on the year sampled, and 

mean values for six of those sites were below the threshold. 

Considering these 6 sites, a total of 38 percent (28 sites) were 

considered biologically degraded. Also note that of the 73 

sites, 36 percent (25 sites) have not been sampled since 2000 

or 2001, so it is likely that assessments may not reflect current 

biological condition at some of these sites. Nonetheless, site 

assessments are considered a screening tool for determining 

where in the ERW biological conditions are good or degraded 

until further investigation confirms otherwise. 

These results may underestimate actual taxa loss and 

sensitivity of O/E values to biological degradation. Sites 

were considered biologically degraded when more than 20 

percent of taxa expected to occur at a site were not observed 

(O/E < 0.80). By necessity, the model was constructed with 

relatively coarse taxonomic resolution (for example, taxonomy 

used in this analysis mostly consisted of genera, some families, midges at subfamily, and few species) (Paul and others, 

2005). As a result, most taxonomic units represented more 

than one species, which likely varied in their individual tolerance to stressors. Such cases can result in O/E models that are 

less sensitive to some environmental stressors than would be 

observed if species level data are used (Hawkins and others, 

2000). 

Site Class and Biological Condition

Site classes were determined primarily from adjacent 

land-cover characteristics. Analysis of variance determined 

that mean O/E values varied significantly among site classes 

(F = 12.415, p < 0.001) based on the most recent samples 

collected at each site. Urban (mean O/E = 0.69) and mixed 

site classes (mean O/E = 0.76) exhibited the most degraded 

biological condition (that is, the lowest mean O/E values); 

whereas, sites in the adjacent road or highway or both class 

(hereafter, ARH; mean O/E = 0.92) were near conditions 

found at undeveloped sites (mean O/E = 1.02) (fig. 5). In 

general, most degraded sites were located along the main stem 

Eagle River and lower Gore Creek; whereas, sites located at 

the higher altitudes in the watershed were in relatively good 

biological condition regardless of site class (fig. 6). Additionally, results remained similar when urban classified sites along 

lower Gore Creek were removed from the analysis. 

In this study, macroinvertebrate communities at urban 

sites were most severely degraded (that is, these sites had 

the lowest mean O/E values compared to other site classes), 

a result that is consistent with many other studies (Paul and 

Meyer, 2001, and citations within). Carlisle and Meador 

(2007) and Carlisle and others (2008) reported similar results 

using O/E and a similar degradation threshold; however, 

they classified urban sites differently. In those two studies, 

sites were classified as urban when upstream watersheds had 

greater than 10- and 25-percent urban-land cover, respectively. 

In the study herein, the highest estimates of watershed urbanland cover did not exceed 2.5 percent, yet similar patterns of 

biological impairment were observed. Most urban-land cover 

in the ERW is concentrated in or near riparian areas and can 

account for as much as 35 percent in some segments (Bledsoe 

and others, 2005). A recent assessment of wadeable streams 

in the United States (U.S. Environmental Protection Agency, 

2006) indicated riparian vegetated cover disturbance as one 

of the main stressors increasing the risk of macroinvertebrate 

impairment and taxa loss in the Western United States. Other 

primary stressors that increased the risk of biological impairment in western streams included nitrogen, phosphorus, and 

suspended sediment concentrations. All of these stressors 

identified in the wadeable streams report (U.S. Environmental 

Protection Agency, 2006) were independently identified by 

others (for example, Bledsoe and others, 2005) as concerns 

affecting the biological integrity of streams in the ERW. 
Macroinvertebrate-Based Assessment of Biological Condition 11

0

0.25

0.50

0.75

1.00

1.25

 OBSERVED/EXPECTED

EXPLANATION

Filled circle indicates Observed/Expected

 value; darker circles represent overlaping 

 values 

Degradation threshold of 0.80

28 sites

65 samples

25 sites

30 samples

15 sites 

21 samples

5 sites 

8 samples

Gore Upper Eagle Middle Eagle Lower Eagle

BASIN SUBREGION

Figure 4. Summary of observed/expected values calculated from 124 

macroinvertebrate samples collected at 73 sites grouped by different subregions 

of the Eagle River watershed, Colorado, 2000–07 (see text for subregion 

definitions).

