Zone Biological Resources on the Rio Grande

in and Near Big Bend National Park, Texas

Water-Resources Investigations Report 02–4106

U.S. Department of the Interior

U.S. Geological Survey
Cover:

Rio Grande just downstream of Santa Elena Canyon in Big Bend National Park, August 1998 

(photograph by Patrick O. Keefe, U.S. Geological Survey).
U.S. Department of the Interior

U.S. Geological Survey

Baseline Assessment of Instream and RiparianZone Biological Resources on the Rio Grande 

in and Near Big Bend National Park, Texas

By J. Bruce Moring

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02–4106

In cooperation with the National Park Service

Austin, Texas

2002
ii

U.S. DEPARTMENT OF THE INTERIOR

Gale A. Norton, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names is for 

descriptive purposes only and does not imply 

endorsement by the U.S. Government.

For additional information write to

District Chief

U.S. Geological Survey

8027 Exchange Dr.

Austin, TX 78754–4733

E-mail: dc_tx@usgs.gov

Copies of this report can be purchased from

U.S. Geological Survey

Information Services

Box 25286

Denver, CO 80225–0286

E-mail: infoservices@usgs.gov
CONTENTS iii

CONTENTS

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

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

Methods of Assessment ....................................................................................................................................................... 4

Site Selection and Establishment .............................................................................................................................. 4

Stream-Habitat Assessment ...................................................................................................................................... 4

Fish-Community Assessment ................................................................................................................................... 11

Benthic Macroinvertebrate Assessment .................................................................................................................... 11

Data Management and Analysis ............................................................................................................................... 12

Results of Assessment .......................................................................................................................................................... 12

Stream Habitat .......................................................................................................................................................... 12

Fish Community ....................................................................................................................................................... 15

Benthic Macroinvertebrates ...................................................................................................................................... 23

Summary .............................................................................................................................................................................. 28

Selected References ............................................................................................................................................................. 29

FIGURES

1. Map showing baseline biological assessment sites in and near Big Bend National Park, Texas ......................... 2

2–6. Planimetric reach maps for:

2. Rio Grande above Colorado Canyon ......................................................................................................... 5

3. Rio Grande below Santa Elena Canyon ..................................................................................................... 6

4. Rio Grande at Johnson Ranch ................................................................................................................... 7

5. Rio Grande above Boquillas Canyon ......................................................................................................... 8

6. Rio Grande below Maravillas Creek (Black Gap Wildlife Refuge) .......................................................... 9

7. Graph showing reach sinuosity for five bioassessment reaches in and near Big Bend National Park, Texas ...... 13

8. Graph showing reach slope for five bioassessment reaches in and near Big Bend National Park, Texas ............ 13

9–13. Boxplots showing:

9. Streamflow velocity and selected stream-habitat measures for five bioassessment reaches in and 

near Big Bend National Park, Texas .......................................................................................................... 15

10. Bank height to channel width ratio for five bioassessment reaches in and near Big Bend 

National Park, Texas .................................................................................................................................. 16

11. Embeddedness of cobble for five bioassessment reaches in and near Big Bend National Park, Texas .... 16

12. Gravel and larger particle size of bed material for five bioassessment reaches in and near Big 

Bend National Park, Texas ......................................................................................................................... 17

13. Bank vegetation coverage for five bioassessment reaches in and near Big Bend National Park, 

Texas .......................................................................................................................................................... 17
iv 

14–26. Graphs showing:

14. Relative abundance of selected species of bank vegetation for five bioassessment reaches in and 

near Big Bend National Park, Texas .......................................................................................................... 18

15. Number of fish species for five bioassessment reaches in and near Big Bend National Park, Texas ........ 20

16. Number of fish versus number of species for five bioassessment reaches in and near Big Bend 

National Park, Texas .................................................................................................................................. 21

17. Menhinick’s species richness for fish communities for five bioassessment reaches in and near Big 

Bend National Park, Texas ......................................................................................................................... 22

18. Results of cluster analysis to indicate similarity of fish communities for five bioassessment reaches 

in and near Big Bend National Park, Texas ............................................................................................... 22

19. Relative abundance of major fish families for five bioassessment reaches in and near Big Bend 

National Park, Texas .................................................................................................................................. 23

20. Relative abundance of major fish trophic groups for five bioassessment sites in and near Big Bend 

National Park, Texas .................................................................................................................................. 24

21. Results of cluster analysis to indicate similarity of benthic aquatic-insect communities for five 

bioassessment reaches in and near Big Bend National Park, Texas ........................................................... 25

22. Comparison of total aquatic-insect taxa and richest targeted habitat (RTH) aquatic-insect taxa for 

five bioassessment reaches in and near Big Bend National Park, Texas .................................................... 25

23. Number of aquatic-insect taxa versus number of aquatic-insect individuals (richest targeted habitat 

[RTH] samples only) for five bioassessment reaches in and near Big Bend National Park, Texas ........... 26

24. Menhinick’s taxa richness for aquatic insects for five bioassessment reaches in and near Big Bend 

National Park, Texas .................................................................................................................................. 26

25. Relative abundance of ephemeroptera and trichoptera taxa (richest targeted habitat [RTH] 

samples only) for five bioassessment reaches in and near Big Bend National Park, Texas ....................... 27

26. Relative abundance of major aquatic-insect trophic groups (richest targeted habitat [RTH] 

samples only) for five bioassessment reaches in and near Big Bend National Park, Texas ....................... 27

TABLES 

1. Name, location, and general description of U.S. Geological Survey Rio Grande bioassessment reaches in 

and near Big Bend National Park, Texas .............................................................................................................. 10

2. Stream-habitat measures for bioassessment reaches on the Rio Grande in and near Big Bend National 

Park, Texas ............................................................................................................................................................ 11

3. Fish taxa and individual counts of fish collected in the Rio Grande in and near Big Bend National 

Park, Texas ............................................................................................................................................................ 19

4. Taxonomic classification of benthic macroinvertebrates and counts for individual taxa collected in the 

Rio Grande in and near Big Bend National Park, Texas ....................................................................................... 31

ABBREVIATIONS

cm, centimeters

km, kilometers

L, liters

m, meters

m/s, meters per second

m2, square meters

m3/s, cubic meters per second

μm, micrometers

mg/L, milligrams per liter

mm, millimeters 
Abstract 1

Baseline Assessment of Instream and RiparianZone Biological Resources on the Rio Grande 

in and Near Big Bend National Park, Texas

By J. Bruce Moring

Abstract

Five study sites, and a sampling reach within 

each site, were established on the Rio Grande in 

and near Big Bend National Park in 1999 to provide the National Park Service with data and 

information on the status of stream habitat, fish 

communities, and benthic macroinvertebrates. 

Differences in stream-habitat conditions and 

riparian vegetation reflect differences in surface 

geology among the five sampling reaches. In the 

most upstream reach, Colorado Canyon, where 

igneous rock predominates, streambed material is 

larger; and riparian vegetation is less diverse and 

not as dense as in the four other, mostly limestone 

reaches. Eighteen species of fish and a total of 474 

individuals were collected among the five reaches; 

348 of the 474 were minnows. The most fish species (15) were collected at the Santa Elena reach 

and the fewest species (9) at the Colorado Canyon 

and Johnson Ranch reaches. The fish community at 

Colorado Canyon was least like the fish communities at the four other reaches. Fish trophic structure 

reflected fish-community structure among the five 

reaches. Invertivores made up at least 60 percent of 

the trophic structure at all reaches except Colorado 

Canyon. Piscivores dominated the trophic structure 

at Colorado Canyon. At the four other reaches, 

piscivores were the smallest trophic group. Eighty 

percent of the benthic macroinvertebrate taxa collected were aquatic insects. Two species of blackfly 

were the most frequently collected invertebrate 

taxon. Net-spinning caddisflies were common at all 

reaches except Santa Elena. The aquatic-insect 

community at the Boquillas reach was least similar 

to the aquatic-insect community at the other 

reaches. 

INTRODUCTION

The National Park Service (NPS) manages 386 

km of the Rio Grande in West Texas, including 190 river 

kilometers in Big Bend National Park (BBNP) and 

another 196 km of the Rio Grande Wild and Scenic 

River downstream from the Park (fig. 1). The Texas 

Parks and Wildlife Department (TPWD) manages 97 

km of the Rio Grande at Big Bend Ranch State Park 

adjacent to and upstream of BBNP. In Mexico, the 

Cañon de Santa Elena Floral and Faunal Protected Area 

borders about 220 km of the Rio Grande from Mulato, 

Mexico, to near the mouth of Mariscal Canyon. The 

Maderas del Carmen Floral and Faunal Protected Area 

borders about 40 km of the Rio Grande upstream of 

Boquillas Canyon to Boquillas, Mexico, at the downstream boundary of BBNP just upstream of La Linda, 

Mexico.

