Summary Report of Responses of Key Resources to the 2000 

Low Steady Summer Flow Experiment, along the Colorado 

River Downstream from Glen Canyon Dam, Arizona

Open-File Report 2011–1220

U.S. Department of the Interior

U.S. Geological Survey
Cover photographs: Glen Canyon Dam, © 2004 Christopher E. Taesali, used with permission; humpback chub 

(Gila cypha), George Andrejko, Arizona Game and Fish Department; sand grains, David M. Rubin, U.S. Geological 

Survey; and whitewater raft, National Park Service.
Summary Report of Responses of Key Resources to 

the 2000 Low Steady Summer Flow Experiment, along 

the Colorado River Downstream from Glen Canyon 

Dam, Arizona 

By Barbara E. Ralston

Open-File Report 2011–1220

U.S. Department of the Interior

U.S. Geological Survey
 

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia 2011

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Suggested citation:

Ralston, B.E., 2011, Summary Report of Responses of Key Resources to the 2000 Low Steady Summer 

Flow Experiment, along the Colorado River Downstream from Glen Canyon Dam, Arizona: U.S. 

Geological Survey Open-File Report 2011–1220, 129 p.

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.
 iii

Contents

Executive Summary....................................................................................................................................................... 1

Chapter 1. Introduction and Background ....................................................................................................................... 5

Chapter 2. Physical Resource Response to the Low Steady Summer Flow Experiment ............................................. 21

Chapter 3. Native and Nonnative Fish Response to the Steady Summer Flow Experiment in the Mainstem 

Downstream from Lees Ferry....................................................................................................................................... 39

Chapter 4. Vegetation Response to Low Steady Summer Flow Experiment................................................................ 53

Chapter 5. Effects of the Low Steady Summer Flow Experiment on Campsite Area, Rafting Safety and

Travel Time, and Overall Recreational Experience...................................................................................................... 63

Chapter 6. Effect of the Low Steady Summer Flow Experiment on Angling Quality in the Lees Ferry 

Trout Fishery.................................................................................................................................................................. 71

Chapter 7. Financial Costs Associated with the Low Summer Steady Flow Experiment ............................................. 87

Chapter 8. Management Implications Associated with the Low Steady Summer Flow Experiment ............................. 93

Appendix. Annotated Bibliography of Studies Conducted During 2000 Low Steady Summer Flow Experiment......... 111

 
 iv

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.09290 square meter (m2)

square inch (in2) 6.452 square centimeter (cm2)

Volume

cubic foot (ft3) 0.02832 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3)

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 10.76 square foot (ft2) 

square centimeter (cm2) 0.1550 square inch (ft2) 

Volume

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8
 

Summary Report of Responses of Key Resources to 

the 2000 Low Steady Summer Flow Experiment, along 

the Colorado River Downstream from Glen Canyon 

Dam, Arizona 

By Barbara E. Ralston 

Executive Summary

In the spring and summer of 2000, a series of steady discharges of water from Glen 

Canyon Dam on the Colorado River were used to evaluate the effects of aquatic habitat stability 

and water temperatures on native fish growth and survival, with a special focus on the 

endangered humpback chub (Gila cypha), downstream from the dam in Grand Canyon. The 

steady releases were bracketed by peak powerplant releases in late-May and early-September. 

The duration and volume of releases from the dam varied between spring and summer. The 

intent of the experimental hydrograph was to mimic predam river discharge patterns by including 

a high, steady discharge in the spring and a low steady discharge in the summer. The hydrologic 

experiment was called the Low Steady Summer Flow (LSSF) experiment because steady 

discharges of 226 m3/s dominated the hydrograph for 4 months from June through September 

2000. 

The experimental hydrograph was developed in response to one of the U.S. Fish and 

Wildlife Service’s Reasonable and Prudent Alternatives (RPA) in its Biological Opinion of 

the Operation of Glen Canyon Dam Final Environmental Impact Statement. The RPA focused on 

the hypothesis that seasonally adjusted steady flows were dam operations that might benefit 

humpback chub more than the Record of Decision operations, known as Modified Low 

Fluctuating Flow (MLFF) operations. Condensed timelines between planning and 

implementation (2 months) of the experiment and the time required for logistics, purchasing, and 

contracting resulted in limited data collection during the high-release part of the experiment that 

occurred in spring. The LSSF experiment is the longest planned hydrograph that departed from 

the MLFF operations since Record of Decision operations began in 1996. 

 As part of the experiment, several studies focused on the responses of physical properties 

related to environments that young-of-year (YOY) native fish might occupy (for example, 

measuring mainstem and shoreline water temperature, and quantifying useable shorelines). The 

part of the hydrograph that included a habitat maintenance flow (a 4-day spike at a powerplant 

capacity of 877 m3/s) and sustained high releases in April and May (averaging 509 m3/s) resulted 

in sediment export to Lake Mead, the reservoir downstream from Glen Canyon Dam, which is 

outside the study area. Some mid-elevation sandbar building (between 566 and 877 m3/s stage 

elevations) occurred from existing sediment deposits rather than from sediment inputs from 
 2

tributaries during the previous winter. Low releases in the summer combined with low tributary 

sediment inputs resulted in minor sediment accumulation in the study area. The September 

habitat maintenance flow reworked accumulated sediment and resulted in increases in the area of 

some backwaters. The mainstem water temperatures in the reach near the Little Colorado River 

during the LSSF experiment varied little from previous years. Mainstem water temperatures in 

western Grand Canyon average 17 to 20°C. During the LSSF, backwaters warmed more than 

other shoreline environments during the day, but most backwaters returned to mainstem water 

temperatures overnight. Shoreline surface water temperatures from river mile (RM) 30 to 72 

varied between 9 and 28°C in the middle of the day in July. These temperatures are within the 

optimal temperature range for humpback chub growth and spawning, which is between 15 and 

24°C. How surface water temperatures transfer to subsurface water temperatures is unknown. 

