Grand Canyon Monitoring and Research Center, 

in cooperation with SWCA Environmental Consultants, Inc. 

Comparisons of Water Quality and 

Biological Variables from Colorado River 

Shoreline Habitats in Grand Canyon, 

Arizona, under Steady and Fluctuating 

Discharges from Glen Canyon Dam 

By Barbara E. Ralston1, Matthew V. Lauretta2, and Theodore A. Kennedy1 

Open-File Report 2007-1195 

2007 

1

Southwest Biological Science Center, USGS, Flagstaff, Arizona 

2

SWCA Environmental Consultants, Inc., Flagstaff, Arizona 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS 

For more information on the USGS—the Federal source for science about the Earth, 

its natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS 

Suggested citation: 

Ralston, B.E., Lauretta, M.V., Kennedy, T.A., 2007, Comparisons of water quality and 

biological variables from Colorado River shoreline habitats in Grand Canyon, Arizona, 

under steady and fluctuating discharges from Glen Canyon Dam: U.S. Geological 

Survey Open File Report 2007-1195 [http://pubs.usgs.gov/of/2007/1195/]. 

Keywords: Colorado River, fluctuating and steady releases, macroinvertebrate 

densities, water quality 

Any use of trade, product, or firm names is for descriptive purposes only and does 

not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the 

individual 

copyright owners to reproduce any copyrighted material contained within this 

report. 

2
Contents 

Abstract................................................................................................................................................. 6

Introduction .......................................................................................................................................... 6

Study Site .......................................................................................................................................... 8

Hydrology ...................................................................................................................................... 9

Hydrology Prior to Each Collection......................................................................................... 10

Methods .............................................................................................................................................. 11

Water Quality Measurements ................................................................................................. 13

Water Velocity Measurements ............................................................................................... 13

Habitat Dimension Measurements......................................................................................... 13

Benthic Macroinvertebrate Collections ................................................................................ 13

Phytoplankton Collections........................................................................................................ 14

Fish Collections .......................................................................................................................... 14

Data Analysis.................................................................................................................................. 14

Results ................................................................................................................................................. 15

Water Quality Measurements ................................................................................................. 15

Benthic Macroinvertebrate and Phytoplankton Collections ............................................. 19

Fish Collections .......................................................................................................................... 23

Discussion .......................................................................................................................................... 26

Recommendations......................................................................................................................... 27

Acknowledgments............................................................................................................................. 28

References Cited ............................................................................................................................... 28

Figures 

Figure 1. Map of sampling area from Lees Ferry, Ariz., to Diamond Creek, Ariz. ...................9

Figure 2. Hydrograph showing daily mean, minimum and maximum discharge at Lees 

Ferry, Ariz., from August 15, 2005 to October 22, 2005. Arrows indicate dates of sample 

collection under fluctuating and steady releases (September 4, 2005–September 16, 2005 

and September 22–October 7, 2005). .............................................................................................11

Figure 3. Water quality parameters: a.) water temperature, b.) salinity, c.) dissolved 

oxygen, d.) conductivity, e.) water velocity, f.) turbidity, and g.) pH measured in 

backwaters (BW mouth, BW center, and BW end) and the mainstem...................................16

Figure 4. Bar graphs comparing macroinvertebrate densities sampled from a.) 

backwaters and b.) shorelines during fluctuating and steady releases in September and 

October 2005 (abbreviations for taxa provided below fig. b).....................................................19

Figure 5. Bar graphs comparing phytoplankton densities sampled from a.) backwaters 

and b.) shorelines during fluctuating and steady releases in September and October 2005.

..............................................................................................................................................................22

3
Figure 6. Benthic invertebrate density by site under fluctuating and steady releases with 

distance from Glen Canyon Dam for a.) backwaters and b.) shoreline habitats (difference 

between treatments were non significant). .................................................................................22

Figure 7. Catch-per-unit-effort (CPUE) for fish species encountered from a.) backwater 

seining and b.) shoreline backpack shocking under fluctuating and steady releases in 

September and October 2005 (abbreviations for species provided below fig. b)..................23

Figure 8. Catch-per-unit-effort (CPUE) for fish species encountered from a.) backwater 

seining and b.) shoreline backpack shocking under fluctuating and steady flow treatments 

with distance from Glen Canyon Dam (difference between treatments were non 

significant). .........................................................................................................................................24

Tables 

Table 1. Maxima, minima, and range of discharge during collection periods September 4, 

2005–October 7, 2005, as recorded at Lees Ferry, Ariz. Data are based on 15-minute 

recorded values (http://www.gcmrc.gov/products/flow_data accessed 06/25/2006). ........11

Table 2. Sample dates, numbers, and locations (river km) of paired shoreline and 

backwater sites. ................................................................................................................................12

Table 3. Average values of water quality parameters from three locations in backwaters 

(mouth, center, and end) and adjacent mainstem (MS) habitats. ............................................17

Table 4. Summary statistics of macroinvertebrate densities from backwaters and 

shoreline habitats under fluctuation and steady releases from September 4 to October 7, 

2005 (abbreviations for taxa provided below table). ...................................................................19

4
Conversion Factors 

Inch/Pound to SI 

Multiply By To obtain 

Length 

inch (in.) 2.54 centimeter (cm) 

mile (mi) 1.609 kilometer (km) 

Area 

square foot (ft2) 929.0 square centimeter (cm2) 

square foot (ft2) 0.09290 square meter (m2) 

square inch (in2) 6.452 square centimeter (cm2) 

Volume 

ounce, fluid (fl. oz) 0.02957 liter (L) 

pint (pt) 0.4732 liter (L) 

quart (qt) 0.9464 liter (L) 

gallon (gal) 3.785 liter (L) 

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate 

foot per second (ft/s) 0.3048 meter per second (m/s) 

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3 s-1) 

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

°F = (1.8×°C)+32. 

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25°C). 

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or 

micrograms per liter (μg/L). 

5
Comparisons of Water Quality and 

Biological Variables from Colorado River 

Shoreline Habitats in Grand Canyon, 

Arizona, under Steady and Fluctuating 

Discharges from Glen Canyon Dam 

By Barbara E. Ralston, Matthew V. Loretta, and Theodore A. Kennedy 

Abstract 

Glen Canyon Dam operations are known to affect mainstem Colorado River 

temperature and shoreline habitats for native fish. Options for ameliorating the impacts 

that operations have on young native fish include changing release volumes and/or 

changing the daily range of releases. Long-term alterations of operations that may 

produce a measurable biological response can be costly, particularly if the treatment 

involves reduced power generation. In September and October 2005, a series of twoweek releases occurred that alternated between daily fluctuations that varied by 76 m3 s-

1 

and steady releases. The purpose of these short-term experiments was to study the effect 

of daily operations on water quality parameters and biotic constituents (phytoplankton, 

macroinvertebrates, and fishes) of associated shoreline habitats. Our results indicate that 

measured biological and physical parameters were, in general, unaffected by flow 

treatments. However, results should be interpreted cautiously as time within and between 

treatments was likely insufficient to affect measured parameters. These results lead to the 

recommendation that studies like this may be more amenable to laboratory experiments 

first and then applied to a large-scale setting, preferably for longer duration. 