The North American Free Trade Agreement 

(NAFTA) of 1992 has resulted in increased population 

and industry upstream of BBNP in both the United 

States and Mexico. Ciudad Juárez, Chihuahua (pop. 

1,200,000), across the Rio Grande from El Paso, Texas, 

has historically discharged untreated urban and industrial waste directly into the Rio Grande.

Dams and flow diversions in the Rio Grande 

Basin have reduced the annual flow volume of the river 

and altered the seasonality of flows, particularly peak 

flows from snowmelt from the mountains of southern 

Colorado and northern New Mexico (Schmidt and 

Everitt, 2000). Currently (2001), peak flows in the 

Rio Grande downstream of the confluence of the Río 

Conchos are about one-half what they were prior to 

1915. For example, the maximum mean daily discharge 

with a 2-year recurrence interval was 217 m3/s prior to 

1915 and the construction of Elephant Butte Reservoir 

in New Mexico and 126 m3/s after its construction 

(Schmidt and Everitt, 2000). The Río Conchos is the 

primary tributary to the Rio Grande upstream of BBNP 
2 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

0 50 100 KILOMETERS

1

Black Gap/Rio Grande below Maravillas

Creek, Black Gap Wildlife Refuge–

River kilometer 1,219

2

Boquillas/Rio Grande above Boquillas

Canyon, Big Bend National Park–

River kilometer 1,260

3 Johnson Ranch/Rio Grande at Johnson

Ranch, Big Bend National Park–River

kilometer 1,377

4

Santa Elena/Rio Grande below Santa Elena

Canyon, Big Bend National Park–

River kilometer 1,414

5

Colorado Canyon/Rio Grande at Colorado 

Canyon below Panther Creek, Big Bend Ranch

State Park–River kilometer 1,468

Canon de Santa

 Elena Floral and

 Faunal Protected

 Area

Maderas

 del Carmen

 Floral and Faunal

 Protected Area

EXPLANATION

Site name/Description

Big Bend Ranch

 State Park

Big Bend

National

Park

Black Gap 

Wildlife Refuge

Panther

 Creek

Maravillas

Creek

1

2

3

4

5

RIO

CONCHOS

ALAMITO

CREEK

RIO

GRANDE

RIO GRANDE

PECOS

RIVER

DEVILS

RIVER

AMISTAD

 INTERNATIONAL

 RESERVOIR

PRESIDIO BREWSTER PECOS TERRELL CROCKETT

VAL VERDE

SUTTON

CHIHUAHUA

COAHUILA

UNITED

STATES

MEXICO

105o

30o

29o

104o 103o 102o 101o

Study area

TEXAS

Presidio

Redford

Lajitas

Dryden

Ciudad

 Juarez

El Paso

Ojinaga

Fort

 Quitman

La Linda

Mulato Boquillas

Santa Elena Canyon Boquillas

 Canyon

Johnson

Ranch

ELEPHANT

 BUTTE

 RESERVOIR

AMISTAD

 INTERNATIONAL

 RESERVOIR

FALCON

 INTERNATIONAL

 RESERVOIR

RIO

RIO

GRANDE

GRANDE

MariscalCanyon

Colorado Canyon

Figure 1. Baseline biological assessment sites in and near Big Bend National Park, Texas.
INTRODUCTION 3

and accounts for almost one-half the entire Rio Grande 

drainage area in Mexico. In recent decades, the Río 

Conchos has contributed 85 percent of its pre-reservoir 

historical flow volumes through Big Bend Ranch State 

Park, BBNP, and the Rio Grande Wild and Scenic River 

(Saunders, 1987). Regulation on the Río Conchos has 

reduced total annual flow volumes and peak flows 

downstream in the Rio Grande and in BBNP.

In addition to historical changes in streamflows in 

the Rio Grande in the Big Bend region, several waterquality issues could be affecting the status and health of 

aquatic biota. First, salinity has increased at a rate of 15 

to 18 mg/L per year downstream of the Río Conchos 

confluence with the Rio Grande (Miyamoto and others, 

1995). At this rate, salinity could reach 1,000 mg/L 

early in the 21st century, about a 30-percent increase 

over average salinity levels in this reach of the Rio 

Grande in 1969. Second, concentrations of dissolved 

arsenic, copper, lead, and mercury in this reach of the 

Rio Grande have exceeded the U.S. Environmental Protection Agency (USEPA) chronic criteria for the protection of aquatic life (Miyamoto and others, 1995), and 

concentrations of metals in fish tissues have exceeded 

the 85th-percentile criterion established by the U.S. 

Fish and Wildlife Service (USFWS), which means that 

the concentrations were greater than 85 percent of the 

concentrations measured throughout the United States. 

Third, the State of Texas has listed segment 2306 of the 

Rio Grande, upstream of Amistad Reservoir, as not 

meeting aquatic-life use designation because of ambient 

toxicity of sediment and water that exceeds criteria for 

the protection of aquatic life (Texas Natural Resource 

Conservation Commission, 1999). Fourth, hydrophobic 

compounds such as the DDT breakdown product p,p'-

DDE, the organophosphate pesticide chlorpyrifos, and 

several polycyclic aromatic hydrocarbons have been 

detected in the water (Moring, 1999) and probably 

occur in the streambed sediment of this reach of the Rio 

Grande. These hydrophobic compounds can bioaccumulate in the tissues of aquatic organisms to levels of 

concern and can accumulate in streambed sediments 

because of their tendency to adsorb to sediment particles and other particulates (Moring, 1999).

The Rio Grande represents the Chihuahuan 

Desert's most extensive aquatic and associated riparian 

environments. Aquatic-life inventories in the Rio 

Grande through the Big Bend region indicate that 4 of 

36 known native fish species from the river and its tributaries in the Big Bend region have been extirpated, 

including the endangered Rio Grande silvery minnow 

(Platania, 1990). Of the remaining native species, the 

Big Bend gambusia is federally endangered, and the Rio 

Grande chub, blue sucker, blotched gambusia, Chihuahua shiner, Mexican stoneroller, proserpine shiner, Rio 

Grande darter, and Rio Grande shiner have been identified by the USFWS as species of concern. Thirty-two 

native fish species persist but compete with 11 nonnative fish species, a number that has steadily increased 

in recent years. Several fish-community surveys have 

been done in the Rio Grande and its tributaries through 

the Big Bend region (Evermann and Kendall, 1894; 

Hubbs, 1940; Trevino-Robinson, 1959; Bestgen and 

Platania, 1988; Platania, 1990; Garrett, 1997; Allan, 

1998). However, no study has been done in the last 5 to 

10 years to quantify the occurrence and distribution of 

native fish species in the Big Bend region or to establish 

long-term benchmark reaches to assess the status and 

trends in fish communities over time. 

The use of aquatic invertebrates, particularly 

insects, as indicators of aquatic ecosystem health is well 

known (Hilsenhoff, 1987; Reynoldson and Rosenberg, 

1996). However, the NPS lacks distribution and abundance data on the majority of Rio Grande invertebrate 

taxa in BBNP. Aquatic invertebrates are integral to 

ecosystem health and are indicators of ecosystem disturbance that could be used to monitor effects of nonnative species, pollution events, reduced streamflow, 

and other anthropogenic disturbances in the Big Bend 

region. No comprehensive characterization of Rio 

Grande aquatic invertebrate communities in the Big 

Bend region has been done, and nothing is in place to 

monitor changes in the invertebrate community in 

BBNP. 

 This study, conducted by the U.S. Geological 

Survey (USGS) in cooperation with the NPS, was 

designed to collect and interpret the essential data 

needed to effectively manage aquatic-life resources of 

the Rio Grande in reaches in and near BBNP—and to 

establish benchmark sites to assess the status and trends 

in stream habitat, fish communities, and benthic macroinvertebrates over time. The key findings from this 

study will be integrated into a comprehensive long-term 

water-quality assessment plan for the Rio Grande. The 

study was part of the Water Quality Partnership Program between the USGS and NPS.

The purpose of this report is to provide the 

NPS as well as policymakers, regulators, and nongovernmental organizations in Texas and Mexico 

with baseline data and supporting interpretations 

on the occurrence and distribution of instream and 
4 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

riparian-zone biological resources of the Rio Grande in 

five reaches in and near BBNP. The report describes the 

field methods of assessment of stream habitat, fish communities, and benthic macroinvertebrates. Results based 

on statistical analyses and graphical comparisons are 

presented.

METHODS OF ASSESSMENT

Site Selection and Establishment

Potential study sites on the main stem of the Rio 

Grande were selected after map-based and aerial reconnaissance in December 1998 and January 1999. Five 

study sites were selected from the potential sites—one 

upstream of BBNP adjacent to Big Bend Ranch State 

Park, three in BBNP, and one downstream of the Park 

adjacent to Black Gap Wildlife Refuge (fig. 1). The 

sites, in downstream order, are Colorado Canyon at river 

kilometer 1,468, Santa Elena at river kilometer 1,414, 

Johnson Ranch at river kilometer 1,377, Boquillas at 

river kilometer 1,260, and Black Gap at river kilometer 

1,219 (table 1). The five sites compose a synoptic network for assessment, and the three sites in BBNP also 

are benchmark sites for proposed long-term monitoring. 