Data collection associated with the response of fish to the 2000 LSSF hydrograph 

focused on fish growth and abundance along the Colorado River in Grand Canyon. The target 

resource, humpback chub and other native fishes, did not respond in a strongly positive or 

strongly negative manner to the LSSF hydrograph during the sampling period, which extended 

from June to September 2000. In 2000, the mean total length of YOY native fishes was similar to 

the mean length from previous years, but the abundance of YOY native fish was greater in 2000. 

The greatest numbers of humpback chub were near the confluence of the Colorado River with 

the Little Colorado River, where the largest spawning population is found. Factors directly 

associated with the LSSF hydrograph, geography, and the abundance of nonnative salmonids in 

the system before the experiment, as well as elements not affected by mainstem hydrology, may 

have contributed to the neutral response observed for native fish. The close proximity of the 

Little Colorado River to Glen Canyon Dam precluded sufficient warming of the mainstem down 

to the confluence with the Little Colorado River (RM 61) to reach optimal growth and spawning 

conditions for humpback chub, unlike shoreline surface water temperatures. The 4-day habitat 

maintenance flow in September interrupted persistent habitats for YOY fishes and may have 

confounded the results. The high abundance of salmonids in the mainstem before the experiment 

and predation by them may have affected the number and size of native fish that were caught. 

Native larval fish survival in the tributaries that is unrelated to mainstem environments and flow 

manipulations also can affect relative abundance observed in the mainstem. Collectively, these 

variables limit understanding the effects of the LSSF hydrograph on young native fish growth 

and survival. 

The complicated hydrograph composed of steady discharges at multiple volumes that 

varied in duration from 4 days to 8 weeks and in magnitude from 226 to 877 m3

/s presented a 

disruption to persistent habitat, which was the intent of the experiment. The longest 

uninterrupted period of persistent habitat for YOY fish was 3 months. YOY fish that entered the 

mainstem in mid-July (for example, humpback chub) had a shorter exposure to persistent habitat. 

Achieving effective high-magnitude discharges for ecological experiments is a challenge in a 

regulated system. The presence of a dam restricts discharge magnitude, and delivery agreements 

among States further restricts annual and monthly volumes releases. 

A change in flow magnitude is the most common element associated with regulation, and 

fish appear to be sensitive to this variable. The spring discharge magnitudes during the LSSF 

experiment were only 25 percent greater than the average MLFFs in the 1990s and 78 percent 

less than the average predam spring discharge. The changes in discharge associated with the 

experimental hydrograph likely were too small compared to standard operations to observe a 

response by fish. The bulk of YOY fish enter the mainstem from tributaries in the summer 
 3

months, with humpback chub YOY entering the mainstem primarily in association with 

monsoons that typically begin in July. Trying to affect life stages (for example, spawning and 

larval development) that primarily are associated with tributaries that have retained their 

hydrology by altering mainstem volumes may be minimally effective. Instead, developing 

experimental flows that can target YOY life stages directly affected by mainstem hydrology and 

temperatures may be more informative. In contrast to experiments involving large volume 

releases that can often only be of short-duration, lower volume releases may be more attainable 

and allow testing of hypotheses about limiting factors in endangered fish species survival in the 

mainstem. 

Other resource responses that were measured during the LSSF experiment included 

seedling establishment of tamarisk (Tamarix spp.), growth of wetland species during the summer, 

recreation safety and perceptions, and the financial costs of the experimental hydrograph to 

recreational businesses and power users. The LSSF hydrograph supported tamarisk seedling 

establishment, as the high-sustained spring flows scoured shorelines and the habitat maintenance 

flow transported tamarisk seeds. The reduced summer hydrograph exposed open shorelines and 

resulted in a proliferation of tamarisk seedlings along the scoured shorelines. The September 

habitat maintenance flow reduced tamarisk seedling densities associated with later season 

germination; those individuals that first established in June likely persisted. 

The experimental hydrograph affected recreational users and businesses, and the 

hydrograph increased the financial costs of power. The low-discharge part of the hydrograph, 

with reduced water velocity, increased travel time for whitewater rafting, reduced time spent at 

attraction sites, increased the availability of low-water camps, and initially increased the number 

of boating accidents at rapids. However, the recreational experience that includes these elements 

and participants’ perceptions likely were affected little by the experimental hydrograph. 