Although most urban sites were considered biologically 

degraded, a few sites had O/E values greater than the degraded 

condition threshold (sites 47, 57, 69, table 1). In a study of 

streams in the eastern United States, Carlisle and Meador 

(2007) observed that some urban sites with intact macroinvertebrate communities (O/E > 0.80) had well protected 

and established riparian corridors. In this study, there were 

no anecdotal explanations to describe why macroinvertebrate 

communities were relatively intact at a few urban sites. Unfortunately, the urban sites where O/E was greater than 0.80 (sites 

47, 57, 69, table 1) have not been sampled since 2000–01, so 

results may not reflect current (2010) biological condition as 

urban development has increased since that time (2000–01) 

near these sites. If macroinvertebrate communities at these 

sites are intact under present-day (2010) land-cover conditions, then further investigation is warranted to better diagnose 

why these sites remain in good biological condition. 

Mean O/E for mixed sites was similar to that of urban 

sites; however, variation in O/E values was largest when 

compared to other site classes (fig. 5). This result likely was 

affected by the inability to isolate the intensity of dominant 

factors or groups of co-occurring factors that affect biota at 

these mixed sites in the classification scheme used in this 

study. Others have suggested (for example, Carlisle and 

Hawkins, 2008) that variation in biological condition likely 

is associated with environmental factors operating at different spatial scales (that is, watershed, segment, and reach). 

Nonetheless, further investigation into causal factors affecting 

biological degradation at mixed sites will require better isolation of stressors through a well-planned study design. Understanding why some sites expected to have degraded biological 

communities (for example, urban and mixed sites), but show 

otherwise, will have clear implications for managing land 

use adjacent to streams and ultimately improving biological 
12 Macroinvertebrate-Based Assessment of Biological Condition at Selected Sites in Eagle River Watershed, CO, 2000–07

0

0.5

1.0

1.5

EXPLANATION

OBSERVED/EXPECTED

SITE CLASS

Undeveloped Adjacent road or 

highway or both

Mixed Urban

F = 12.415, p < 0.001

A

A

A

B

BC

18

18

15 26 14

90th percentile

75th percentile

50th percentile

25th percentile

10th percentile

Open circle indicates outlier 

Number of observations

Statistically different distributions (p < 0.05)

Distributions with different letters are statistically different 

Degradation threshold of 0.80

Figure 5. Distribution of macroinvertebrate observed/expected (O/E) values among 

undeveoped, adjacent road or highway or both, mixed, and urban site classes.

condition in the ERW at selected sites through mitigation 

efforts. 

Mean O/E values from ARH sites were expected to 

be lower than undeveloped sites, but they were found to be 

indistinguishable (fig. 5). This result was somewhat surprising 

as many of the ARH sites previously were noted as being 

affected by sediment deposition associated with FS road 

networks (Laurie, 2003) or Colorado Department of Transportation’s road sanding operations during the winter months 

(December through March) (for example, Black Gore Creek; 

Healy, 2008). Effects of sedimentation on macroinvertebrate 

communities are well documented (Waters, 1995 and citations 

within), causing significant reductions in macroinvertebrate

density, biomass, and diversity (Lenat and others, 1981; 

Newcombe and MacDonald, 1991; Angradi, 1999). The results 

in this report likely were affected by the inability to quantify 

the intensity of sedimentation at each ARH site in the classification scheme used herein. Other factors potentially affecting 

macroinvertebrate community response include the degree 

to which adjacent roads were buffered from adjacent streams 

by riparian vegetation, or whether disturbed areas associated 

with roads were directly connected to streams (see Wemple 

and others, 1996); however, neither was quantified in this 

study. Additionally, O/E measures taxa loss which may not be 
Macroinvertebrate-Based Assessment of Biological Condition 13

!

! ! !

! !

!!

!

!

! ! !

!

8

7

5

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2

1

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72

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70

69

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49

48

40

33

32

31

30

28 27

26

23

18

19

!

! !! !

!

!

!