A sampling reach was selected at each site on the 

basis of the occurrence and frequency of geomorphic 

channel features. Reaches were selected in segments of 

the river where riffle-pool geomorphic sequences were 

common, and the channel had a classical meandering 

morphology (Leopold and others, 1964; Gordon and 

others, 1992). A meandering channel was selected for 

each reach to maximize the variability in streamflow 

velocities, depth, bed substrate, and in-channel structure 

over the shortest length of channel possible. Each reach 

had at least two each of the three geomorphic channel 

units: riffles, runs, or pools (Meador, Hupp, and others, 

1993). Reach length varied from about 352 m at Santa 

Elena to about 726 m at Black Gap. Upstream and 

downstream boundaries for each reach were selected, 

and monuments were established to identify them by 

driving a 30- by 1.25-cm reinforcement-bar stake with 

an aluminum surveyor cap on top stamped with 

“USGS.” Additional stakes and caps were driven into 

the ground to establish markers for each of four 

transects corresponding to the four geomorphic channel 

units in the reach. All monuments and markers were 

established on or above what was determined by on-site 

visual inspection to be the high-bank terrace on the U.S. 

bank. If a stake could not be driven, a chisel-square 

mark was made on the rock and painted blue to indicate 

a reach boundary monument or orange to indicate a 

transect marker. Coordinates were recorded for each 

monument and marker using a hand-held GPS unit with 

an accuracy of +3.0 m. 

All of the transects were located in reference to a 

permanent marker that was georeferenced. Coordinates 

and a transect bearing and distance were recorded in reference to this marker. This permanent marking of each 

transect will allow subsequent investigators to return 

and make measurements and comparisons over time of 

all of the reach and transect-based measures.

Stream-Habitat Assessment

Synoptic assessments at each reach included the 

construction of a detailed planimetric reach map 

(figs. 2–6) to highlight channel features and reach shape 

and the characterization of channel and riparian-zone 

habitat features (Meador, Hupp, and others, 1993; and 

Moring and others, 1998), which include depths, 

velocities, and type, frequency, and extent of geomorphic channel units (riffles, runs and pools).

Several reach-based and within-reach, transectbased measures were taken (table 2). Four channel cross 

sections extending from the right bank (Mexican bank) 

high-bank terrace to the left bank (U.S. bank) high-bank 

terrace were selected in each reach corresponding to the 

four geomorphic channel units that were selected. Six or 

more transects per reach have been suggested (Cuffney 

and others, 1993). However, in this study, the selection 

of four cross sections in each reach was thought to be 

sufficient to characterize channel morphology and 

therefore variability in stream velocity, depth, and substrate types and distribution. In addition to noting the 

dominant and subdominant bed substrate, a 100-point 

“pebble count” (Goudie, 1981) was done in riffles in 

each reach to quantify the types and relative abundance 

of the coarser substrates such as gravel, cobble, and 

boulders.

A Sokia Leitz Set 4A laser-operated total 

station was used to survey all transects and the entire 

reach to produce many of the stream habitat measures 

and the planimetric maps. All survey data were stored 

on site in a datalogger that was electronically linked to 

the total station. The data were imported into an electronic spreadsheet and sorted, and computations were 

performed to determine linear reach length, curvilinear 

reach length, bank slope, bank height, channel width, 

and median depth (Moring and others, 1998).
METHODS OF ASSESSMENT 5Figure 2. Planimetric reach map for Rio Grande above Colorado Canyon.

Upstream boundary

Transect 1

Transect 2

Transect 3

Transect 4

Downstream boundary

Riffle

Pool

Water

Thalweg

N

Note: Width and depth measurememeters and velocity measurements meters per second.For location see figure 1.

0 43 

18.59

146.13

Transect 1

Average depth: 0.50Average velocity: 0.42

U.S. high-bankmonument

Not to scale

Not to scale

153.46

43.89

Transect 3

Average depth: 0.43Average velocity: 0.54

94.47

Transect 4

Not to scale

Transect 2

21.33

152.84

Average depth: 0.41Average velocity: 0.78

U.S. high-bankmonument
6 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

Figure 3. Planimetric reach map for Rio Grande below Santa Elena Canyon.

N

Note: Width and depth measuremmeters and velocity measuremenmeters per second.For location see figure 1.

Upstreamboundary

Transect 1

Transect 2

Transect 3

Thalweg

Island

Point bar

0 

Riffle

Transect 2Average depth: 0.49Average velocity: 0.84

U.S. high-bankmonument

17.67

137.96

Not to scale

U.S. high-bank

monument

Transect 1

Average depth: 0.62Average velocity: 0.40

40.84

133.55

Not to scale

Transect 3

Not to scale

U.S. high-bankmonument

Average depth: 0.42Average velocity: 0.96

18.89

96.46

Transect 4

Average depth: 0.50Average velocity: 0.51

101.52
METHODS OF ASSESSMENT 7Figure 4. Planimetric reach map for Rio Grande at Johnson Ranch. Note: Width and depth measurements in meters and velocity measurements in meters per second. For location see figure 1.

N

Upstream boundary

Transect 1

Transect 2

Transect 3

Tr

Water

Thalweg

Island Riffle

0 

U.S. highbankmonument

Average depth: 0.47Average velocity: 0.65

Transect 1

52.02

16.45 18.28

Not to scale

Not to scale

Not to scale

U.S. highbank monument

Average depth: 0.65Average velocity: 0.49

Transect 2

28.95

96.18

Transect 3

Average depth: 0.55Average velocity: 0.78

119.72

U.S. highbank monument

Average depth: 0.67Averge velocity: 0.23

Transect 4

142.09

28.95
8 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

Figure 5. Planimetric reach map for Rio Grande above Boquillas Canyon.

Upstream boundary

Transect 1 Transect 2

Transect 3

Transect 4

Water Thalweg

Riffle

Downstream boundary

Island

Note: Width and depth measurements in meters and velocity measurements in meters per second.For location see figure 1. 0 32 64 METERS

N

U.S. highbankmonument

Average depth: 0.43Average velocity: 0.22

Transect 1

75.66103.73

U.S. highbankmonument

Transect 2

Average depth: 0.45Average velocity: 0.37

77.36

101.39

Average depth: 0.35Average velocity: 0.83

Transect 3

12

U.S. high-bank monument

Transect 4

Average depth: 1.36Average velocity: 0.37

60.30

108.22

Not to scale

Not to scale

Not to scale
METHODS OF ASSESSMENT 9Figure 6. Planimetric reach map for Rio Grande below Maravillas Creek (Black Gap Wildlife Refuge).

Upstream boundary

Transect 1

Transect 2

Trans

Transect 4

Downstream boundary

Water

Riffle

Thalweg

N

Note: Widin meters in meters For locati

0 50 100 METERS

Boulder

78.29

27.31

Transect 3

Average depth: 0.83Average velocity: 0.34

U.S. high-bank monument

Not to scale

102.76

30.48

U.S. highbankmonument

Transect 1

Average depth: 0.31

Not to scale Average velocity: 1.01

Transect 2

Average depth: 0.41Average velocity: 0.91

130.35

Not to scale

76.44

32.00

Transect 4

Average depth: 2.60Average velocity: 0.11

U.S. high-bankmonument
10 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

Table 1. Name, location, and general description of U.S. Geological Survey Rio Grande bioassessment reaches in and nePark, Texas[km, kilometers; --, not available] 

Reach

name

Reach

location

River

kilometer

at upstream

boundary

Coordinates

for upstream

boundary

monument

Geomorphic channel unit coordinates for reach 

Transect 1

monument

Transect 2

monument

Transect 3

monument

Tra

mo

Colorado 

Canyon

1.6 km above “Big Hill” turnoff on 

U.S. 170

1,468

N 29°17'52.89"W 103°57'23.49"

Riffle

N 29°17'52.68"

W 103°57'18.74" 

Riffle

N 29°17'52.42"

W 103°57'17.08"

Pool

N 29°17'52.46"

W 103°57'16.76"

R

N 29°W 103

Santa Elena 1.6 km below Terlingua Creek 

confluence at river take-out

1,414

N 29°09'20.38"W 103°35'54.88"

Run

N 29°09'19.40"

W 103°35'54.15"

Riffle

N 29°09'17.60"

W 103°35'52.66"

Riffle

N 29°09'16.04"

W 103°35'51.45"

N 29°0W 103

Johnson 

Ranch 

International Boundary and Water 

Commission gaging station near Johnson Ranch

1,377

N 29°02'05.10"W 103°23'22.47"

Riffle

N 29°01'59.72"

W 103°23'18.88"

Run

N 29°02'01.12"

W 103°23'16.04"

Riffle

N 29°01'58.10"W 103°23'13.61"

N 29°0W 103

Boquillas 1.6 km above mouth of Boquillas 

Canyon

1,260

N 29°11'30.80"W 102°55'14.87"

Riffle

N 29°11'28.65"W 102°55'10.19"

Run

N 29°11'28.32"

W 102°55'08.02"

Riffle

N 29°11'28.98"W 102°55'06.13"

N 29°W 102

Black Gap 4.8 km below confluence of 

Maravillas Creek in Black Gap Wildlife Refuge

1,219

--

Riffle

--

Riffle

N 29°35'34.49"

W 102°46'07.79"

Run

N 29°35'38.24"

W 102°46'09.11"

N 29°3W 102
METHODS OF ASSESSMENT 11

Fish-Community Assessment

The fish community at each reach was sampled 

once in early spring 1999 and once in late summer 1999. 