Introduction 

Glen Canyon Dam operations are known to affect mainstem Colorado River 

temperature and shoreline habitats for fish. Recruitment failure by native fish in the 

Colorado River in Grand Canyon has been attributed to habitat loss associated with 

reduced mainstem temperatures and unstable shorelines, among other factors (Kaeding 

and Zimmerman, 1983; Gorman and Stone, 1999). Cold mainstem temperatures (8–

12°C), influenced by reservoir elevations and release volume (Vernieu and others, 2005), 

reduce growth rates of larvae and young-of-year fishes and prevent or reduce mainstem 

spawning success (Robinson and Childs, 2001; Korman and others, 2005). In Glen 

Canyon, habitat instability associated with daily fluctuations or changes in base flow is 

6
connected with displacement of nonnative young-of-year rainbow trout (Oncorhynchus 

mykiss), which results in mortality or exposure of young fish to predation (Korman and 

others, 2005); the same may be true for native fishes downstream of Lees Ferry (Korman 

and others, 2004). Stone and Gorman (2006) found that young-of-year humpback chub 

(Gila cypha), an endangered species, occupied stable, shallow, near-shore habitats in the 

Little Colorado River. 

Options for ameliorating Glen Canyon Dam impacts on young fish include 

altering how much water is released and/or changing the daily range of the releases. 

Reducing the discharge of releases allows for greater mainstem warming, while 

stabilizing or reducing daily fluctuations in discharge allows for greater nearshore 

warming. To date, several short duration tests of alternative releases have occurred from 

Glen Canyon Dam. Short duration, experimental high flow releases beyond power plant 

capacity have occurred twice, in 1996 (7 days in March at 1274 m3 s-1) and 2004 (60 

hours in November at 1160 m3 s-

1), principally to study sediment transport and storage 

(Rubin and others, 2002; Topping and others, 2006). Additionally, power plant capacity 

maintenance flows (24 hours at 877 m3 s-

1) occurred once in 1997 and twice in 2000. 

These high flow releases have increased our understanding of physical resource 

responses to dam operations. However, the responses of biological resources to these 

short duration operations remain unclear. Cause and effect relationships are more 

difficult to identify for biological resources as these short-term flow events may 

differentially affect life stages with the responses expressed several years later, if at all. 

Longer term alterations of operations (e.g., months), which may produce a measurable 

biological response, can be costly, particularly those that reduce power generating 

potential. As with short term events, their impact may not be observable for several 

years. 

A single longer-term experiment of steady flows that targeted native fish species 

occurred in the summer of 2000 (Trammell and others, 2002). These flows may have 

increased mainstem spawning success and survivorship of juvenile native fishes by 

increasing mainstem and nearshore water temperatures and stabilizing nearshore habitats. 

The Grand Canyon fish monitoring program started observing higher densities of adult 

native fish in 2005 (Melis and others, 2006), indicating that recruitment into the adult 

native fish populations, an important measure of whether an experiment or management 

action benefits native fishes, may have increased as a result of the 2000 experiment. The 

five-year response lag measured for native fish compared with the relatively 

instantaneous response by sediment resources illustrates how treatment duration 

influences response and how our ability to measure response varies by resource. 

Reconciling and balancing resource needs associated with adaptive management 

for Glen Canyon Dam is an ongoing process. Improvement of downstream resources 

associated with managed operations can only be achieved through experimental 

approaches coupled with resource monitoring that include variation in time of treatment 

as prescribed by the adaptive management process (Holling, 1978). In an attempt to 

determine whether minor daily fluctuations in discharge that allow for greater power 

generation, compared to steady flow releases, have the same effects on water quality and 

aquatic organisms, the Bureau of Reclamation implemented a series of two-week 

operations that alternated between steady and fluctuating flows in September and 

October 2005. The alternating two-week scenario followed summer operations (June–

August) in which daily operations fluctuated by approximately 226 m3 s-

1. 

7
The motivation for this experiment was to determine whether nearshore physical 

and biological variables are affected by steady flows relative to low daily fluctuations 

(daily range of 74 m3 s-1) in flow. We hypothesized that during steady flows, nearshore 

environments would be more stable and allow for greater nearshore warming. Stable 

nearshore environments might also promote growth of lower trophic levels, leading to 

higher abundance or biomass. Daily fluctuations might force young fish to move among 

and within habitats, so we also hypothesized that relative abundance of young fishes 

occupying nearshore environments might be higher during stable flows relative to low 

fluctuations. For example, Korman and others (2005) have shown that changing base 

flow conditions and greater daily range in volumes result in significantly lower relative 

abundance for young-of-year (YOY) rainbow trout in Glen Canyon. We also 

hypothesized that backwaters might show a larger change in water quality and biological 

parameters during steady flows relative to other nearshore environments because steady 

flows will increase the residence time of water there, thereby promoting warming. 

To evaluate these hypotheses we quantified water quality and biological variables 

in backwaters and adjacent nearshore environments during the September and October 

2005 experimental flows. The null hypothesis was that there would be no difference in 

water quality or biological parameters among habitats or among flow treatments. 

Antecedent conditions of measured parameters were not measured prior to the start of the 

first flow treatment of low fluctuations. 

Study Site 

Data were collected at 12 different sites between Lees Ferry [river kilometer (RK) 

15] and Diamond Creek (RK 360; fig. 1) under each treatment for a total of 24 sites. 

Along this reach, the river channel passes through 13 bedrock-controlled reaches that 

vary in width and depth as influenced by the underlying bedrock (Schmidt and Graf, 

1990). Within Grand Canyon there are 740 tributaries that produce debris-fan eddy 

complexes (Schmidt and Graf, 1990; Griffiths and others, 2004), which form 

constrictions and associated rapids along the river. Most tributaries are intermittent rather 

than having a permanent source of water entering the mainstem Colorado River. The 

compass orientation of the river in the canyon varies from east-west and north-south, 

which affects available light for primary production (Stevens and others, 1997; Yard and 

others, 2005). 