Financial costs to the downstream commercial rafting industry included repair and replacement 

of equipment damaged by exposed rocks and customer refunds associated with trip evacuations 

because of stranding in rapids. Commercial fishing guides in Lees Ferry lost business during the 

habitat maintenance flows because they could not access desired fishing locales. Lastly, Federal 

power users incurred increased financial costs because the experiment occurred when higher than 

normal daily market prices had to be paid to supplement power needs. Reallocating water 

delivery to other months and in the subsequent water year (12 month delivery of water delivery 

from October to the end of September) to accommodate the hydrograph also increased costs to 

power users. The timing of the 2000 LSSF experiment was coincident with the onset of a 

drought in the American Southwest, an energy crisis in California, and market manipulation by 

energy suppliers that collectively affected daily market prices for power translated to increased 

costs to power users. 

The 2000 LSSF experiment was the first seasonally based experiment using Glen Canyon 

Dam releases that focused on biological resources, primarily humpback chub and other native 

fish. Implementing such an ecosystem-scale experiment created an opportunity to learn about 

resource responses and identify flaws and barriers that limit experimental success. The short 

amount of time available for planning and implementation and the lack of long-term monitoring 

were apparent flaws of the 2000 LSSF experiment. Future experiments would benefit from 

sufficient planning, long-term monitoring, and testable hypotheses for resource responses that 

can be measured and are appropriate for the duration of the experiment. Future experiments 

would also benefit from publishing results and findings in peer-reviewed reports and journal 

articles that can be summarized for stakeholder use in a timely fashion. Reports by cooperators 
 4

who collected and analyzed data are the first step in the process of incorporating knowledge but 

not the final step. Having citable literature, which can be incorporated into future experimental 

efforts, is critical to building a solid, peer-reviewed basis for documenting results and furthering 

experimental planning and decisionmaking by resource managers. 

Basin hydrology and reservoir elevations greatly affect experimental capacity in the 

Colorado River downstream from Glen Canyon Dam. Taking advantage of unexpected sediment 

inputs to the system or increased water temperatures because of reduced inflows and associated 

reservoir elevations can be used to advance the understanding of how manipulated flow variables 

benefit downstream resources. If experiments were approached opportunistically, flexibility also 

would need to extend to administrative tasks associated with launch schedules, collection 

permits, and use of motorized equipment. 

Experimental flexibility necessitates the implementation of long-term monitoring that 

provides a consistent data stream for long-term resource response. Immediate measures of 

response may be meaningless in the longer term, particularly for long-lived species, if consistent 

monitoring is absent after the experiment. A lack of response observed for 1 year may not mean 

the treatment was ineffective. Multiple years of data collection may be necessary for a response 

to be measurable or understood. 
 5

Chapter 1. Introduction and Background

Introduction

Adaptive management acknowledges the inherent uncertainty of ecosystem responses to 

systemic management actions and embraces the notion of “learning by doing” (Walters and 

Holling, 1990). The U.S. Department of the Interior uses adaptive management approaches for 

several ecosystem-scale landscapes, including the Colorado River downstream from Glen 

Canyon Dam (fig. 1-1; Williams and others, 2007). The adaptive management program for Glen 

Canyon Dam, established in 1996 (U.S. Department of the Interior, 1996), was developed in 

response to the Operation of Glen Canyon Dam Final Environmental Impact Statement 

(GCDEIS; U.S. Department of the Interior, 1995) and the Grand Canyon Protection Act of 1992 

(GCPA; title XVIII, secs. 1801–1809, of Public Law 102-575). The GCDEIS identified 

uncertainties of downstream resource responses to water release patterns from the dam. The 

GCPA directed the Secretary of the Interior to implement long-term monitoring programs and 

research activities to ensure that the dam was operated “in such a manner as to protect, mitigate 

adverse impacts to, and improve the values for which Grand Canyon National Park and Glen 

Canyon National Recreation Area were established, including, but not limited to natural and 

cultural resources and visitor use” (GCPA, sec. 1802(a)). 

Short-duration, high-discharge floods from Glen Canyon Dam are the predominant 

ecosystem experiments implemented by the Secretary of the Interior since 1996 (U.S. 

Department of the Interior, 1996; Melis, 2011). These floods (discharges as much as 1,274 cubic 

m3/s) have advanced understanding about sediment storage and transport within the river 

corridor (Webb and others, 1999; Topping and others, 2000b; Rubin and others, 2002; Schmidt 

and others, 2004; Hazel and others, 2006; Wright and others, 2008). The responses of biological 

resources to these floods are not always immediately evident and may not be measurable 

depending on the life history of the organism and other factors, such as food availability and the 

effects that tributaries may have on the survival of organisms (Melis and others, 2006a; Coggins 

and Walters, 2009; Kennedy and Ralston, 2011). 

A response to a multiple-day event by an organism does not necessarily identify the longterm fate of the population of the organism. Many investigators associated with an experimental 

flood conducted in 1996 noted immediate responses by fish or other organisms to the flood 

pulse. The responses were in the form of movement by fish or short-term reduction in algal and 

plant biomass through scour and transport (Kearsley and Ayers, 1999; McKinney and others, 

1999; Stevens and others, 2001). For organisms that evolved in environments subject to annual 

flooding that lasted a month or more, the likelihood of a short-duration “flood” eliciting a longterm measurable biological response such as increased recruitment is low, and the reported 

results support this observation (Blinn and others, 1999; Kearsley and Ayers, 1999; McKinney 

and others, 1999). More recently, however, data from the 2008 high-flow experiment (HFE) 

suggest that rainbow trout in the Lees Ferry reach may have an increased recruitment response to 

these experiments (Korman and others, 2011). A longer-duration flow experiment intended to 

improve native fish growth and recruitment was implemented in the spring and summer of 2000. 