Samples were collected in different seasons to account 

for any seasonal sampling bias because of differences in 

fish recruitment and local distribution and to provide the 

most accurate information on species composition and 

relative abundance for each reach. 

Various electrofishing and netting techniques 

were used to assess fish-community composition and 

structure (Meador, Cuffney, and Gurtz, 1993; Moring 

and others, 1998). Backpack and barge electrofishing 

equipment were used to sample fish at each reach. Two 

electrofishing passes per reach were made, and the time 

in seconds that the electrofishing unit was operating was 

recorded to monitor electrofishing sampling. A 7.5- by 

3.0-m by 0.64-cm mesh seine was used to supplement 

electrofishing sampling, particularly in riffles, and at 

least one seine-haul was made in each type of geomorphic channel unit present in the reach.

All collected fish were identified to species. In 

addition, total and standard lengths in millimeters, total 

weight in grams, and the type, frequency, and location 

of any external anomalies such as lesions, tumors, or 

parasites were noted (Meador, Cuffney, and Gurtz, 

1993). All minnows were fixed in 10-percent buffered 

formalin and returned to the USGS office in Austin, 

Tex., for verification of identification. 

Benthic Macroinvertebrate Assessment

Benthic macroinvertebrate samples were collected at the five sampling reaches using reach-based 

compositing methods developed by the USGS National 

Water-Quality Assessment (NAWQA) Program 

(Cuffney and others, 1993; Moring and others, 1998). 

Two sample types, a richest targeted habitat (RTH) and 

a qualitative multihabitat (QMH), were collected at 

each reach in early March 1999. In addition to the 

NAWQA-style samples, a USEPA Rapid Bioassessment Method (Plafkin and others, 1989) traveling 

Table 2. Stream-habitat measures for bioassessment reaches on the Rio Grande in and near Big Bend National 

Park, Texas 

[m, meters; m/s, meter per second; mm, millimeters] 

Habitat measure Colorado

Canyon

Santa

Elena

Johnson

Ranch Boquillas BlackGap

Linear reach length (m) 362.6 323.9 349.4 397.7 609.5

Curvilinear reach length (m) 487.6 352.5 403.7 593.1 726.6

Reach sinuosity 1.34 1.09 1.16 1.49 1.19

Reach slope .0048 .0024 .0017 .0012 .0017

Median bank slope .102 .079 .061 .072 .187

Median bank height (m) 20.4 18.0 19.9 15.7 22.8

Median high-bank channel width (m) 149.5 117.5 116.8 105.9 90.5

Median bank height to high-bank channel 

width ratio

.117 .042 .042 .047 .077

Median depth (m) 1.19 .503 .609 .548 .695

Median velocity (m/s) .799 .587 .492 .309 .625

Median embeddedness of cobble (percent) 40 20 60 50 65

Dominant streambed substrate boulder cobble cobble cobble cobble

Median canopy angle (degrees) 153 138 158 168 162

Median particle size (mm) 110.0 26.5 47.0 57.0 50.0

Median bank vegetation coverage (percent) 17.5 72.5 92.5 40.0 85.0
12 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

kick-seine sample was collected at each reach at the 

time the RTH samples were collected. 

Five RTH samples were collected at each reach 

and composited. Each RTH sample was taken by selecting five sampling locations distributed between two 

or more riffles in the reach and placing a 0.25-m2

sampling quadrat frame in the streambed at each location. A modified Surber sampling net with a 425-μm 

mesh net and a 425-μm attached plankton bucket was 

placed on the downstream end of the frame, and the 

streambed in the frame was disturbed for 60 seconds to 

dislodge benthic organisms into the net. The contents 

of the net and plankton bucket were rinsed into a 19-L 

bucket, processed through a 425-μm sieve to remove 

entrained organic and inorganic debris, and placed in 

a 1-L polypropylene jar and fixed with 10-percent 

buffered formalin (Cuffney and others, 1993).

A QMH sample was collected at each reach by 

using a 205-μm mesh d-frame net. Individual QMH 

samples were collected in multiple habitats at each 

reach and composited into the QMH sample. Contents 

of the d-frame net were placed in a 19-L bucket, field 

processed through a 205-μm mesh sieve, and placed in 

a 1-L polypropylene jar and fixed with 10-percent 

buffered formalin. 

All benthic samples were shipped to a contract 

laboratory for identification and enumeration of taxa. 

If a sample contained 500 or fewer organisms, all the 

organisms in the sample were sorted, counted, and identified. If the sample contained more than 500 organisms, 

a 500-count subsampling routine was used (Lester, 

1999). Most taxa, except for some midges and noninsect macroinvertebrates, were identified to genus. A 

reference collection with at least one specimen of each 

taxon identified was provided to the USGS by the contract laboratory.

Data Management and Analysis

All field data were recorded on forms printed on 

waterproof paper. All field data were checked for datarecording anomalies and consistency in units of measurement prior to transfer to digital format, then keyed 

into an electronic spreadsheet and reviewed for accuracy. Survey data were recorded in digital format in the 

field, printed and reviewed, and uploaded to an electronic spreadsheet pending map production and data 

analysis.

Selected stream-habitat measures (table 2) were 

compared among the five reaches. All quantitative 

transect-based stream-habitat measures (those for 

which medians are listed in table 2) were compared 

among reaches using a multisample median test (Zar, 

1984). The objective of the test was to determine if the 

median measures differ significantly among the five 

reaches. The test indicates whether at least one median 

is significantly different but does not indicate which 

reach median is different. If the result of a median test 

indicated a significant difference, then a nonparametric 

Tukey-type multiple-comparison test (Zar, 1984) was 

used to determine which median (or medians) was different. The multiple-comparison test compares all possible combinations of reach medians and indicates 

which medians are similar or different from others. The 

tests were done and graphical comparisons were generated using a statistical software package (StatSoft, 

1998).

All fish and benthic macroinvertebrate data were 

analyzed using an electronic spreadsheet to compute 

community metrics or were transferred to the statistical 

software package for statistical analysis and generation 

of graphical comparisons. In addition to the computation of summary statistics (means, medians, percentiles, 

maximums, and minimums), cluster analyses were 

done using abundance data to compare overall fishcommunity similarity and benthic aquatic-insect 

similarity among the five reaches. Cluster analysis is a 

statistical classification technique that places similar 

objects (reaches in this application) into groups or 

“clusters” (Ludwig and Reynolds, 1988).

RESULTS OF ASSESSMENT

Stream Habitat

Reach sinuosity, determined as the ratio of curvilinear reach length divided by linear reach length, varied 

from a minimum of 1.09 at Santa Elena to a maximum 

of 1.49 at Boquillas (fig. 7). Reach slope varied from a 

minimum of 0.0012 at Boquillas to a maximum of 

0.0048 at Colorado Canyon (fig. 8). Sinuosity can be 

interpreted as the degree to which a channel meanders 

over a given distance of channel length. Presumably, the 

greater the sinuosity, the greater the variability in stream 

velocities, channel substrate types and sizes, and overall 

habitat variability and availability for aquatic life. The 

larger sinuosity of the Boquillas reach reflects the wider 

floodplain of the Rio Grande just upstream of Boquillas 

Canyon; whereas the smaller sinuosity of the Santa 

Elena reach might be influenced by confining canyon 

walls, at least on the Mexico side. 
RESULTS OF ASSESSMENT 13

1.0

1.1

1.2

1.3

1.4

1.5

0.9

0.8

0.2

0.3

0.4

0.5

0.6

0.7

0.1

0

SINUOSITY

REACH

Colorado

 Canyon

Santa

Elena

Johnson

 Ranch

Boquillas Black

 Gap

REACH

SLOPE

0

0.001

0.002

0.003

0.004

0.005

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

Figure 7. Reach sinuosity for five bioassessment reaches in and near Big Bend National Park, Texas.