8
Figure 1. Map of sampling area from Lees Ferry, Ariz., to Diamond Creek, Ariz. 

The shoreline consists of sand, cobble, talus/boulder, and ledges. The sandy 

shorelines can be vegetated and, if associated with a debris fan, may form return current 

channel sandbars in an up- or downstream direction that is termed a “backwater.” 

Backwaters along the Colorado River in Grand Canyon differ from backwaters in alluvial 

systems (e.g., the Green River near Vernal, Utah) by being smaller in size (e.g., 100–500 

m2

 vs. 3,000 m2 or greater), consistently open to exchange with the mainstem, and are 

more ephemeral depending on the amount of sand storage in the eddy complex and 

discharge volume (Goeking and others, 2003). 

Hydrology 

Discharge of the Colorado River through Glen Canyon National Recreation Area 

and Grand Canyon National Park is regulated by Glen Canyon Dam. In 1995 (U.S. DOI, 

1995), operational constraints were placed on Glen Canyon Dam, and since then, the 

median annual discharge measured at the U.S. Geological Survey gage at Lees Ferry 

(USGS gage number 09380000) has been 378 m3 s-1 (Topping and others, 2003). The 

median daily range associated with daily fluctuations around the discharge has been 138 

m3 s-1 since 1996. 

Side tributaries periodically contribute sediment and coarse organic matter to the 

main channel. Generally these inputs are delivered from July to September (Griffiths and 

9
others, 2004) and are associated with summer convective thunderstorms created by moist 

air originating in the Gulf of Mexico or the Pacific Ocean, locally called “monsoonal 

rains.” While not all storms result in debris flows, the amount of rain falling is sufficient 

to generate local sediment and coarse particulate matter inputs or “spates” periodically 

within the Colorado River ecosystem (CRE). The directional pattern of monsoon storms 

trends from southwest to northeast across the canyon. Tributaries within the CRE have 

greater sediment loads than those tributaries associated with the Colorado River above 

Glen Canyon Dam or the Green River (Andrews, 1991). 

Hydrology Prior to Each Collection 

Releases in August, immediately prior to the experimental flow treatments we 

studied, had a daily peak discharge of 532 m3 s-1, a daily minimum of 294 m3 s-1, and a 

daily range of 237 m3 s-1. From September 1 to 3, 2005, the daily range and daily minima 

were slowly reduced to transition between the higher volume and wider fluctuating 

August releases and the more constrained September release pattern (fig. 2). During the 

months of September and October 2005, water was released from Glen Canyon Dam that 

alternated between fluctuating and steady releases. The dates for fluctuating releases 

were from September 4 to 22, 2005, and steady releases occurred from September 22 to 

October 7, 2005 (fig. 2). Data were collected between the dates of September 4–16, 2005 

and September 22–October 7, 2005. Release maxima and minima and range for data 

collection dates are provided in table 1. Antecedent hydrology, which can influence 

biological and water quality parameters, for the first treatment included large fluctuations 

from August and potential sediment inputs associated with monsoonal activity. The 

antecedent conditions for the steady flow treatment include the reduced flows that began 

on September 4, 2005. 

10
Collection 

period for 

fluctuating 

release

Collection 

period for 

steady 

releases 

Figure 2. Hydrograph showing daily mean, minimum and maximum discharge at Lees 

Ferry, Ariz., from August 15, 2005 to October 22, 2005. Arrows indicate dates of sample 

collection under fluctuating and steady releases (September 4, 2005–September 16, 2005 

and September 22–October 7, 2005). 

Table 1. Maxima, minima, and range of discharge during collection periods September 4, 

2005–October 7, 2005, as recorded at Lees Ferry, Ariz. Data are based on 15-minute 

recorded values (http://www.gcmrc.gov/products/flow_data.aspx accessed 06/25/2006). 

Collection Dates 

Flow 

Release 

Pattern 

Maximum 

Release 

(m3 s-1)

Minimum 

Release 

(m3 s-1) 

Daily range (m3 

s-1) 

Median Release 

(m3 s-1) 

09/04/2005-

09/16/2005 Fluctuating 263 189 74 250 

09/22/2005-

10/07/2005 Steady 255 227 27 236 

Methods 

Sample collection included water quality, water velocity, and habitat dimensions 

for backwaters, benthic invertebrate and plankton density and composition, and fish 

composition and catch per unit effort (CPUE), a measure of relative abundance. Samples 

11
were collected at selected sites distributed along 360 river kilometers of shoreline of the 

CRE from Lees Ferry to Diamond Creek (fig. 1). Sampling included backwaters and 

adjacent shorelines. Sampling was conducted at 12 paired sites, each composed of a 

backwater and nearby shoreline habitat. Sites were selected by stratified random 

sampling within each logistic sampling reach (table 2). Shoreline habitats consisted of 

vegetated shorelines, talus slopes, boulders, or ledges. In order not to bias the samples 

from the effects of previous sampling, a random sample of habitats without replacement 

was selected for each of the trips. 

Table 2. Sample dates, numbers, and locations (river km) of paired shoreline and 

backwater sites. 

Sample 

Date 

Sample Number (site 

number) 

River km and side1

(Left or Right) 

09/04/2005 1 (1) 49.3R 

09/05/2005 2 (2) 71.7L 

09/06/2005 3 (3) 82.7L 

09/07/2005 4 (4) 105.0L 

09/10/2005 5 (5) 192.7R 

09/10/2005 6 (6) 193.8R 

09/11/2005 7 (7) 215.4R 

09/11/2005 8 (8) 221.5L 

09/13/2005 9 (9) 276.4L 

09/13/2005 10 (10) 277.7L 

09/14/2005 11 (11) 305.3R 

09/15/2005 12 (12) 342.7R 

09/22/2005 13 (1) 5.4L 

09/25/2005 14 (2) 57.2R 

09/25/2005 15 (3) 70.4L 

09/27/2005 16 (4) 82.7L 

09/27/2005 17 (5) 110.3L 

09/29/2005 18 (6) 150.9R 

09/30/2005 19 (7) 191.1R 

10/01/2005 20 (8) 213.1R 

10/02/2005 21 (9) 269.5R 

10/03/2005 22 (10) 284.6R 

10/04/2005 23 (11) 291.8R 

10/04/2005 24 (12) 326.4R 

12
Water Quality Measurements 

Because backwater habitats are connected to the mainstem, we were also 

interested in how water quality parameters might change longitudinally along the 

backwater. Water quality measurements were taken in three locations at each backwater 

site (mouth, center, and foot of the backwater) and three locations at other shoreline sites. 