53 29

10

9

6

52

51

50

47

46

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34

25

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11

106°15'W

106°45'W

39°45'N

39°30'N

58

Lower Eagle

Basin subregions

Base from U.S. Geological Survey 

digital data, 1983, Universal 

Transverse Mercator projection 

Zone 13

0

0 5 10 20 KILOMETERS

5 10 20 MILES

EXPLANATION

Scale 1:400,000

Gore

Middle Eagle Upper Eagle

Number corresponds to site 

 identification in table 1

Urban

Streams

Site class

Adjacent road 

 and/or highway

Undeveloped

Mixed

Good Degraded

Figure 6. Site location, site class, biological condition, and Eagle River watershed subregions, Colorado. 

Biological condition was based on the most recent samples collected between 2000 and 2007.
14 Macroinvertebrate-Based Assessment of Biological Condition at Selected Sites in Eagle River Watershed, CO, 2000–07

sensitive enough to indicate the intensity levels of sedimentation that differentially occurred across ARH sites. Furthermore, the most recent samples collected at each site were 

used in this analysis; and therefore, those samples may reflect 

sediment mitigation efforts implemented at some of these sites 

the year the samples were collected (for example, sites along 

Black Gore Creek) or may be a result of a decreased need to 

apply traction sand during that year (for example, a milder 

winter with less snow). 

Annual Variability of Observed/Expected 

Annual variability of O/E values over a 3- to 5-year 

period visually differed between undeveloped sites and sites 

along Lower Gore and Black Gore Creeks (fig. 7). Assessments of biological condition at undeveloped sites consistently 

were above the 0.80 threshold (mean O/E, 1.01; Standard 

Deviation (SD), 0.07; Coefficient of Variation (CV), 6.85), 

indicating that these sites were consistently in good condition 

and stable over the sampling period; whereas, assessments 

made along Black Gore Creek (mean O/E, 0.85; SD, 0.017; 

CV, 20.67) and lower Gore Creek sites (mean O/E, 0.67; SD, 

0.14; CV, 20.41) were comparatively variable and year depedent (table 2).

At lower Gore Creek urban sites (table 1, sites 38, 39, 41, 

and 52), most of the annual variability (large CV values) can 

be attributed to differences between O/E values from samples 

collected in 2000 (O/E near 1.0) and those samples collected 

during 2004 to 2007 (O/E < 0.80). These results indicate that 

either sampling error (that is, error because of field sampling, 

laboratory processing, modeling, or natural variability) or 

some other unexplained sources of variability affected O/E 

values, or major changes occurred between 2000 and 2004 

along Gore Creek that severely altered macroinvertebrate 

community structure. Regardless of what affected O/E values 

between 2000 and 2004 at sites along lower Gore Creek, all 

but one assessment made during 2004 and 2007 have indicated 

degraded biological condition as defined herein. 

Relatively high annual variability also was observed 

at most Black Gore Creek sites (fig. 7, table 2). Black Gore 

Creek sites (table 1, sites 30, 31, 34, and 36) used in this 

analysis were located along the section of stream listed in 

2002 as one of Colorado’s impaired waters under the Clean 

Water Act of 1972 (303d) for sediment deposition (Healy, 

2008). Although most O/E values calculated from these sites 

were above the impairment threshold, several were below, and 

a few were among the lowest observed in this study. Potential 

factors influencing variability in biological condition among 

years at these sites is possibly associated with interacting 

annual differences among the amount of traction sand and 

deicing chemical applied, timing and maintenance of sediment 

retention basins, sediment control or stabilization projects, 

or differences in rainfall, snowmelt, and associated stream 

hydrology. Identifying the sources of variability affecting 

assessments of biological condition was beyond the scope of 

this study; however, understanding assessment variability is 

imperative to mitigation efforts (for example, sediment control 

efforts currently (2010) being lead by CDOT that began in 

2002 (Healy, 2008) so that the uncertainty affecting any future 

mitigation efforts are minimized. 

Table 2. Site and data description where 3 to 5 years of annual macroinvertebrate samples were collected in the Eagle River 

watershed, Colorado, 2000-07. 