Figure 8. Reach slope for five bioassessment reaches in and near Big Bend National Park, Texas.
14 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

Stream velocity (χ2 = 2.67, p = 0.61), bank height 

(χ2 = 7.00, p = 0.14), and bank slope (median test 

statistic [χ2

] = 4.00, p-value = 0.41) were not significantly different among the five reaches (fig. 9). Too few 

data were available for statistical testing of high-bank 

channel width. Median bank slope was largest at Black 

Gap (0.187) and smallest at Johnson Ranch (0.061). 

Bank heights generally were larger at the Colorado 

Canyon and Black Gap reaches. Median high-bank 

channel widths ranged from 90.5 m at Black Gap to 

149.5 m at Colorado Canyon. Median stream velocities 

were relatively uniform among all five reaches, less than 

1 m/s. A maximum stream velocity of 3.25 m/s was 

recorded at Colorado Canyon in Panther Rapids at the 

upstream end of the study reach. 

The bank height to high-bank channel width ratio 

was computed for each reach as a measure of channel 

incision. On the basis of the median test, this ratio was 

not the same among all reaches (χ2

 = 16.00, p = 0.003); 

thus the multiple comparison test was done. Channel 

incision ratio was significantly different at the 0.05 

level between Colorado Canyon and Santa Elena and 

between Colorado Canyon and Johnson Ranch (fig. 10). 

The median incision ratio of 0.117 for Colorado Canyon 

was greater than the ratios for Santa Elena and Johnson 

Ranch by a factor of almost 3. The Colorado Canyon 

reach is bordered on both sides by the downstream end 

of Colorado Canyon. This confinement has created a 

narrow floodplain, a relatively high bank terrace, and 

the largest bank height to channel width ratio of the five 

reaches.

The percent embeddedness of cobble substrate 

was not the same among the five reaches (χ2 = 18.02, 

p = 0.0012). Percent embeddedness was significantly 

different at the 0.05 level between the Santa Elena and 

Black Gap reaches (fig. 11). Median percent embeddedness varied from 20 percent at Santa Elena to 65 percent 

at Black Gap. Increased embeddedness might indicate a 

channel that is not in equilibrium with its floodplain 

because of increasing bed loads that are not being distributed downstream or into the adjacent floodplain. 

Embeddedness also can be interpreted as a measure of 

the extent of siltation in a reach and can affect organism 

dispersal, occlusion of respiratory membranes (such as 

gills), egg development, and hatch success.

Gravel and larger particle sizes were compared 

among the reaches. Particle size was not the same 

among the five reaches (χ2

 = 205.4, p = 0.0001). 

Median particle size for the Colorado Canyon reach and 

for the Santa Elena reach were significantly different at 

the 0.05 level from that for other reaches (fig. 12). 

Median particle size was not significantly different 

among the Johnson Ranch, Boquillas, and Black Gap 

reaches. The significantly different (larger) median particle size at Colorado Canyon is caused by the presence 

of more small cobbles or boulders of igneous rock. 

Igneous rock weathers more slowly than the limestone 

that characterizes the surficial geology of the four other 

reaches farther downstream. No apparent factor was 

responsible for the significantly different (smaller) 

median particle size at Santa Elena.

The density and composition of bank vegetation 

can influence bank stability, rates of bank erosion, and 

composition and rate of allochthonous input to a stream 

channel (Hynes, 1970). In addition, a number of exotic 

species of vegetation have encroached on BBNP and the 

surrounding region. Accounting for the occurrence and 

distribution of indigenous and non-indigenous species 

of riparian vegetation along the Rio Grande is important 

because the river can act as a major corridor of dispersal 

for exotics as well as native species.

The percent of the bank covered by rooted vegetation was not the same (fig. 13) among the reaches 

(χ2

 = 13.00, p = 0.01). Percent bank vegetation coverage 

was significantly different at the 0.05 level between 

Colorado Canyon and Johnson Ranch and between 

Johnson Ranch and Boquillas. Median percent coverage 

ranged from 17.5 percent at Colorado Canyon to 92.5 

percent at Johnson Ranch. The Santa Elena, Johnson 

Ranch, and Black Gap reaches all had percent coverages from 1.8 to 5.3 times greater than those of Colorado Canyon and Boquillas. The banks at the Colorado 

Canyon reach were dominated by igneous rock, and 

vegetation was much more sparse on the Colorado 

Canyon banks than on the banks dominated by the more 

weathered limestone rocks at the four other reaches. 

Santa Elena, Johnson Ranch, and Black Gap all had 

bermuda grass covering much of the bank surface, 

which accounts for the higher percentage of bank vegetation coverage for these reaches.

The relative abundance of the dominant native 

and non-native species common to the banks of the 

Rio Grande varied among the five reaches (fig. 14). 

Of the native species, honey mesquite, groundseltree 

(Baccharis sp.), giant reed (Arundo donax), and common reed (Phragmites australis) were the most common. Like the common and giant reeds, groundseltree 

often was found in dense stands, possibly excluding 

seeding and germination by other species. The giant 

reed was more commonly encountered within and 
RESULTS OF ASSESSMENT 15

between the reaches than the common reed. Common 

reed was encountered only along the transects at the 

Colorado Canyon and Johnson Ranch reaches. The nonnative species salt cedar (Tamarisk sp.) and bermuda 

grass (Cynodon dactylon) accounted for about 25 to 40 

percent of the vegetation on the banks of the five 

reaches. Salt cedar was as much as 17 percent of the 

vegetation at Santa Elena and not present along the 

transects at Johnson Ranch. Bermuda grass ranged from 

a minimum of 16 percent at Boquillas and Black Gap to 

a maximum of 26 percent at Colorado Canyon. Where 

bermuda grass exists on the banks of the Rio Grande, it 

was usually the dominant herbaceous ground cover and, 

in some instances, covered the majority of the bank. 

This was most obvious on the U.S. bank at Johnson 

Ranch on both sides of several transects. 

Fish Community

Eighteen species of fish were collected among the 

five reaches (table 3). A total of 474 fish were collected 

Figure 9. Streamflow velocity and selected stream-habitat measures for five bioassessment reaches in and near 

Big Bend National Park, Texas.

Bank height

REACH

BANK HEIGHT (METERS)

0

10

20

30

40

50

60

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

A

A

A

A

A

(8)

(8) (8) (8)

(8)

REACH

Stream velocity 

 STREAM VELOCITY (METERS PER SECOND)

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

(12)

(12) (12) (12)

(12)

A A A A A

Hig h-bank channel width

REACH

HIGH-BANK CHANNEL WIDTH (METERS)

60

80

100

120

140

160

180

200

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

Bank slope 

REACH

BANK SLOPE 

-0.05

0.05

0.15

0.25

0.35

0.45

0.55

0.65

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

A

A A A

A

(8) (8) (8)

(8)

(8)

Maximum

Minimum

75th percentile

25th percentile

Median

EXPLANATION

(12) Number of samples Datapoint shown when

less than six samples

Note: Boxes with same letter are

not significantly different.
16 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

Figure 10. Bank height to channel width ratio for five bioassessment reaches in and near Big Bend National Park, 

Texas.

REACH

BANK HEIGHT TO CHANNEL WIDTH RATIO

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

(8)

(8) (8) (8)

A (8)

AB

AB

B B

Maximum

Minimum

75th percentile

25th percentile

Median

EXPLANATION

(8) Number of samples

Note: Boxes with same letter are

not significantly different.

Figure 11. Embeddedness of cobble for five bioassessment reaches in and near Big Bend National Park, Texas.

REACH

EMBEDDEDNESS (PERCENT)

0

20

40

60

80

100

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

(12) (12)

(12)

(12)

(12)

A

AB

AB

B

AB

Maximum

Minimum

75th percentile

25th percentile

Median

EXPLANATION

(12) Number of samples

Note: Boxes with same letter are

not significantly different.
RESULTS OF ASSESSMENT 17

Figure 12. Gravel and larger particle size of bed material for five bioassessment reaches in and near Big Bend 

National Park, Texas.

Maximum

Minimum

75th percentile

25th percentile

Median

EXPLANATION

(478) Number of samples

Note: Boxes with same letter are

not significantly different.

REACH

PARTICLE SIZE (MILLIMETERS)

 0

100

300

500

700

900

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

(478)

(190)

(371) (395)

(403)

A

B C C C

200

400

600

800

Figure 13. Bank vegetation coverage for five bioassessment reaches in and near Big Bend National Park, Texas.

REACH

BANK VEGETATION COVERAGE (PERCENT)

0

20

40

60

80

100

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

(8)

(8)

(8)

(8)

(8)

AC

ABC

B

AC

ABC

Maximum

Minimum

75th percentile

25th percentile

Median

EXPLANATION

(8) Number of samples

Note: Boxes with same letter are

not significantly different.
18 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

among the reaches, and of these, 348 were minnows. 

Among the minnows, the most commonly collected 

species was the red shiner (Cyprinella lutrensis). Red 

shiners were the most frequently collected species from 

a mid-1980s survey of the same region (Platania, 1990). 