For comparisons between habitats we compared means of the three backwater 

measurements to means of shoreline temperatures across sites. Measurements were taken 

in backwaters after a block net was placed across the mouth and prior to any other 

sampling. Water temperature, specific conductivity, pH, dissolved oxygen, and salinity 

were measured using a Eureka Amphibian data logger© with a Eureka Mantis probe©. 

The data logger was calibrated against standards for water quality measures prior to each 

trip. Turbidity measurements were taken at each site using a Hydrolab Quanta©. Air 

temperature was also recorded using a standard thermometer. Water temperature loggers 

(Hobos©) were placed along five separate shoreline habitats to record water temperature 

continuously for the duration of the sampling trips. These temperature data are part of a 

separate data delivery submitted by Kaplinski (2006). 

Water Velocity Measurements 

Three water velocity measurements were taken at each site using a MarshMcBirney Flo-Mate©. In backwaters, after the block net was removed, water velocity 

was measured at three locations: near the mouth, in the center, and near the foot. 

Habitat Dimension Measurements 

The maximum length, maximum width, and maximum depth were recorded at 

each backwater. Three additional width and length measurements were taken at the 

mouth, center, and foot to calculate the average backwater width and depth. A quick 

sketch of the backwater was drawn, the location marked on ortho-photographic maps, 

and location information recorded. 

Benthic Macroinvertebrate Collections 

Macroinvertebrates were collected at each backwater using a Petit Ponar Grab 

Sampler© with a sampling area of 0.0231 m2

 (Wildlife Supply Company, Buffalo, N.Y.) 

and at each shoreline using a Hess Sampler© (0.33 m in diameter and 0.45 m high, 

sampling area = 0.0855 m2) equipped with a 500-micron mesh net. Macroinvertebrates 

were collected following the water quality measurements. Backwater samples were 

strained through a 500-micron sieve so that Ponar and Hess samples would be 

comparable. It was not possible to use the same sampler in both habitats because Hess 

samplers perform best in environments with current (i.e., shorelines), and a Ponar 

sampler is only suitable for sandy substrates (i.e., backwaters). At shoreline sites, the 

Hess sampler was pushed into the substrate and the substrate bottom stirred up. The 

organisms were washed into the collection bag by the current flowing through the front 

screen opening. Each sample was stored in a 1.89-L plastic jar with 70% ethanol added to 

13
preserve the organisms. Macroinvertebrate samples were identified to family, and counts 

per area sampled were calculated. 

Phytoplankton Collections 

Plankton samples were collected at 10 paired sites during each sampling trip (20 

samples per trip) after water quality and macroinvertebrate samples were collected. 

Plankton was collected at each site by straining 50 L of water through a Fieldmaster 

Student Plankton Net© (0.20-m diameter, 1.29-m length, and 80-micron mesh). A 10-L 

bucket was used to collect water that was poured through the plankton net. A wash bottle 

was used to rinse the strained sample to the bottom of the net, rinsing from the outside of 

the net to insure that no additional plankton was caught during rinsing. The plankton was 

collected in a 250-ml sample bottle attached to the end of the net. Lugol’s iodine solution 

was added as a sample preservative. Aquatic Consulting and Testing Inc. conducted 

phytoplankton analyses including composition and counts/ml of sample. The counts were 

converted to numbers per volume sampled. When available, algae grab samples were 

collected at each site to identify species. 

Fish Collections 

The mouth of each backwater was blocked with a 9.14-m block net (1.83-m 

height x 25-mm mesh size), and seining was conducted at each backwater using a 9.14-m 

bag seine (25-mm mesh size, 1.83- x 1.83- x 1.83-m bag) after other sample collections. 

Three depletion passes with the 9.14-m bag seine were taken at each backwater in order 

to estimate fish density. Seine haul length and three depth and width measurements were 

recorded for each sample. All fish were identified and total lengths measured. The CPUE 

of each species was calculated for each sample [# of fish/backwater area seined (m2), first 

pass]. 

Adjacent shorelines were sampled with a Smith Root LR-24 backpack 

electrofisher©. One electrofishing pass was made along the shoreline and CPUE 

(fish/100 s) calculated. Fish were handled following established protocols for Grand 

Canyon. Additionally, fish specimens were collected for otolith analyses and included up 

to 20 speckled dace (Rhinichthys osculus), 20 flannelmouth suckers (Catostomus 

latipinnis), all fathead minnow (Pimephales promelas), all plains killifish (Fundulus 

zebrinus), and all rainbow trout per trip. All fish collected were less than 60 mm in total 

length for daily otolith growth analysis. Preservation of fish specimens follows Korman 

and others (2005). These samples remain preserved for analysis at a future date if the 

availability of funds allows. 

Data Analysis 

To determine if measured parameters differed between treatments, we used a onesided Student’s t-test with unequal variances. Analysis was done using Excel algorithms 

(Microsoft® Office Excel, 2003). To determine if there was any dissimilarity between 

habitats for macroinvertebrates or fishes, we ran a Mantel test in PC-ORD (McCune and 

Mefford, 1999). The Mantel test used Pearson’s correlation to relate the distance matrices 

(shoreline vs backwaters under fluctuating and steady flow treatments). The distance 

14
measures were obtained with Sorensen (Bray-Curtis) distances. The runs consisted of 999 

permutations using the Monte Carlo randomization method. 

Results 

Water Quality Measurements 

Except for turbidity ( X turb treatment 1 = 22.1 ntu, X turb treatment 2 = 6.48 ntu, t (0.05,(1),11) = 

3.51, p = 0.02), water quality parameters did not differ significantly among treatments 

(table 3; fig. 3a–g). Temperatures in backwaters were generally lower during steady 

flows relative to low fluctuations, but the difference was not significant. Water 

temperatures among backwaters were less variable during steady flows relative to low 

fluctuations ( X temp treatment 1 = 19.5°C, var. = 4.6 vs. X temp treatment 2 = 18.5°C, var. = 1.9), 

but again the difference was not statistically significant. Within a given backwater the 

temperature range averaged 1oC during fluctuating flows and 1.4oC during steady flows 

with the warmest water located the farthest from the backwater opening. Under 

fluctuating flows, dissolved oxygen concentrations declined farther from the backwater 

opening. Under steady flows, dissolved oxygen concentrations increased with distance 

from the backwater opening, but the differences in dissolved oxygen values between 

mouth and ends of backwaters recorded were within the error limits of the instrument. 

Water quality parameters did not differ significantly along shorelines among treatments 

except for turbidity, which was significantly lower during the second treatment along the 

shoreline ( X turb treatment 1 = 25.2 ntu, X turb treatment 2 = 3.87 ntu, t (0.05,(1),11) = 3.74, p = 0.001). 