The experiment was called the Low Steady Summer Flow (LSSF) experiment. This report 

describes key results of the effects of the LSSF experiment on mainstem and shoreline water 
 6

temperatures, sediment reworking and mainstem young-of-year (YOY) fish rearing 

environments, YOY fish growth and abundance, riparian vegetation response, campsite area and 

recreation satisfaction, and the financial costs of the experiment. Also included is a review of the 

process associated with the implementation of the experiment, a discussion of the management 

implications associated with long-duration experiments, and questions that remain unresolved 

following the LSSF experiment. 

Figure 1-1. Map of the Colorado River downstream from Glen Canyon Dam. River mile (RM) 

designations occur at 25-mile intervals starting from Lees Ferry, designated as RM 0. Large tributaries to 

the mainstem, including the Little Colorado River, are identified. 
 7

Background

Native fish species, particularly the endangered humpback chub (Gila cypha; fig. 1-2), 

are a resource of concern for the stakeholders in the Glen Canyon Dam Adaptive Management 

Program. The species’ declining population numbers and their rarity were noted before the 

establishment of the Endangered Species Act (Minckley, 1991). Multiple factors contributed to 

the decline of the humpback chub, including predation by nonnative species, competition for 

resources, reduced mainstem temperatures (temperatures below 15°C), and unstable shoreline 

habitats (Minckley, 1991; Gloss and Coggins, 2005; Yard and others, 2011). Aspects of Glen 

Canyon Dam operations that are hypothesized to affect native fish recruitment are coldwater dam 

releases that average 10.5°C (Vernieu and others, 2005; Voichick and Wright, 2007) and daily 

fluctuating discharges that result in unstable shoreline habitats. The result is reduced spawning, 

growth, and subsequent recruitment of humpback chub and other native fish species in the 

Colorado River mainstem downstream from Glen Canyon Dam (Hamman, 1982; Douglas and 

Marsh, 1996; Clarkson and Childs, 2000). 

Figure 1-2. Image of adult humpback chub (Gila cypha), an endangered species with a population that 

persists in the Colorado River downstream from Glen Canyon Dam (George Andrejko, Arizona Game and 

Fish Department). Adult humpback chub range in size from 120 to 380 mm (Marsh and Douglas, 1997).

The U.S. Fish and Wildlife Service issued a Biological Opinion (U.S. Fish and Wildlife 

Service, 1995) and a series of Reasonable Prudent Alternatives (RPAs) in association with 

the GCDEIS to improve conditions for endangered native fish, principally humpback chub. One 

RPA directed the Bureau of Reclamation to initiate a program of experimental dam releases 

consisting of high, steady flows in the spring and low steady flows in the summer. The 

experimental flows were to be implemented when annual water delivery between the upper and 
 8

lower Colorado River Basin States (not shown) was at the minimum of 8.23 million acre feet 

(maf). The assumptions of the required flows were that long-duration, steady flows would reduce 

the risk of further jeopardizing the continued existence of the endangered humpback chub by 

stabilizing shoreline habitats for young humpback chub, and that mainstem temperatures would 

increase in association with low-volume, steady releases, thus improving spawning and growth 

of this species in the mainstem (U.S. Fish and Wildlife Service, 1995). 

In response to the RPA, a flow plan that considered habitat needs associated with 

humpback chub life-history stages (for example, larvae, YOY, juveniles) and other native fish 

was developed by SWCA Environmental Consultants, Inc. (Valdez and others, unpub. report, 

20001). Valdez and others (unpub. report, 2000) recommended different discharges and release 

patterns for each life stage and divided flows into three time periods: March–May (high steady

flows of 594 m3/s with a 4-day 877 m3/s spike); June–September (low steady flows of 226 m3/s 

ending with a 4-day 877 m3/s spike); and October–February (steady flows of 226 m3/s). The 

three time-periods (fig. 1-3) are respectively coincident with spawning and emergence in 

tributaries, summer migration into the mainstem and subsequent growth, and overwintering in 

the mainstem. Valdez and others (unpub. report, 2000) identified the following objectives for

experimental flow plan: 

• To enhance survival and growth of young native fishes by providing stable, warm, 

productive shoreline nursery habitats 

• To increase recruitment of native fish 

• To minimize adverse effects of nonnative fish 

• To contribute to the recovery of endangered humpback chub 

 

1

 Although the U.S. Geological Survey does not typically cite unpublished reports, this report 

makes reference to several unpublished reports to provide the reader with important background 

information not otherwise available. Copies of unpublished reports are available upon request by 

contacting the Center Director, U.S. Geological Survey, Southwest Biological Science Center, 

2255 N. Gemini Drive, Flagstaff, Ariz. 86001. 
 9

Figure 1-3. Experimental hydrograph of dam releases measured in cubic meters per second (m3/s) from 

Glen Canyon Dam recommended by Valdez and others (adapted from Valdez and others, unpub. report, 

2000).

Valdez and others (unpub. report, 2000) identified the objective of each flow period and 

the potential response by physical and aquatic resources (table 1-1). They identified possible 

negative and positive effects to sediment because sediment conservation is affected directly by 

discharge volume, and sediment in eddies affects some fish habitat (for example, backwaters). 