The least frequently collected species were the western 

mosquitofish (Gambusia affinis) and the freshwater 

drum (Aplodinotus grunniens). The introduced species, 

common carp (Cyprinus carpio), was collected at all 

reaches except Santa Elena. At least one of two species 

of suckers, the blue sucker (Cycleptus elongatus) or the 

river carpsucker (Carpiodes carpio), was collected at all 

reaches. The blue sucker is listed by the State of Texas 

as a threatened species; this species was collected at all 

reaches except Johnson Ranch. The only sunfish collected was the longear sunfish (Lepomis megalotis). 

Other species of sunfish, including the green and redear 

sunfish, have been collected in BBNP in the past but 

not in the main stem of the Rio Grande (Hubbs, 1958; 

Platania, 1990). 

The most fish species (15) were collected at 

Santa Elena and the fewest species (9) at Colorado 

Canyon and Johnson Ranch (fig. 15). Unlike the four 

other reaches, the Santa Elena reach was wadeable 

throughout, and this could account for the larger number 

of species collected at this reach. The other reaches had 

deep pools and runs and some rapids that made fishcommunity assessment more difficult than at the Santa 

Elena reach. 

The number of species collected can be influenced by the number of individuals collected, and the 

number of individuals can introduce bias into any interpretation of biological diversity indices or other metrics 

(Ludwig and Reynolds, 1988). The relation of sample 

Figure 14. Relative abundance of selected species of bank vegetation for five bioassessment reaches in and near 

Big Bend National Park, Texas.

REACH

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

RELATIVE ABUNDANCE (PERCENT)

 0

 15

 30

 45

 60

 75

 90

Honey mesquite

Groundseltree

Giant reed

Common reed

Bermuda grass

EXPLANATION

Note: Totals less than 100 

percent because not all 

vegetation identified.

Salt cedar
RESULTS OF ASSESSMENT 19

Table 3. Fish taxa and individual counts of fish collected in the Rio Grande in and near Big Bend National Park, Texas 

Group Common name Family Scientific name

Study reaches on the Rio Grande by site 

Colorado

Canyon

Santa

Elena 

Johnson

Ranch Boquillas 

Gars Lepisosteidae

Longnose gar Lepisosteus osseus 0 1 00 

Herrings Clupeidae

Gizzard shad Dorosoma cepedianum 2 0 04 

Minnows Cyprinidae

Red shiner Cyprinella lutrensis 0 31 35 32 

Rio Grande shiner Notropis jemezanus 0 31 4 0 Tamaulipas shiner Notropis braytoni 0 31 25 32 Common carp Cyprinus carpio 1 0 21 Longnose dace Rhinichthys cataractae 0 5 00 Speckled chub Macrhybopsis aestivalis 0 31 0 11 

Suckers Catostomidae

Blue sucker Cycleptus elongatus 2 1 0 2 

River carpsucker Carpiodes carpio 2 2 1 16 Smallmouth buffalo Ictiobus bubalus 0 1 00 

Catfish Ictaluridae

Blue catfish Ictalurus furcatus 7 1 1 7 

Channel catfish Ictalurus punctatus 2 1 2 8 Flathead catfish Pylodictis olivaris 8 4 2 6 

Killifishes Cyprinodontidae

Plains killifish Fundulus zebrinus 0 4 20 

Livebearers Poeciliidae

Western mosquitofish Gambusia affinis 0 1 00 

Sunfishes Centrarchidae

Longear sunfish Lepomis megalotis 2 3 00 

Drums Sciaenidae

Freshwater drum Aplodinotus grunniens 1 0 00 

Number of fish 27 148 74 119 Number of fish species 9 15 9 10 
20 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

size to the number of species is illustrated by the high 

linear correlation (Pearson’s r = 0.793, p = 0.109) 

between these variables, even with the non-significant 

p-value, because of the small sample size of five 

(fig. 16). 

Channel widths and stream order are similar 

among the reaches. The number of taxa often increase 

with increasing stream size (Vannote and others, 1980). 

The number of species can be standardized by computing species richness as the number of species 

related in some way to sample size or to the number of 

individuals collected. For example, Menhinick’s species 

richness (R) (Menhinick, 1964) was largest at Colorado 

Canyon and smallest at Santa Elena (fig. 17)—the 

opposite of the result when only the number of species 

is considered. Menhinick’s R takes into account the 

number of fish collected in addition to the number of 

species. It is computed as the number of species collected (S) divided by the square root of the total number 

of individuals collected (N). The lower R for Santa 

Elena is influenced by the fact that 63 percent of the fish 

collected were three species of minnows, whereas the 

higher R for Colorado Canyon is influenced by a more 

even distribution of the number of fish sampled among 

the nine species collected (table 3). 

A cluster analysis was done using absolute abundance data to compare overall fish-community similarity among the five reaches, and euclidean (linkage) 

distance was used as the distance measure for each cluster node (fig. 18). Distance in this context refers to a 

quantification of the similarity or dissimilarity between 

samples. The more similar samples from different 

reaches are in species composition and quantity, the 

closer their linkage distance (Ludwig and Reynolds, 

1988). Colorado Canyon was the least similar to the 

other reaches. Johnson Ranch, Boquillas, and Black 

Gap were more similar to one another than any one of 

these three reaches was to the Santa Elena reach in overall fish-community structure.

Figure 15. Number of fish species for five bioassessment reaches in and near Big Bend National Park, Texas.

REACH

NUMBER OF SPECIES

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap
RESULTS OF ASSESSMENT 21

Figure 16. Number of fish versus number of species for five bioassessment reaches in and near Big Bend 

National Park, Texas.

NUMBER OF FISH COLLECTED AT INDICATED REACH

0 20 40 60 80 100 120 140 160

NUMBER OF SPECIES

9

11

12

13

15

10

14

8

16

Colorado Canyon

Santa Elena

Johnson Ranch

Boquillas

Black Gap

Pearson's r = 0.793, p = 0.109

Minnows dominated fish-community structure at 

all reaches except Colorado Canyon (fig. 19). Minnows 

accounted for 89 percent of the community at Johnson 

Ranch, 88 percent at Santa Elena, and a minimum 

of about 4 percent at Colorado Canyon. Minnow populations generally are much larger than other fish populations in streams, and the dominance of minnows is 

not surprising. Red shiners and Tamaulipas shiners 

(Notropis braytoni) dominated the minnow community 

at all of the sites except Colorado Canyon where one 

common carp was the only minnow species collected. 

Catfish, particularly flathead catfish (Pylodictis 

olivaris), dominated the fish community at Colorado 

Canyon. In general, fish-community structure was 

similar among Santa Elena, Johnson Ranch, Boquillas, 

and Black Gap, and most different at Colorado Canyon. 

Colorado Canyon was more problematic to sample 

because of swift riffles and rapids, large cobble and 

boulders, and one very deep pool in the reach. 

As with community structure, fish trophic structure varied among the reaches (fig. 20). Trophic structure largely reflected community structure. Invertivores, 

predominantly minnows, made up at least 60 percent of 

the trophic structure at all reaches except Colorado Canyon. Piscivores, weighted by blue catfish, dominated the 

trophic structure at Colorado Canyon. At the four other 

reaches, piscivores were the smallest trophic group. 

This finding is consistent with the trophic structure of 
22 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

LINKAGE DISTANCE

BLACK GAP

BOQUILLAS 

JOHNSON RANCH

SANTA ELENA

COLORADO CANYON

10 15 20 25 30 35 40 45

REACH

Figure 18. Results of cluster analysis to indicate similarity of fish communities for five bioassessment reaches in 

and near Big Bend National Park, Texas.

REACH

MENHINICK'S RICHNESS

0

0.5

1.0

1.5

2.0

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

Figure 17. Menhinick’s species richness for fish communities for five bioassessment reaches in and near Big 

Bend National Park, Texas.
RESULTS OF ASSESSMENT 23

most aquatic communities. The principal carbon and 

energy source in large rivers such as the Rio Grande in 

the Big Bend region are the finer particulate organic 

materials, making the river a heterotrophically based 

system rather than autotrophically based (Vannote and 

others, 1980; Minshall and others, 1983). Fish communities in large heterotrophic rivers often are dominated 

by omnivorous species such as suckers, many species of 

catfish, smallmouth buffalo, and common carp. These 

omnivores feed extensively on detritus that constitutes a 

large percentage of the finer particulate organics in the 

river.

Benthic Macroinvertebrates 

A cumulative total of 92 benthic macroinvertebrate taxa were collected among the five reaches 

sampled (table 4 at end of report). Seventy-four of these 

taxa were aquatic insects, and eighteen were non-insect 

taxa including freshwater worms (Oligochaeta), clams 

(Bivalvia), snails (Gastropoda), ostracods (Ostracoda), 

leeches (Hirudinea), mites (Acarina), and flat worms 

(Turbellaria). Sampling in the study was intentionally 

biased for the collection of aquatic insects. Therefore, 

results discussed below will compare aquatic-insect 

communities among the five reaches. 