Shoreline temperatures did not differ significantly between sampling treatments. Mean 

shoreline temperatures averaged 0.8°C lower during the steady flow treatment relative to 

low fluctuating treatment. The range of the mean sample variance was smaller during the 

second treatment, but not significantly different ( X temp treatment 1 = 18.1°C, var. = 1.73 vs. 

X temp treatment 2 = 17.2°C, var. = 0.79). Water velocity along shorelines was higher during 

steady flows than fluctuating flows, but the difference was not significant. 

15
Figure 3. Water quality parameters: a.) water temperature, b.) salinity, c.) dissolved oxygen, d.) conductivity, e.) w

turbidity, and g.) pH measured in backwaters (BW mouth, BW center, and BW end) and the mainstem. 

B.

Salinity

0.300.350.400.45

BW Mouth BW Center BW End Mainstem

PSS

Fluctuating FlowSteady Flow

D.

Conductivity

700750800850900950

BW Mouth BW Center BW End 

μS/cm

FluSte

A.

Surface Water Temperatures

15.016.017.018.019.020.021.022.0

BW Mouth BW Center BW End Mainstem

Temperature (C)

Fluctuating FlowSteady Flow

C.

Dissolved Oxygen

8.08.59.09.5

10.010.511.0

BW Mouth BW Center BW End 

mg/L

FluSt

16
e. g.

pH

7.07.58.08.59.0

BW Mouth BW Center BW End 

c

17 

Water Velocity

0.050.000.100.150.200.25

BW Mouth BW Center BW End Mainstem

m/s

Fluctuating FlowSteady Flow

f.

Turbidity

05

101520253035

BW Mouth BW Center BW End Mainstem

NTU

Fluctuating FlowSteady Flow

Figure 3 continued 
18

Table 3. Average values of water quality parameters from three locations in backwaters 

(mouth, center, and end) and adjacent mainstem (MS) habitats. 

Averages Surface H2OTemp (°C) Dissolved Oxygen (mg/L) 

 Mouth Center End MS Mouth Center End MS 

mean 19.0 19.5 20.0 18.1*1 9.3 9.2 9.1 9.5 

Stdev 1.659 2.209 2.881 1.316 0.811 0.588 0.852 0.335 

SE 0.5 0.6 0.8 0.3 0.2 0.1 0.2 0.1 

Fluctuating flow 

CI (2SE) 1.0 1.2 1.5 0.7 0.4 0.3 0.4 0.2 

 

mean 17.9 18.4 19.3 17.3* 9.6 9.7 10.1 9.4 

Stdev 1.3 1.5 1.8 0.9 0.7 0.8 1.1 0.8 

SE 0.4 0.4 0.5 0.2 0.2 0.2 0.3 0.2 

Steady flow 

CI (2SE) 0.7 0.8 0.9 0.5 0.4 0.4 0.5 0.4 

Averages Conductivity (μS/cm) pH 

 Mouth Center End MS Mouth Center End MS 

mean 873.3 862.6 879.9 821.7 8.2 8.2 8.2 8.4 

Stdev 136.1704 47.53 59.39 52.18 0.379 0.323 0.338 0.254 

SE 30.4 10.4 12.7 10.9 0.1 0.1 0.1 0.0 

Fluctuating flow 

CI (2SE) 60.9 20.7 25.3 21.8 0.2 0.1 0.1 0.1 

 

mean 808.5 816.3 826.1 808.5 8.5 8.5 8.4 8.4 

Stdev 56.5 66.3 81.8 57.7 0.7 0.7 0.7 0.7 

SE 12.6 14.5 17.4 12.0 0.1 0.1 0.1 0.1 

Steady flow 

CI (2SE) 25.3 28.9 34.9 24.1 0.3 0.3 0.3 0.3 

Averages Salinity (pss) Turbidity (NTU) Velocity (m/s) 

 Mouth Center End MS Mouth Center End MS Mouth Center End MS 

mean 0.4 0.4 0.4 0.4 24.6 21.6 20.1 25.3 0.02 0.04 0.02 0.13 

St dev 0.074 0.043 0.043 0.045 16.64 14.01 15.64 19.7 0.028 0.034 0.04 0.142

SE 0.0 0.0 0.0 0.0 2.9 2.4 2.7 3.3 0.0 0.0 0.0 0.0 

Fluctuating flow 

CI (2SE) 0.0 0.0 0.0 0.0 5.9 4.9 5.4 6.7 0.0 0.0 0.0 0.0 

 

mean 0.4 0.4 0.4 0.4 6.1 5.6** 7.7 3.9**2 0.04 0.02 0.02 0.17 

St dev 0.0 0.0 0.1 0.0 4.6 2.4 5.6 2.2 0.0 0.0 0.0 0.1 

SE 0.0 0.0 0.0 0.0 0.8 0.4 1.0 0.4 0.0 0.0 0.0 0.0 

Steady flows 

CI (2SE) 0.0 0.0 0.0 0.0 1.6 0.9 1.9 0.8 0.0 0.0 0.0 0.0 

* p<0.05 for one-tailed t-test of differences in means between habitats. 

** p<0.05 for one-tailed t-test of difference in means between flow treatments within habitats. 

Shoreline temperatures were significantly lower than backwater temperatures by 

1oC (t.

treatment 1 (0.05,(1),11) = 1.88, p = 0.037; t.treatment 2 (0.05,(1),11) = 2.65, p = 0.007) during both 
treatments. Turbidity along the shoreline was significantly lower during steady flow 

treatment ( X turb backwater = 6.48 ntu, X turb shoreline = 3.87 ntu, t (0.05,(1),11) = 2.27, p = 0.016). 

Benthic Macroinvertebrate and Phytoplankton Collections 

The density of benthic invertebrates in backwaters did not differ significantly 

between treatments (t (0.05,(1),11), p > 0.05; table 4). In contrast, plankton densities were 

significantly higher during the fluctuating flow treatment (t (0.05,(1),11), p < 0.05) in 

backwaters. Annelids were the most abundant species found in backwaters followed by 

Chironomid larvae and pupae (table 4; fig. 4a, b). 