The plan did not speculate on the effects of the hydrograph on resources that did not appear to 

support fish directly (for example, riparian vegetation and recreation). Funding was provided by 

the Bureau of Reclamation to study the response of these resources as well as those identified by 

Valdez and others (unpub. report, 2000). 
 10

Table 1-1. Hypothesized effects of experimental flows on physical and aquatic resources (Valdez and 

others, unpub. report, 2000), and identification of associated studies conducted during the 2000 Low 

Steady Summer Flow (LSSF) experiment.

Benefits to physical and aquatic 

resources

Risks to physical and aquatic 

resources

Studied during LSSF/

author

Period 1: March to May

Scour backwater eddies and 

increase area for shoreline fish 

habitat

No negative risk identified YES

Goeking and others, unpub. report, 

2003; Hazel and others, 2006; 

Schmidt and others, 2007.

Mobilize and store sand and 

sediment in campsites/increase 

campsite area

Export sediment, reduce campsite 

areas

YES

Hazel and others, 2006; Schmidt 

and others, 2007.

Create warm, low-velocity areas at 

tributary mouths

No negative risk identified YES

Protiva and others, 2010

Ponding as thermal refuges for 

drifting larvae and young fish

Warm ponded tributaries attract 

nonnative fish predators/

competitors

NO

Destabilize habitats to 

disadvantage nonnative fishes 

No negative risk identified NO

Redistribute nutrients No negative risk identified NO

Reset community primary 

production

No negative risk identified NO

Spike flows flush nonnative fish 

from nearshore habitats

No negative risk identified NO

Period II: June to September

Store sediment in river channel No negative risk identified Hazel and others, 2006; Schmidt 

and others, 2007.

Expand campsite beach area No negative risk identified YES

Kaplinski and others, 2005; Hazel 

and others, 2006; Schmidt and

others, 2007.

September habitat maintenance 

flows resuspend and store sand 

from summer tributary inputs

September habitat maintenance 

flows export sand and sediment 

instead of storing it

YES

Kaplinski and others, 2005; Hazel 

and others, 2006; Schmidt and

others, 2007.

Create stable shorelines for fish 

habitat

No negative risk identified YES

Korman and others, 2004

Increased growth and survival of 

young native fishes 

Increased growth and survival of 

nonnative fishes

YES 

Trammell and others, unpub. 

report, 2002; Speas and others, 

unpub. report, 2003, 2004; 

Coggins and Walters, 2009

Increased autotrophic algal and 

macroinvertebrate production

Decreased drift of food for fish YES 

Yard and Blinn unpub. report, 

2001; Rogers and others, unpub. 

report, 2003;Benenati and others, 

unpub. report, 2002
 11

Table 1-1. Hypothesized effects of experimental flows on physical and aquatic resources (Valdez and 

others, unpub. report, 2000), and identification of associated studies conducted during the 2000 Low 

Steady Summer Flow (LSSF) experiment. —Continued

Benefits to physical and aquatic 

resources

Risks to physical and aquatic 

resources

Studied during LSSF/

author

Possible mainstem hatching 

success

Mainstem reproduction by 

nonnative fishes

YES 

Trammell and others, unpub. 

report, 2002

September habitat maintenance 

flows flush nonnative fishes from 

nearshore habitats

Minimized thermal plume at RM 

30 may reduce survival of young 

humpback chub

NO

September habitat maintenance 

flows flush nonnative fishes from 

nearshore habitats

Increased water clarity leads to 

increased predation of native fish 

by sight predators

NO

September habitat maintenance 

flows flush nonnative fishes from 

nearshore habitats

Increased infestation of parasites 

and diseases

NO

Period III: October to February

Retain sediment with low volume 

discharge

No negative risk identified NO

Increased survival of young native 

fishes

Possible overwinter survival and 

expansion of nonnative fishes

YES

Trammell and others, unpub. 

report, 2002; Speas and others, 

unpub. report, 2004. Modified 

Low Fluctuating Flow operations 

implemented in October 2000 and 

overwinter survival under steady 

flows not tested

Maintain stable winter conditions 

to minimize energy expenditure

Possible greater spawning success 

of downstream populations of 

trout

NO 

Maintain overwinter autotrophic 

production in mainstem, 

shorelines, backwaters

Decreased drift of food for fish NO

Maintain overwinter autotrophic 

production in mainstem, 

shorelines, backwaters

Increased predation by sight 

feeders

NO 

Modified Low Fluctuating Flow 

operations implemented in 

October 2000 and studies 

discontinued

The experimental plan (Valdez and others, unpub. report, 2000) recommended that longterm monitoring of resources be in place for trend detection before experimentation. They also 

suggested data collection should occur for a year of Modified Low Fluctuating Flow (MLFF) 

operations for an 8.23 maf water year (WY; water delivery for 12 months from October to the 

end of September) before implementing the experimental LSSF hydrograph. Additionally, the 

authors outlined a yearlong implementation process for the experiment that included stakeholder 

approval, regulatory environmental assessment and permitting, and data collection and reporting. 

Before the LSSF experimental hydrographic planning document was finalized (Valdez and 

others, unpub. report, 2000), discussions began among the Bureau of Reclamation, Grand 

Canyon Monitoring and Research Center (GCMRC), and other U.S. Department of the Interior 
 12

partners in January 2000 about implementing the LSSF experimental hydrograph in April 2000. 