The most frequently collected aquatic insects 

were two blackfies—Simulium sp. and Cnephia sp. 

(table 4). Three species (Polypedilum convictum, 

Polypedilum scalaenum, and Telopelopia okoboji) 

were predominant among midges at the five sites. 

Midges, family Chironomidae, often are the dominant 

aquatic-insect taxon in number of species, number of 

individuals, and in biomass in large rivers such as 

the Rio Grande (Resh and Rosenberg, 1984). The netspinning caddisfly, Cheumatopsyche sp., was common 

at all reaches except Santa Elena and was the third 

most frequently collected aquatic insect at the Boquillas 

reach. This taxon, a few additional caddisfly families, 

and members of the family Simulidae commonly are the 

most abundant filter feeders in the coarse substrate such 

as cobble or boulders in fast-flowing streams (Wallace 

and Merritt, 1980) that characterized the five reaches.

Figure 19. Relative abundance of major fish families for five bioassessment reaches in and near Big Bend 

National Park, Texas.

0

20

40

60

80

100

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

REACH

RELATIVE ABUNDANCE (PERCENT)

Drums

Sunfish

Livebearers

Killifish

Catfish

Suckers

Minnows

Herrings

Gars

EXPLANATION
24 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

A cluster analysis using absolute abundance data 

indicated that the benthic aquatic-insect communities 

were most alike between the Santa Elena and Black 

Gap reaches (fig. 21). Colorado Canyon, Santa Elena, 

Black Gap, and Johnson Ranch were more similar in 

aquatic-insect community structure than any one of 

these reaches was to Boquillas. Boquillas had fewer 

species of midges than the other reaches, which 

accounted for the dissimilarity with the other reaches, as 

indicated by the relatively large linkage distance in the 

cluster analysis (fig. 21). 

A maximum of 51 aquatic-insect taxa were collected at Black Gap, and a minimum of 41 were collected at Santa Elena (fig. 22). These total numbers of 

aquatic-insect taxa are cumulative from all three methods of collection—the QMH, RTH, and traveling kickseine. The RTH method is a more quantitative method 

of sampling than the QMH (Cuffney and others, 1993) 

or traveling kick-seine method. RTH sampling for 

benthic macroinvertebrates was done to provide a standardized method of collection that would allow reliable 

comparisons among the reaches. Therefore, all comparative discussions below are based on findings from 

the RTH method. The other methods, QMH and traveling kick seine, are less quantitative and were done in 

addition to the RTH method to obtain the most comprehensive information on taxa richness for each reach. 

Aquatic-insect data from the RTH method were used to 

compute aquatic-insect community metrics for the five 

reaches. Sample size, or the total number of individual 

insects collected at each reach, and the number of individuals per taxon can bias community metrics such as 

species diversity and richness (Ludwig and Reynolds, 

1988). Generally the number of species or number of 

taxa will correlate positively to the number of individuals collected. This was the case in comparing the number of species of fish to the number of individuals 

collected per reach (fig. 16). However, the number of 

aquatic-insect taxa was inversely correlated (Pearson’s 

r = -0.73, p = 0.045) to the number of individuals collected at each reach (fig. 23). The relatively large number of individuals in contrast to the smaller number of 

taxa for the Boquillas and Santa Elena reaches is attributed to the relatively large numbers of Simulium sp. 

Piscivores

Omnivores

Invertivores

REACH

RELATIVE ABUNDANCE (PERCENT)

0

10

20

30

40

50

60

70

80

90

100

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

EXPLANATION

Figure 20. Relative abundance of major fish trophic groups for five bioassessment sites in and near Big Bend 

National Park, Texas.
RESULTS OF ASSESSMENT 25

EXPLANATION

Total aquaticinsect taxa

RTH aquaticinsect taxa

REACH

NUMBER OF TAXA

0

10

20

30

40

50

60

70

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

Figure 22. Comparison of total aquatic-insect taxa and richest targeted habitat (RTH) aquatic-insect taxa for five 

bioassessment reaches in and near Big Bend National Park, Texas.

Figure 21. Results of cluster analysis to indicate similarity of benthic aquatic-insect communities for five 

bioassessment reaches in and near Big Bend National Park, Texas.

LINKAGE DISTANCE

BOQUILLAS

JOHNSON RANCH

BLACK GAP

SANTA ELENA

COLORADO CANYON

100 200 300 400 500 600 700 800 900 1,000

REACH
26 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

REACH

MENHINICK'S TAXA RICHNESS

0.95

1.05

1.15

1.25

1.35

1.45

1.55

1.65

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

Figure 24. Menhinick’s taxa richness for aquatic insects for five bioassessment reaches in and near Big Bend 

National Park, Texas.

NUMBER OF INDIVIDUALS COLLECTED AT INDICATED REACH

NUMBER OF INSECT TAXA COLLECTED

Colorado Canyon

Santa Elena

Johnson Ranch

Boquillas

Black Gap

22

24

26

28

30

32

34

36

510 520 530 540 550 560 570

Pearson's r = -0.73, p = 0.045

580 590 600

Figure 23. Number of aquatic-insect taxa versus number of aquatic-insect individuals (richest targeted habitat 

[RTH] samples only) for five bioassessment reaches in and near Big Bend National Park, Texas.
RESULTS OF ASSESSMENT 27

REACH

 0

 5

10

15

20

25

30

35

40

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

RELATIVE ABUNDANCE (PERCENT)

EXPLANATION

Trichoptera

Ephemeroptera

Figure 25. Relative abundance of ephemeroptera and trichoptera taxa (richest targeted habitat [RTH] samples 

only) for five bioassessment reaches in and near Big Bend National Park, Texas.

EXPLANATION

REACH

RELATIVE ABUNDANCE (PERCENT)

0

10

20

30

40

50

60

70

80

90

100

Colorado Canyon Santa Elena Johnson Ranch Boquillas Black Gap

Shredders

Scrapers

Predators

Gatherers

Filterers

Figure 26. Relative abundance of major aquatic-insect trophic groups (richest targeted habitat [RTH] samples 

only) for five bioassessment reaches in and near Big Bend National Park, Texas.
28 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas 

collected at Boquillas (1,075) and Cnephia sp. collected 

at Santa Elena (745). Considering sample size as a factor, Menhinick’s taxa richness, computed as the number 

of RTH taxa divided by the square root of the RTH sample size (Menhinick, 1964), was highest for Black Gap 

and lowest for Santa Elena (fig. 24). 

The Ephemeroptera, Plecoptera, and Trichoptera 

(EPT) index often is used as an indicator of the ecological health of streams (Loeb and Spacie, 1994). The 

index is the sum of the relative abundance of each of 

these three taxa expressed as a percentage of all identified taxa. The index has been found to be sensitive to 

changes in water quality and often is less variable seasonally and perennially than other metrics such as taxa 

richness (Lenat and Barbour, 1994). No plecopterans 

were collected at any of the five reaches. Therefore, the 

EPT index for the five reaches is the sum of the percentages of ephemeropterans and trichopterans only. The 

EPT index was largest for Colorado Canyon and smallest for Boquillas (fig. 25). Trichopterans (caddisflies) 

were a larger percentage than ephemeropterans (mayflies) at Colorado Canyon, Santa Elena, and Johnson 

Ranch; and mayflies were more abundant at Boquillas 

and Black Gap. The net-spinning caddisflies and many 

of the mayflies often are more abundant on larger, more 

stable bed materials. The large cobble and boulders that 

are characteristic of Colorado Canyon should be more 

stable than the smaller cobble common at Johnson 

Ranch and Boquillas. 

The trophic structure of the aquatic-insect 

community of a stream is controlled by many factors 

including channel size, depth, stream velocity, nutrient 

composition and availability, and composition of the 

bed materials (Hynes, 1970). Large rivers like the Rio 

Grande are dominated by filter feeders and gatherers 

(fig. 26) that feed on particulate organic material that is 

suspended in the water column or abundant in depositional areas such as pools. Filter feeders such as the netspinning caddisfly (Cheumatopsyche sp.) were the most 

abundant trophic group at Colorado Canyon, Johnson 

Ranch, and Boquillas. The filter feeders were dominated by Simulium sp. at the Boquillas reach and were 

more abundant at Boquillas than at any of the four other 

reaches. The majority of the mayfly taxa are collectorgatherers; the mayflies were a major trophic component 

at the Colorado Canyon and Santa Elena reaches. 