Backwater macroinvertebrate density during varied flows

0

500

1000

1500

2000

2500

ANN

AMISC

MITE

CASE

CHIL

CHIP

CYCL

DET

GAM

GAS

HYD

HYDL

NEM

NZM

SIML

SIMP

TCHI

TCOL

TDIP

TSPI

TMISC

Family

Density (# / sq m)

Fluctuating flow

Steady flow

Shoreline macroinvertebrate density during varied flows

0

200

400

600

800

1000

1200

1400

ANN

AMISC

MITE

CHIL

CHIP

DIPL

GAM

GAS

HYD

HYDL

NEM

NZM

SIML

SIMP

TCHI

TCOL

TDIP

TSPI

TMISC

Family

Density (# / sq. m)

Fluctuating flow

Steady flow

ANN=Annelida, AMISC=Aquatic Misc., MITE=Arachnida, CASE=Unknown case, CHIL=Chironomidae larvae, 

CHIP=Chironomidae pupae, CYCL=Cyclopedia, DET=Detritus, DIPL=Diptera larvae, GAM=Gammarus, GAS=Gastropod, 

HYD=Hydroptilidae case, HYDL=Hydroptilidae larvae, NEW=Nematoda, NZM=New Zealand mudsnail, SIML=Simulidae larvae, 

SIMP=Simulidae pupae, TCHI=Terrestrial Chironomidae, TCOL=Terrestrial Coleptera, TDIP=Terrestrial Diptera, TSPI=Terrestrial 

Spider, TMISC=Terrestrial Misc. 

Figure 4. Bar graphs comparing macroinvertebrate densities sampled from a.) 

backwaters and b.) shorelines during fluctuating and steady releases in September and 

October 2005 (abbreviations for taxa provided below fig. b). 

19
Ponar Macroinvert Summary Statistics 

ANN AMISC MITE CASE CHIL CHIP CYCL DE

Mean 772.18 10.83 7.22 3.61 90.21 158.77 0.00 s.e. 437.69 7.77 4.87 3.61 63.84 68.12 0.00 Sum 9266.20 129.90 86.60 43.30 1082.50 1905.20 0.00 Count 12.00 12.00 12.00 12.00 12.00 12.00 12.00 

Fluctuating flow (treatment 1) 

Confidence Level (95.0%) 963.36 17.10 10.71 7.94 140.51 149.92 0.00 

Mean 703.63 18.04 7.22 0.00 220.11 93.82 3.61 s.e. 544.30 9.91 7.22 0.00 65.81 54.31 3.61 Sum 8443.50 216.50 86.60 0.00 2641.30 1125.80 43.30 17Count 12.00 12.00 12.00 12.00 12.00 12.00 12.00 

Steady flow (treatment 2) 

Confidence Level (95.0%) 1198.00 21.82 15.88 0.00 144.85 119.54 7.94 2

Hess Macroinvert Summary Statistics

ANN AMISC MITE CHIL CHIP DIPL GAM GAS 

Mean 89.70 2.93 15.60 37.05 33.15 0.00 8.78 0s.e. 82.53 1.53 11.00 17.57 14.96 0.00 8.78 0Sum 1076.40 35.10 187.20 444.60 397.80 0.00 105.30 11Count 12.00 12.00 12.00 12.00 12.00 12.00 12.00 12

Fluctuating flow (treatment 1) 

Confidence Level (95.0%) 181.66 3.36 24.21 38.67 32.92 0.00 19.31 2

Mean 611.59 31.91 4.25 92.54 98.92 2.13 9.57 0s.e. 574.54 30.76 3.26 37.94 71.45 2.13 6.47 0Sum 6727.50 351.00 46.80 1017.90 1088.10 23.40 105.30 0Count 11.00 11.00 11.00 11.00 11.00 11.00 11.00 11

Steady flow (treatment 2) 

Confidence Level (95.0%) 1280.15 68.53 7.27 84.54 159.21 4.74 14.42 0

Table 4. Summary statistics of macroinvertebrate densities from backwaters and shoreline habitats under fluctuareleases from September 4 to October 7, 2005 (abbreviations for taxa provided below table).

20
Table 4 continued

Ponar (backwater) Macroinvert Summary Statistics 

HYDL NEM NZM SIML SIMP TCHI TCOL 

Mean 10.83 1490.24 364.44 241.76 7.22 0.00 7.22 s.e. 7.77 807.18 169.84 76.21 4.87 0.00 7.22 Sum 129.90 17882.90 4373.30 2901.10 86.60 0.00 86.60 Count 12.00 12.00 12.00 12.00 12.00 12.00 12.00 

Fluctuating flow (treatment 1) 

Confidence Level (95.0%) 17.10 1776.59 373.82 167.74 10.71 0.00 15.88 

Mean 321.22 109.55 151.04 1.06 0.00 0.00 0.00 s.e. 188.51 86.69 110.78 1.06 0.00 0.00 0.00 Sum 3533.40 1205.10 1661.40 11.70 0.00 0.00 0.00 Count 11.00 11.00 11.00 11.00 11.00 11.00 11.00 

Steady flow (treatment 2) 

Confidence Level (95.0%) 420.04 193.15 246.83 2.37 0.00 0.00 0.00 

Hess (Shoreline) Macroinvert Summary Statistics 

NEM NZM SIML SIMP TCHI TCOL TDIP TSPI 

Mean 35.10 75.08 27.30 0.00 0.98 0.98 1.95 1.95s.e. 20.16 43.10 12.16 0.00 0.98 0.98 1.31 1.31Sum 421.20 900.90 327.60 0.00 11.70 11.70 23.40 23.40Count 12.00 12.00 12.00 12.00 12.00 12.00 12.00 12.00

Fluctuating flow (treatment 1) 

Confidence Level (95.0%) 44.38 94.85 26.77 0.00 2.15 2.15 2.89 2.89

Mean 321.22 109.55 151.04 1.06 0.00 0.00 0.00 1.06s.e. 188.51 86.69 110.78 1.06 0.00 0.00 0.00 1.06Sum 3533.40 1205.10 1661.40 11.70 0.00 0.00 0.00 11.70Count 11.00 11.00 11.00 11.00 11.00 11.00 11.00 11.00

Steady flow (treatment 2) 

Confidence Level (95.0%) 420.04 193.15 246.83 2.37 0.00 0.00 0.00 2.37

ANN=Annelida, AMISC=Aquatic Misc., MITE=Arachnida, CASE=Unknown case, CHIL=Chironomidae larvae, CHIP=Chironomidae pupae, CYCL=Cyclopedia, DETDIPL=Diptera larvae, GAM=Gammarus, GAS=Gastropod, HYD=Hydropsychidae case, HYDL=Hydroptilidae larvae, NEW=Nematoda, NZM=New Zealand mudsnail, SIML=Simulidae larvae, SIMP=Simulidae pupae, TCHI=Terrestrial Chironomidae, TCOL=Terrestrial Coleptera, TDIP=Terrestrial Diptera, TSPI=Terrestrial Spider, TMISC=Terrestrial Misc.