As a result, the LSSF experiment proceeded without the recommended year of baseline 

monitoring under MLFF operations in an 8.23-maf WY. 

Plan Implementation and Data Collection Limitations

The precipitation and snowpack conditions in the upper Colorado River Basin (not 

shown) and the desire to meet Biological Opinion requirements associated with the GCDEIS 

converged in the winter of 2000. The convergence resulted in a compressed timeline for the 

implementation of an LSSF experiment in 2000. In January 2000, the Bureau of Reclamation 

identified a high likelihood that the 2000 WY (October 1999–September 2000) would be an 8.23 

maf delivery year and that an LSSF experiment might be implemented. The Bureau of 

Reclamation identified March 2000 as the time at which a final decision about implementing the 

LSSF experiment would take place, because by that time more data would be available about the 

snowpack and predicted inflows to Lake Powell, the reservoir formed by Glen Canyon Dam (fig. 

1-1). The experimental flow plan by Valdez and others (unpub. report, 2000) provided a 

hydrograph (fig. 4), but lacked specific field approaches to evaluate resource responses to the 

experimental hydrograph. There was no consistent monitoring plan in place for many of the 

downstream resources in 2000; instead, several resource programs were initiating reviews of 

previous data-collection efforts for implementing long-term monitoring. In early February 2000, 

GCMRC convened a meeting of existing cooperators to explain the experimental hydrograph and 

identify potential projects, approaches, and logistics needs for the LSSF experiment. In March 

2000, the Bureau of Reclamation determined that an LSSF experiment would proceed. The 

decision in March 2000 to implement the LSSF experiment provided some investigators, Grand

Canyon National Park (GRCA), GCMRC, and the Bureau of Reclamation 1 month to prepare for 

and implement the experiment. Work associated with implementation included identifying 

logistics needs, securing necessary permits, modifying and establishing new agreements among 

cooperators, and buying equipment. Some of the hypotheses and data-collection approaches 

identified in the plan (Valdez and others, unpub. report, 2000; table 1-1) could not be addressed 

because of the compressed timeline for planning and implementation. For example, no baseline 

data were collected during a year of MLFF operations with an annual release of less than or 

equal to 8.23 maf. 

The hydrograph implemented during the 2000 LSSF experiment differed from the 

hydrograph proposed by Valdez and others (unpub. report, 2000). The high, steady discharge in 

spring was reduced to 481 m3/s in volume and shortened in duration by a month, starting in April 

rather than March (fig. 1-4). The reduction in the volume and duration of the hydrograph 

compromised the ponding hypotheses associated with Period I (table 1-1); although, the velocity 

and temperatures at tributary mouths could be measured under different discharge volumes, the 

duration of ponding and its effects on larval retention could not be measured as originally 

intended. The proposed fall steady flows were terminated and MLFF operations were resumed 

on October 1, 2000 (fig. 1-4). As a result, the effects of stable shorelines on growth and survival 

of juvenile and YOY native fish in the fall period were not measured. Instead of developing 

alternative hypotheses associated with the return of MLFF operations and young fish survival or 

other resource responses, data collection for most resources ended when MLFF operations 

resumed. With respect to environmental compliance needs, an environmental assessment of the 

experiment was unnecessary because the hydrograph did not exceed powerplant capacity and fell 

within Record of Decision flow criteria. As a result, implementation proceeded quickly. 
 13

Figure 1-4. Actual release pattern of the experimental flow period from March to October 2000 (dotted 

line) compared with the proposed experimental hydrograph (solid line) from Valdez and others (unpub. 

report, 2000). 

LSSF research projects dealt with individual resources and addressed questions related to

separate periods of the LSSF hydrograph. Physical, biological, and cultural resources were 

studied, including sediment transport and reworking, changes in water temperature, aquatic 

productivity, mainstem fish growth and abundance, riparian vegetation response, recreation 

safety and perceptions, and the financial costs of the experiment. Many of the LSSF projects 

were expanded efforts of existing research projects. For example, the sandbar and sediment 

mass-balance project scheduled to collect sandbar area and volume data in the summer of 2000 

expanded the number of trips and data-collection efforts to capture information about sediment 

response for each experimental flow period (Schmidt and others, 2007). Some projects only 

collected data during a single period within the experiment. For example, an algal production 

study only occurred during Period II (Yard and Blinn, unpub. report, 2001). Other projects 

collected data for a 2-year period, as in the case of riparian vegetation (Porter, 2002), or for a 9-

year period following the experiment, as in the case of humpback chub population estimates 

(Coggins and Walters, 2009). 

The status of monitoring program development for resources along the Colorado River 

affected data collection approaches, intensity, and the ability of researchers to draw conclusions 

about experimental effects. Plans for long-term monitoring of resources for Record of Decision 

operations were still in development in 2000 (U.S. Geological Survey, 1999; Urquhart and 

others, unpub. report, 2000; Anders and others, unpub. report, 2001). A comprehensive longterm dataset for resources responding to different hydrographs was not available, which limited 
 14

researchers’ abilities to compare resource responses to the LSSF hydrograph with Record of 

Decision operations. However, the LSSF experiment provided opportunities to test sampling 

strategies for long-term monitoring (Schmidt and others, 2007) because steady flow operations 

removed one variable that can affect sampling for fish and sediment resources. In 2000, a fishmonitoring program that estimated fish abundance downstream from Glen Canyon Dam was not 

well developed. The fisheries and physical science programs used the experiment to refine 

sampling approaches. As of this writing (2011), long-term monitoring programs for resources 

remain in various states of development. 