[ID, identification; O/E, ratio of observed to expected macroinvertebrate taxa, >0.80, biologically good, <0.80 biologically degraded; >, greater than; <, 

less than; CV, coefficient of variation] 

Site ID 

(fig. 2) Stream Years of data

Site 

class O/E Mean O/E

CV of 

O/E

18 Cross Creek 3 Undeveloped > 0.80 1.04 4.03

27 Gore Creek 3 Undeveloped > 0.80 1.14 3.46

28 Gore Creek 3 Undeveloped > 0.80 0.95 7.69

29 Gore Creek 5 Undeveloped > 0.80 1.01 8.76

32 Polk Creek 3 Undeveloped > 0.80 0.94 10.66

33 Polk Creek 3 Undeveloped > 0.80 1.00 6.49

30 Black Gore Creek 4 Adjacent road or highway, or both Year dependent 0.70 26.72

31 Black Gore Creek 4 Adjacent road or highway, or both Year dependent 0.86 13.39

34 Black Gore Creek 5 Adjacent road or highway, or both Year dependent 0.87 35.36

36 Black Gore Creek 4 Adjacent road or highway, or both > 0.80 0.96 7.23

38 Gore Creek 3 Urban < 0.80 0.61 13.32

39 Gore Creek 5 Urban Year dependent 0.76 25.69

41 Gore Creek 5 Urban Year dependent 0.71 26.73

52 Gore Creek 5 Urban < 0.80 0.60 15.90
Summary and Conclusions 15

0

0.25

0.50

0.75

1.00

1.25

EXPLANATION

OBSERVED/EXPECTED

Undeveloped

sites

Black Gore Creek

sites

Gore Creek

sites

Degradation threshold of 0.80

Unfilled circle indicates sites sampled 

 in 2000

Filled circle indicates individual samples; 

 darker colors represent overlap in 

 Observed/Expected values

Number corresponds to SiteID

 in table 1

18, 27, 28, 29, 

32, 33

18

30, 31, 34, 36 38, 39, 41, 52

Figure 7. Distribution of observed/expected values collected 

from 14 sites (6 undeveloped sites, 20 samples; 4 Black Gore 

Creek sites, 17 samples; 4 Gore Creek sites, 18 samples) 

representing 3 to 5 years of annual variability between 2000 

and 2007.

Observed and Expected Macroinvertebrate Taxa 

at Urban Sites

At urban sites, the occurrence of many macroinvertebrate 

taxa statistically was different than expected (fig. 8). Of the 

173 taxa evaluated, 21 were observed or predicted to occur 

at 7 or more sites (that is, one-half of the total number of 

urban sites). Of these 21 taxa, 9 were observed less frequently 

(intolerant decreasers) and 4 were observed more frequently 

(tolerant increasers) than expected. Additionally, there were 

six taxa apparently tolerant (nonresponsive) to the conditions 

at urban sites as they were observed nearly as frequently as 

predicted. Combined, these 19 taxa provide an opportunity to 

enhance the interpretation of future studies in the ERW but 

will require better insight into the responses of these taxa to 

specific stressors.

In general, the patterns of tolerant increaser (hereafter 

increaser) and intolerant decreaser (hereafter decreaser) taxa 

are consistent with the findings of other studies. Many of the 

decreaser taxa observed belong to groups known to be sensitive (for example, mayflies, stoneflies, and caddisflies), and 

many of the increasers belong to groups known to be tolerant 

(for example, Oligochaeta, Turbellaria, Chironomidae) to 

general disturbance (Barbour and others, 1999). It is emphasized that these results are specific to this study for a variety of reasons (for example, small sample size, taxonomic 

resolution, and small geographic scale) and should not be 

applied outside the ERW until they are further studied. Nonetheless, the list of decreaser taxa (fig. 8) provides a starting 

point for identifying causal factors at urban sites responsible 

for degraded biological condition if further effort is made 

to better understand the ecological requirements of these 

taxa and their tolerance to more specific stressors. Integrating results of this study with other approaches such as field 

and laboratory experiments greatly will enhance abilities to 

identify causal factors associated with degraded biological 

condition (Clements and others, 2002). Information generated from such integrative studies will be imperative for well 

targeted mitigation efforts at urban sites in the ERW. 