21
Benthic invertebrate densities along shorelines did not differ significantly 

between treatments (t (0.05,(1),11), p > 0.05). Plankton density along shorelines was 

significantly higher during the fluctuating flow treatment (t (0.05,(1),11), p < 0.05), a pattern 

similar to that found for backwaters (fig. 5a, b). Annelids were the most abundant species 

found in shoreline samples during both treatments, followed by Simulidae larvae and 

Chironomid pupae (table 4; fig. 4a, b). 

a.

Backwater phytoplankton density during varied flows

0

1000

2000

3000

4000

5000

6000

7000

Achnanthes

Biddulphia

Ceratium

Chlamydomonas

Chroomonas

Cladophora

Cocconeis

Cruceginia

Cryptomonas

Cyclotella

Cymbella

Diatoma

Dinobryon

Elakathorix

Fragilaria

Glenodinium

Gomphonema

Gyrosigma

Merismopedia

Navicula

Oscillatoria

Rhoicosphenia

Scenedesmus

Spyrogyra

Staurastrum

Synedra

Tetraedron

Species Trachelomonas

# / liter

Fluctuating flow

Steady flow

Shoreline phytoplankton density during varied flows 

0

500

1000

1500

2000

2500

3000

Achnanthes

Ceratium

Chroomonas

Cocconeis

Cryptomonas

Cymbella

Dinobryon

Fragilaria

Gomphonema

Merismopedia

Oscillatoria

Scenedesmus

Staurastrum

Tetraedron

Species

# / liter

Fluctuating flow

Steady flow

Figure 5. Bar graphs comparing phytoplankton densities sampled from a.) backwaters 

and b.) shorelines during fluctuating and steady releases in September and October 2005. 

Although the number of individuals for particular species differed between 

habitats, benthic communities were similar based on a non significant result for the 

Mantel test for all comparisons between habitats and treatments (p > 0.05). Patterns 

associated with benthic density with distance from the dam were similar between habitats 

(fig. 6a, b). 

22
a.

Comparison of benthic density from backwater sites 

0

50

100

150

200

250

300

350

400

450

0 1 2 3 4 5 6 7 8 9 10 11 12 13

site

g/sq m

Fluctuating flow

Steady flow

Comparison of benthic density from shoreline sites 

0

200

400

600

800

1,000

0 1 2 3 4 5 6 7 8 9 10 11 12 13

site

g/sq m

Fluctuating flow

Steady flow

Figure 6. Benthic invertebrate density by site under fluctuating and steady releases with 

distance from Glen Canyon Dam for a.) backwaters and b.) shoreline habitats (difference 

between treatments were non significant). 

Fish Collections 

The CPUE for all native fish (speckled dace, flannelmouth sucker, and humpback 

chub) was not significantly different in backwaters among sampling treatments (t 0.1,(1),11, p 

> 0.05; fig 7a) with the exception of bluehead suckers (t 0.1,(1),11 = 1.92, p = 0.04). The 

23
CPUE was greatest at a site located above the Little Colorado River during the first 

treatment (Site 4, fig. 8a, b). Under the steady flow treatment, greatest CPUE was 

recorded below the Little Colorado River in Site 7 (fig. 8a, b) with CPUE showing an 

increasing trend from sample Sites 5 through 7. There is a positive relationship between 

increasing CPUE with distance from the dam, but this was not significant. Fathead 

minnows were the most abundant fish, followed by flannelmouth suckers, during both 

treatments. 

A.

Backwater Seining Catch-per-unit-effort

0.0

0.2

0.4

0.6

0.8

1.0

1.2

BBHBHS

CCFCRP

FHMFMSHBC

PKF

RBT

RSH

SPD

Species

CPUE (fish/m2)

Fluctuating flow

Steaady flow

B.

Shoreline Electrofishing Catch-per-unit-effort 

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

BBHBHS

CCFCRP

FHMFMSHBC

PKF

RBT

RSH

SPD

Species

CPUE (fish/s)

Fluctuating flow

Steady flow

BBH=black bullhead, BHS=bluehead sucker, CCF=channel catfish, FHM = fathead minnow, 

FMS=flannelmouth sucker, HBC=humpback chub, PKF=plains killifish, RBT=rainbow trout, 

RSH=red shiner, SPD=speckled dace 

Figure 7. Catch-per-unit-effort (CPUE) for fish species encountered from a.) backwater 

seining and b.) shoreline backpack shocking under fluctuating and steady releases in 

September and October 2005 (abbreviations for species provided below fig. b). 

24
a.

Comparison of seining CPUE of fishes in backwater sites 

0

2

4

6

8

0 1 2 3 4 5 6 7 8 9 10 11 12 13

site

CPUE

Fluctuating flow

Steady flow

b.

Comparison of electrofishing CPUE of fishes along the shoreline sites

0

0.01

0.02

0.03

0.04

0.05

0.06

0 1 2 3 4 5 6 7 8 9 10 11 12 13

site

CPUE

Fluctuating flow

Steady flow

Figure 8. Catch-per-unit-effort (CPUE) for fish species encountered from a.) backwater 

seining and b.) shoreline backpack shocking under fluctuating and steady flow treatments 

with distance from Glen Canyon Dam (difference between treatments were non 

significant). 

The CPUE for native fish did not change significantly during the two treatments 

along the shoreline. Speckled dace was the most abundant fish captured along the 

shoreline during fluctuating flows followed by fathead minnows (fig. 7b). During the 

steady flow period, fathead minnows were the most common fish species captured, 

followed by speckled dace. Total CPUE during the second treatment increased by 120%. 

Although CPUE differed between habitats, fish communities were similar between 

habitats based on non significant Mantel test results for all comparisons between habitats 

and treatments (p > 0.05). 

25
Discussion 

Our results indicate that there were some differences in the water quality and 

biological parameters that we measured in backwaters and along shorelines between low 

fluctuating and steady flow treatments. Water quality parameters were not significantly 

different among treatments except for turbidity, which was significantly lower during the 

second treatment. This may be due to reduced tributary inputs during the second 

treatment rather than operations, however. Plankton density was greater during the 

fluctuating treatment than during the steady flow treatment, while macroinvertebrate 

densities were not significantly different among treatments. Total macroinvertebrate 

density was greater during the steady flow treatment, but the difference was not 

statistically significant. Macroinvertebrate communities did not differ between habitats or 

treatments, but invertebrate density was higher in backwaters relative to shorelines. The 

CPUE for native fish was also not different between these treatments except for an 

increase in bluehead suckers in backwaters during the steady flow treatment (fig. 7a). 

Differences between habitat types were observed during both treatments. During 

fluctuating flows, backwaters were warmer than the shoreline habitat by 1°C as 

compared to during steady flows (table 3). Warmer temperatures in backwaters relative 

to other shoreline types have been reported elsewhere (Arizona Game and Fish 

Department 1996; Kaplinski, 2006; Korman and others, 2006). Water temperature was 

less variable during steady flows relative to fluctuating flows. This may have been due to 

shorter day length during the steady flow treatment, which limited the amount of 

warming, and hence variability in temperature, that could occur. 