SUMMARY

The NPS manages a 386-km segment of the Rio 

Grande in the Big Bend region of West Texas. The Rio 

Grande in the region represents the Chihuahuan Desert's 

most extensive aquatic and associated riparian environments. Increases in population and industrial development in the region associated with the North American 

Free Trade Agreement, flows altered by upstream dams 

and diversions, and several water-quality issues could 

be affecting the status and health of aquatic biota in the 

river. This study was designed to collect and interpret 

the essential data needed to effectively manage aquaticlife resources of the Rio Grande in reaches in and near 

BBNP—and to establish benchmark sites to assess the 

status and trends in stream habitat, fish communities, 

and benthic macroinvertebrates over time.

Five study sites, and a sampling reach within 

each, were selected: one upstream of BBNP adjacent to 

Big Bend Ranch State Park, three in BBNP, and one 

downstream of the Park adjacent to Black Gap Wildlife 

Refuge. The three sites in BBNP are benchmark sites, 

marked with identifying monuments, for proposed 

long-term monitoring. Several reach-based and withinreach, transect-based stream-habitat measures were 

taken. The fish community at each reach was sampled 

once in early spring 1999 and once in late summer 1999. 

Benthic macroinvertebrate samples were collected 

once, in early March 1999.

Differences in stream-habitat conditions and 

riparian vegetation reflect differences in surface geology among the five reaches. The Colorado Canyon 

reach is dominated by igneous rock; streambed material 

is larger, and riparian vegetation is less diverse and not 

as dense as at the four other sampling reaches downstream. The Santa Elena, Johnson Ranch, Boquillas, 

and Black Gap reaches each have a limestone channel 

that is more weathered and thus characterized by 

smaller streambed material and more diverse and dense 

stands of riparian vegetation. The relative abundance of 

the dominant native and non-native species common to 

the banks of the Rio Grande varied among the five 

reaches. Of the native species, honey mesquite, giant 

reed, common reed, and groundseltree were the most 

common. The non-native species salt cedar and bermuda grass accounted for about 25 to 40 percent of the 

vegetation on the banks of the five reaches.

Eighteen species of fish and a total of 474 individuals were collected among the five reaches; 348 of the 

474 were minnows. The most fish species (15) were 
SELECTED REFERENCES 29

collected at Santa Elena and the fewest species (9) at 

Colorado Canyon and Johnson Ranch. The fish community at Colorado Canyon was least like the fish communities at the four other reaches. Minnows dominated 

fish-community structure at all reaches except Colorado 

Canyon. The common carp was collected at all reaches 

except Santa Elena; and the blue sucker, listed as threatened by the State of Texas, was collected at all reaches 

except Johnson Ranch. 

Fish trophic structure reflected fish-community 

structure among the five reaches. Invertivores, predominantly minnows, made up at least 60 percent of the 

trophic structure at all reaches except Colorado Canyon. 

Piscivores, weighted by blue catfish, dominated the 

trophic structure at Colorado Canyon. At the four other 

reaches, piscivores were the smallest trophic group.

Eighty percent of the benthic macroinvertebrate 

taxa collected were aquatic insects. Two species of 

blackfly, Simulium sp. and Cnephia sp., were the most 

frequently collected invertebrate taxon. Net-spinning 

caddisflies such as Cheumatopsyche sp. were common 

at all reaches except Santa Elena. The aquatic-insect 

community at the Boquillas reach was least similar to 

the aquatic-insect community at the other reaches. The 

EPT index, an indicator of the ecological health of 

streams, was largest for the Colorado Canyon reach and 

smallest for the Johnson Ranch reach. The larger 

streamflow velocities and larger, more stable substrates 

at Colorado Canyon might facilitate a larger relative 

abundance of EPT taxa than at the other reaches.

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Table 4 31

Table 4 Table 4. Taxonomic classification of benthic macroinvertebrates and counts for individual taxa collected in the Rio Grande 

National Park, Texas—Continued

Class Order Family Subfamily Tribe Genus or scientific name

Study 

Colorado

Canyon

Santa

Elena

Joh

Ra

Turbellaria 0 0Aphanoneura Araeolaimida Aelosoma sp. 2 0 

Ostracoda 3 2Bivalvia Pelecypoda Corbiculidae Corbicula fluminea 0 0

Sphaeriidae 1 0 

Sphaerium sp. 23 0 

Gastropoda Limnophila Physidae 0 1 

Stenophysa sp. 3 0 

Hirudinea Pharyngobdellida Erpobdellidae 0 0 Oligochaeta Haplotaxida Tubificidae Branchiura sowerbyi 0 0

Limnodrilus hoffmeisteri 1 47 

Lumbricidae Lumbricina 1 0Enchytraeidae 0 0 Naididae Nais pardalis 23 0 

Paranais sp. 1 0 Pristina breviseta 2 0

Dero sp. 0 0 

Arachnida Acarina Acari 0 0Insecta Ephemeroptera Baetidae Callibaetis sp. 0 0 

Camelobaaetidius sp. 0 0 Fallceon quilleri 0 0

Heptageniidae Neochoroterpes sp. 32 51 

Leptophlebiidae 1 12 

Thraulodes sp. 15 5 Thraulodes gonzalesi 18 16 Traverella sp. 5 6 Traverella presidiana 3 0 

Tricorythidae Tricorythodes sp. 44 20 

Odonata Coenagrionidae Argia sp. 3 3 

Table 4. Taxonomic classification of benthic macroinvertebrates and counts for individual taxa collected in the Rio Grande 

National Park, Texas[Number of individuals per taxon shown for each site.]
32 Baseline Assessment of Instream and Riparian-Zone Biological Resources on the Rio Grande in and Near Big Bend National

Park, Texas Insecta—Cont. Odonata—Cont. Calopterygidae Hetaerina americana 1 0

Gomphidae 6 1 

Erpetogomphus sp. 0 1 Stylurus sp. 0 0 

Macromiidae Macromia sp. 0 0 

Hemiptera Belostomatidae Abedus sp. 0 1 

Corixidae 2 14 

Trichocorixa sp. 1 3 

Naucoridae Ambryus sp. 1 1 

Cryphocricos sp. 4 1 

Veliidae 1 0 

Trochopus sp. 0 0 

Megaloptera Corydalidae Corydalus sp. 6 3 

Trichoptera 0 0

Glossomatidae 0 0 

Protoptila sp. 0 0 

Hydropsychidae 1 0 

Cheumatopsyche sp. 64 1 Smicridea sp. 25 10 

Hydroptilidae 0 0 

Hydroptila sp. 2 0 Mayatrichia sp. 89 58 Ochrotrichia sp. 20 0 

Lepidoptera Pyralidae Petrophila sp. 9 0 

Coleoptera Dytiscidae Laccophilus sp. 0 0 

Hydrophilidae 0 0 Elmidae Heterlimnius sp. 0 0 

Hexacylloepus sp. 0 0 Microcylloepus sp. 0 0 Microcylloepus pusillus 75 6 

Dryopidae Helichus sp. 10 2 

Ceratopogonidae Ceratopogoninae 1 0 

Table 4. Taxonomic classification of benthic macroinvertebrates and counts for individual taxa collected in the Rio Grande 

National Park, Texas—Continued

Class Order Family Subfamily Tribe Genus or scientific name

Study 

Colorado

Canyon

Santa

Elena

Joh

Ra
Table 4 33

Insecta—Cont. Coleoptera—Cont. Hemerodromia sp. 2 0 

Ephydridae 0 3 Simulidae 6 15 

Simulium sp. 0 48 2Cnephia sp. 423 745 3

Tabanidae 1 1 Chironomidae Tanypodinae Pentaneurini 0 0 

Ablabesmyia sp. 6 28 Labrundinia sp. 13 2 Thienemannimyia gr. sp. 1 0 Thienemannimyia sp. 0 0 Telopelopia okoboji 167 155 2

Orthocladiinae Orthocladiini Cricotopus bicinctus gr. 15 12 2

Cricotopus sp. 42 11 Cricotopus trifascia 0 0Nanocladius sp. 42 0 Nanocladius distinctus 48 89 Orthocladius rivicola gr. 4 13 Orthocladius complex 6 4Parakiefferiella sp. 193 133 Thienemanniella sp. 6 3 

Chironominae Chironomini 0 0 

Chironomus sp. 12 21 Cryptochironomus sp. 2 5 Dicrotendipes sp. 0 2 Paralauterborniella 

nigrohalteris

15 0 

Polypedilum 6 0 Polypedilum convictum 0 5 

Polypedilum scalaenum 44 48 1Polypedilum tritum 6 0Tanytarsus sp. 4 5 

Table 4. Taxonomic classification of benthic macroinvertebrates and counts for individual taxa collected in the Rio Grande 

National Park, Texas—Continued

Class Order Family Subfamily Tribe Genus or scientific name

Study 

Colorado

Canyon

Santa

Elena

Joh

Ra
Moring— Baseline Assessment of Instream and Riparian-Zone Biological Resources on 

the Rio Grande in and Near Big Bend National Park, Texas

District Chief U.S. Geological Survey 8027 Exchange Dr.Austin, TX 78754–4733