Summary and Conclusions

Local entities in the Eagle River watershed (ERW) are 

interested in better understanding how specific land-management activities affect biological communities and water 

quality in order to guide watershed-wide water-resource use 

management. In response to these needs, the U.S. Geological Survey (USGS), in cooperation with the Colorado River 

Water Conservation District, Eagle County, Eagle River 

Water and Sanitation District, Upper Eagle Regional Water 

Authority, Colorado Department of Transportation, City of 

Aurora, Town of Eagle, Town of Gypsum, Town of Minturn, 

Town of Vail, Vail Resorts, Colorado Springs Utilities, 

Denver Water, and the Department of U.S. Agriculture Forest 

Service (FS), compiled previously collected macroinvertebrate data in the ERW from selected USGS and FS studies 

from 2000–07. Specific objectives were to: (1) assess the 

biological condition of selected sites in the ERW using 

macroinvertebrate communities and (2) evaluate whether the 

dominant types of land cover were associated with biological 

condition. Additionally, short-term annual variability (3 to 

5 years) in O/E values was described and macroinvertebrate 

taxa that occurred more or less frequently than predicted at 

urban sites were identified.

Sites were classified into one of four classes (undeveloped, adjacent road or highway or both, mixed, and urban) 

using a combination of riparian land-cover characteristics, examination of topographic maps and aerial imagery, 
16 Macroinvertebrate-Based Assessment of Biological Condition at Selected Sites in Eagle River Watershed, CO, 2000–07

0

0 2 4 6 8 10 12 14 16

2

4

6

8

10

12

14

16

 NUMBER OF SITES OBSERVED (Of

)

EXPLANATION

1:1 line to aid interpretation

Tolerant increaser taxon that statistically occurred at more sites than predicted

Tolerant taxon that occurred nearly as frequently as predicted, and considered 

 impartial to conditions at urban sites

Intolerant decreaser taxon that statisically occurred less frequently than predicted

NUMBER OF SITES EXPECTED (EF ) 

Heterlimnius

Chloroperlidae

Rhyacophila sibirica group Zapada

Drunella doddsii

Rhithrogena

Cinygmula Rhyacophila brunnea group

Epeorus

Oligochaeta

Diamesinae

Brachycentrus

Turbellaria

Drunella grandis

Baetis

Arctopsyche

Chironominae

Orthocladiinae

Simuliidae

Figure 8. Frequencies of observed (Of) and expected (Ef) occcurrence of 19 invertebrate taxa 

that statistically occurred more or less frequently than predicted or were impartial to conditions 

at urban sites in the Eagle River watershed, Colorado, 2000–07.

screening for exceedances in water-quality standards, and best 

professional judgment. 

An independently developed predictive model was used 

to calculate a measure of taxonomic completeness for 124 

macroinvertebrate samples collected from 73 sites in the ERW, 

where taxonomic completeness was expressed as the ratio of 

the number of observed (O) taxa collected to those expected 

(E) to occur at each site in the absence of environmental 

degradation. These data were analyzed to assess the biological condition (that is, biologically “degraded” or “good”) of 

selected sites. Analysis of variance was used to determine if 

site class accounted for variability in mean macroinvertebrate 

O/E values. Macroinvertebrate communities were considered degraded at sites where O/E values were less than 0.80, 

indicating that at least 20 percent of expected taxa were not 

observed. This threshold also was statistically based as 0.80 

was near the 10th percentile O/E value (0.79) of the distribution of reference site data used to develop the model. Also, 

macroinvertebrate taxa that increased (tolerant increasers), 

decreased (intolerant decreasers), or were nonresponsive to 

conditions at urban sites were identified. It is emphasized that 

these assessment results are specific to the models used herein 

and are not directly comparable to assessments made with different indicators, methods, models, or thresholds.

This study represents the first standardized assessment 

of biological condition of selected sites distributed across 

the ERW. Of the 73 sites evaluated, just over one-half (55 

percent) were considered in good biological condition (O/E 
References Cited 17

greater than 0.80). The remaining sites either were biologically 

degraded (30 percent; O/E less than 0.80) or conditions were 

dependent on the year sampled (15 percent; O/E less than 

or greater than 0.80 depending on the year sampled). These 

assessments may not reflect current (2010) biological condition because 35 percent (26 sites) of these sites have not been 

sampled since 2000 or 2001. Nonetheless, site assessments 

are considered a screening tool for determining where in the 

ERW biological conditions are good or degraded until further 

investigation confirms otherwise. 