Under both treatments, macroinvertebrate densities declined in both habitats with 

distance from the dam (fig. 6 a, b). This pattern may be related to factors other than 

operations, but suggests further study may be warranted. Comparisons of species within 

habitats showed a pattern of increased densities among some macroinvertebrates (e.g., 

annelids, New Zealand mudsnails) along shoreline habitats under the steady flow 

treatment (fig. 4b), while there was a slight decline in the densities of some 

macroinvertebrates in backwaters (fig. 4a). While neither of these patterns were 

statistically significant they do suggest data collection under longer term flow treatments 

that compare habitats may be beneficial to understand food availability and habitat 

interactions. 

Our results should be interpreted cautiously as the time within and between 

treatments was insufficient to determine the effects of flow alone on the sampled 

organisms. The combination of antecedent environmental conditions associated with 

summer operations, localized storm events, and life histories of organisms likely 

overwhelmed any measurable short-term effects of the two-week flow treatments. Larger 

volume releases prior to data collection associated with August operations, the gradual 

reduction in release volumes during the first three days of September, and inputs from 

tributaries contribute to ambiguities in interpretation of the results for each treatment 

period. Just as the first treatment’s antecedent conditions included higher volumes, the 

second treatment’s conditions were potentially influenced by these releases as well as by 

the previous two weeks of reduced flows. The lower plankton densities observed during 

the second treatment may be an artifact of the reduced flow volume initiated on 

September 4, 2005, while the first treatment’s data, which immediately followed reduced 

26
releases, were still likely associated with plankton densities released under larger 

volumes prior to the treatment period. Tributary inputs may have also contributed to 

changes in plankton density. Because there was no period of equilibration allowed 

between treatments, interpretation of the results relative to flow treatments is limited. 

Monsoonal rains that prompted tributary inputs likely contributed to increased 

turbidity values recorded during the first treatment. During the first treatment, both the 

Paria and Little Colorado Rivers had small spates that probably contributed to increased 

turbidity. The sediment inputs preclude determining the effect of operations on turbidity 

levels for these habitats. If the flow treatments had been extended past the time that 

monsoon rains were most probable to occur, then questions regarding water clarity and 

discharge pattern might be apparent. 

With respect to macroinvertebrate densities, the effect of suspended sediment 

inputs that occurred during the first treatment cannot be excluded as a factor affecting 

densities of macroinvertebrates observed during either the first or second treatments. 

Moreover, the observed density differences, while not significant, may also represent 

system-wide variability that might be observed at any time. The Arizona Game and Fish 

Department (1996) reported that benthic invertebrate densities varied significantly 

between years, seasons, and reaches from 1991 to 1994, suggesting the variables 

observed from these two treatments may be typical rather than attributable to changes in 

flows. A study by Hofknecht (1981) indicates that lower velocities and deposition of 

detritus are more likely to occur in a return channel, and these conditions favor increased 

numbers of benthic invertebrates. These physical conditions were observed during the 

second treatment in this study. Had steady flows continued for a longer period of time, 

changes in invertebrate densities may have been greater than average for the season. 

Small fishes exhibited similar variability in CPUE between treatments as was 

observed for macroinvertebrates. The variability in catch effort could be attributed to 

tributary spates potentially influencing spatial distributions more than short-term changes 

in flows. Increases in fish abundance coincided with locations below the Little Colorado 

River, a major tributary to the mainstem (Sites 4–12 for fluctuating flows; Sites 5–12 for 

steady flows; fig. 8). Fish introductions from tributaries associated with monsoonal 

storms confound our ability to assess flow variables independently. While we did observe 

some slight differences in temperature and CPUE between habitats and treatments, our 

ability to make general statements about how slight fluctuations versus steady flows 

might affect shoreline habitat variables is limited by the short time permitted for data 

collection. 

Recommendations 

To determine the effect of an experimental treatment, such as different dam 

operations, on organisms, whether phytoplankton or fish, treatments must be roughly 

equivalent in length to the life-span of the organism in question. The only organisms we 

studied that have life-spans of ~ two weeks, the duration of the experimental treatments, 

were phytoplankton. Although we observed minor differences in fish CPUE and 

invertebrate densities among treatments, it is impossible to determine what effect the 

treatments have on food availability for fishes or fish growth and survival because the 

treatments were not of sufficient duration. 

27
If the objective of the experiment is to identify a directional response by an 

organism to a treatment, which is assumed for adaptive management purposes, then the 

data collected should be associated with life history parameters (e.g., mortality, 

survivorship, and reproduction). Collecting life history parameters for aquatic organisms 

in the Colorado River is difficult given the size of the watershed, the influence tributaries 

can have on mainstem interactions, and the transient nature of aquatic organisms. Life 

history studies related to flow treatments may be more amenable to laboratory or other 

controlled environments first and then applied in a large scale setting (e.g., the CRE) for 

longer periods of time. 

A laboratory setting can control external variables and identify likely trends 

associated with a particular flow treatment that can be subsequently measured and 

reliably tested in a field setting. The lab setting may also be used to identify the length of 

time needed to execute a field-based effort, identify the minimum amount of change in 

flow (or other parameters) required to elicit a measurable response by the target 

organism, and identify the lag time of response by an organism. The identification of 

time requirements for treatment and response by a target organism promotes wellplanned field-based efforts that collect data that are trophically linked and may improve 

commitment on the part of stakeholders to longer-term flow treatments that are required 

to measure responses by biological organisms. 

Alternative collecting methods may also be used in a field-based study. Sampling 

might be focused on specific reaches of the river rather than a system-wide approach. In 

this manner sample frequency per site could increase and possibly reduce variance 

around the samples. This would require a more concerted logistical effort than a systemwide approach, but the variability observed in the system-wide approach may be reduced 

by sampling fewer sites more intensively, and the information gained may be more useful 

for answering questions regarding flow effects on habitats of young fishes and aquatic 

food base linkages. 

Acknowledgments 

Funding for this work was provided through Interagency Agreement 

02AA406750 from the Bureau of Reclamation to USGS/GCMRC and Cooperative 

Agreement 04WRAG0011 between GCMRC and SWCA Environmental Consultants, 

Inc. We thank Carol Fritzinger, GCMRC, for her logistic coordination associated with 

the field collection dates and Matthew Andersen, Scott Vanderkooi, and two anonymous 

reviewers for their comments that improved this document. Christy Parry provided 

valuable editorial assistance. 

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