Report Organization, Place Names, and Units

This report summarizes the hydrologic design and implementation of the LSSF 

experiment and describes the response of the key resources that were monitored during the 

experiment. A conceptual diagram (fig. 1-5) developed for this report identifies the relations of 

resources that were studied during the LSSF experiment. This report includes an annotated 

bibliography summarizing individual publications associated with the LSSF experiment 

(appendix). The final section of this report identifies the successes and failures of the LSSF 

experiment and identifies aspects of resource responses to the experimental hydrograph that 

remain unknown. 

Throughout the report, “Grand Canyon” is used broadly to refer to the Colorado River 

corridor between Glen Canyon Dam and the western boundary of Grand Canyon National Park, 

including Glen, Marble, and Grand Canyons. The study area is referred to as the “Colorado River 

ecosystem” (CRE).

In this report, metric units are used for all units, with the exception of river miles. By 

convention, locations along the Colorado River below Glen Canyon are identified by river mile 

(RM) and by side of the river (left (L) or right (R)). Lees Ferry, which is the dividing point 

between the upper and lower Colorado River Basin, is the starting point RM 0 (fig. 1-1), with 

mileage measured in upstream (-) and downstream directions. 
 15

Figure 1-5. Diagram of the resources studied during the Low Steady Summer Flow Experiment (ovals) 

and variables that affect the resource response (squares) with arrows identifying connections between 

resources and variables. 

Acknowledgments

This report was funded by hydropower revenues provided by the Bureau of Reclamation 

as part of the Glen Canyon Dam Adaptive Management Program (GCDAMP). The GCDAMP 

was established in 1997 to provide for long-term research and monitoring of downstream 

resources affected by Glen Canyon Dam. Scientific information such as that contained in this 

report, is used as the basis for developing recommendations for dam operations and management 

actions. More information on the GCDAMP is available at http://www.gcdamp.gov/. 

Several people provided input on this document and greatly improved the quality and 

readability of this document. They are, in no particular order, Scott Wright, Theodore Kennedy, 

Mike Yard, Matthew Andersen, John Hamill, Marjike van Heeswijk, and two anonymous 

reviewers. Special thanks go to Ronald Kirby, Lara Schmit, and Meredith Hartwell for their 

dedication and commitment to excellence, which improved the quality and readability of this 

document. 

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and Melis, T.S., 2002, Recent sediment studies refute Glen Canyon Dam hypothesis: Eos 

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Chapter 2. Physical Resource Response to the Low Steady 

Summer Flow Experiment

Introduction

Water temperature affects growth of young fish, and water velocity affects the 

environments they can occupy by affecting their swimming endurance. Water volume affects the 

warming potential of water and the velocity of water as it moves downstream. To benefit early 

life stages of native fish, particularly humpback chub (Gila cypha), the Low Steady Summer 

Flow (LSSF) experiment hydrograph incorporated different discharge volumes to affect 

velocities along shorelines, rework shorelines associated with sediment deposition, and to affect 

the warming potential of the water released from Glen Canyon Dam. The high, steady discharge

in the spring was intended to pond tributaries to improve tributary spawning and retain larvae by 

creating low-velocity areas at the confluence areas. These high volumes of water would mobilize 

and rework sediment along shorelines that young fish could occupy later in the summer after 

entering the mainstem (Valdez and others, unpub. report, 2000). The lower water volume (226 

m3/s) in the summer was intended to create warm, low-velocity shoreline environments for 

young-of-year (YOY) and juvenile native fish. Ideally, warmer and slower velocity water along 

shoreline environments would result in greater growth of young native fish occupying mainstem 

shorelines; although, increased abundances of fish could limit growth of fish because of densitydependent effects. 

Several researchers studied the effects of discharge on the temperature and velocity of 

water at the confluence with the Little Colorado River (LCR; fig. 2-1), in the mainstem, and 

along shorelines in 2000 (Davis, 2002; Korman and others, 2004; Protiva and others, 2010). 

Subsequently, Wright and others (2008) developed a model for mainstem warming potential that 

helps explain the mainstem warming patterns observed in 2000. The physical effects of the 

experimental hydrograph on sediment transport and shoreline reworking, specifically backwater 

rejuvenation, were studied in 2000 (Goeking and others, unpub. report, 2003; Schmidt and 

others, 2007), but primarily in association with the second habitat maintenance flow. Results 

from these studies are described in this chapter. 