Sites primarily affected by urbanization were among the 

most severely degraded (urban sites had the lowest mean O/E 

values) when compared to other site classes. Although most 

urban sites were among the most severely degraded (that is, 

lowest mean O/E values relative to other site classes), a few 

sites had nearly intact macroinvertebrate communities (O/E 

near 1.0). Similar observations were noted among mixed 

classified sites. At least some within-class variability was 

possibly associated with errors inherent to classifying sites 

(that is, the inability to equally isolate dominate stressors at 

each site), especially for sites in the mixed class. Nonetheless, 

understanding why some sites were expected to have degraded 

biological communities, but showed otherwise, will have clear 

implications for mitigating biologically degraded streams in 

the ERB and elsewhere. 

Apparent changes occurred in biological condition (that 

is, from good to degraded) between 2000 and 2004 at urban 

sites along Gore Creek because nearly all annual samples 

collected during 2004 to 2007 have indicated degraded conditions. At undeveloped sites, biological condition remained 

stable during 2000–05, whereas conditions were year dependent at some adjacent road or highway or both (ARH) sites 

along Black Gore Creek. Understanding the associated causes 

of annual variability in macroinvertebrate communities among 

Black Gore Creek sites, as well as apparent biological degradation along Gore Creek, is warranted to effectively target 

mitigation efforts in the Gore Creek watershed. 

Thirteen macroinvertebrate taxa were identified that 

occurred more or less frequently than expected (that is, 4 taxa 

were tolerant increasers and 9 were intolerant decreasers) 

at urban sites. Additionally, six other taxa were impartial 

(tolerant) to the same conditions (that is, they occurred nearly 

as often as predicted). Combined, these 19 taxa provide an 

opportunity to enhance the interpretation of future studies in 

the ERW but will require better insight into the responses of 

these taxa to specific stressors. As a starting point, developing 

a better understanding of the ecological requirements and tolerance to specific stressors of the 9 decreaser taxa can enhance 

the ability to identify causal factors associated with degraded 

biological condition, ultimately directing mitigation efforts at 

urban sites in the ERW. 

In conclusion, these results represent the first standardized assessment of biological condition of selected sites in the 

ERW. More than one-half of the sites sampled were in good 

biological condition; however, nearly one-third of the sites 

sampled were considered biologically degraded. In most cases, 

sites in good biological condition were associated with undeveloped parts of the watershed (higher elevations); whereas, 

sites primarily affected by urbanization were among the most 

severely degraded (that is, lowest mean O/E values when compared to other site classes). Understanding the sources of variability affecting biological assessments along with why some 

sites expected to be degraded, but showed otherwise, will have 

clear implications for mitigation efforts. Integrating results of 

this study with other approaches such as field and laboratory 

experiments will enhance abilities to identify causal factors 

associated with degraded biological condition. Information 

generated from such integrative studies will be imperative for 

well targeted mitigation efforts in the ERW.

Acknowledgments

The authors thank the Colorado River Water Conservation District, Eagle County, Eagle River Water and Sanitation 

District, Upper Eagle Regional Water Authority, Colorado 

Department of Transportation, City of Aurora, Town of 

Eagle, Town of Gypsum, Town of Minturn, Town of Vail, 

Vail Resorts, Colorado Springs Utilities, and Denver Water 

for their continued support of this program. The authors also 

thank all U.S. Geological Survey (USGS), U.S. Department of 

Agriculture Forest Service employees, and others who assisted 

in collecting field data evaluated in this study. Daren Carlisle 

(USGS Reston, Va.) and several other reviewers provided 

useful comments that greatly improved earlier versions of this 

manuscript. 

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Zuellig and others—Macroinvertebrate-Based Assessment of Biological Condition at Selected Sites in Eagle River Watershed—USGS/SIR 2010–5148

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