Period I: Tributary Ponding 

Before the regulation of the Colorado River by Glen Canyon Dam, the average annual 

spring flood volume of the river in Grand Canyon was 2,407 m3/s: runoff began in March and 

peaked in May and June (Carothers and Brown, 1991; Topping and others, 2003). The timing of 

native fish spawning in tributaries is coincident with these historical flood peaks (Carothers and 

Brown, 1991; Valdez and Carothers, unpub. report, 1998; Gorman and Stone, 1999). Typically, 

during times of historical flooding, mainstem water would extend into tributary mouths and 

create extensive low-velocity pools that lasted for several months for larval and subsequent YOY 

fish (Robinson and others, 1998). Before regulation, mainstem water temperatures in April and 

May at Lees Ferry were likely equal to or greater than the average of 14.2°C recorded between 

1955 and 1965 for these months upstream at the Green River gaging station near Green River, 

Utah (not shown; sta. number 09315000) (U.S. Geological Survey, 2010). This gage is located 
 22

several hundred miles upstream from Lees Ferry and water temperatures may have increased as 

it traveled downstream. Although volumes would be high, the water in the mainstem would be 

approaching the optimal rearing and spawning temperatures of humpback chub, which are 

greater than or equal to 15°C and 16°C, respectively (Bulkley and others, 1981), as it ponded in 

tributaries downstream from Lees Ferry. Robinson and others (1998) and Clarkson and Childs 

(2000) speculated that the younger fish would grow in these warmer tributary pools to a size that 

would allow them to manage mainstem velocities when the fish entered the receding mainstem 

flow in July. 

Figure 2-1. Map of the Colorado River downstream from Glen Canyon Dam. River mile (RM) 

designations occur at 25-mile intervals starting from Lees Ferry, designated as RM 0. Large tributaries to 

the mainstem, including the Little Colorado River, are identified. 

Both the water temperatures and the maximum planned spring releases from Glen 

Canyon Dam in 2000 limited the potential benefit of tributary ponding. The high, steady 

discharges in Period I (April and May) of the LSSF hydrograph included discharges of 538 and 

877 m3

/s (fig. 1-4); these volumes were, respectively, 75 and 70 percent lower than predam 
 23

volumes. In 2000, the maximum tributary ponding occurred for 4 days at the end of May during 

the 877 m3/s habitat maintenance flow (HMF). The 4-day ponding duration is a 93-percent 

reduction compared to 2 months of ponding before regulation. Furthermore, in April and May of 

2000, water temperatures at the Lees Ferry gaging station (sta. number 09380000), 15 miles 

downstream from Glen Canyon Dam, varied between 9.1 and 10.3°C (fig. 2-2; U.S. Geological 

Survey, 2010) and were 4 to 10°C lower than those upstream at the Green River gage for the 

same period (fig. 2-3; U.S. Geological Survey, 2010). The hypolimnetic releases (releases from 

the midpoint in the reservoir’s elevation) from Glen Canyon Dam result in water temperatures 

that rarely exceed 12°C any time of the year except when the reservoir volume is reduced 

(Voichick and Wright, 2007). In 2000, the reservoir was full and water release temperatures in 

April and May were typical (Vernieu and others, 2005). High-volume discharges released in the 

spring intended to pond tributaries in the postdam Colorado River are unlikely to create a 

warmwater environment because colder water released from Glen Canyon Dam dominates the 

ponded areas. 

Figure 2-2. Mean daily temperatures (°C) recorded at the gage at Lees Ferry, Ariz. (USGS 09380000), 

from April 1 through May 31, 2000. 
 24

F

f

igure 2-3. Mean daily temperatures (°C) recorded at the gage at Green River, Utah (USGS 09315000), 

rom April 1 through May 31, 2000. 

Protiva and others (2010) studied the effect of increased mainstem discharge on water 

elocities and temperatures at the confluence of the LCR and the mainstem in the spring and 

ummer of 2000 (fig. 2-1). They determined that as mainstem discharge volumes increase, the 

vailability of low-velocity areas decrease at the confluence area (figs. 2-4 A–C). The greatest 

vailable low-velocity area occurred when discharges were between 226 and 509 m3/s (fig. 2-

B). The cobble island that is situated at the confluence affects the availability of low-velocity 

reas. Mainstem volumes that force LCR inflow into the channel on the south side of the island 

ncreased low velocity (<0.2 m/s) water when the tributary was at base flow. Similarly, Protiva 

nd others (2010) determined that warmwater areas (>15°C) are limited to areas directly affected 

y the inflow from the LCR because the mainstem temperatures (typically 12°C even in the 

ummer months) are lower than those of the LCR (≥16°C; Voichick and Wright, 2007). In the 

pring, the potential benefit of ponding that might occur in the LCR decreases with higher 

ischarges because the colder mainstem waters enter the ponded tributary mouth and reduce the 

rea of warm water (fig. 2-5 A–C). 

v

s

a

a

4

a

i

a

b

s

s

d

a
 25

Figure 2-4. Available low velocity area (≤0.2 m/s and ≤1 m deep) at the confluence of the mainstem 

Colorado River and the Little Colorado River (LCR). Low-velocity areas (pink colored polygons) are 

superimposed on a black and white image of the confluence area taken in September 2000 when the 

mainstem discharge was 226 m3/s. The light colored plume of sediment coming from the LCR in (A) shows 

the direction the LCR inflow follows under a low mainstem discharge. When the mainstem discharge 

increases to 368 m3/s (B), ponding at the north side of the island occurs, LCR water is concentrated into 

the southern channel of the cobble bar, and low-velocity area increases and includes areas in the 

mainstem. When mainstem discharge reaches 509 m3/s (C), low-velocity area is significantly reduced at 

the confluence. At discharges >509 m3/s (not shown), low-velocity area is limited to the LCR. Adapted from 

Protiva and others, 2010.
 26

Figure 2-5. Available warmwater area (≤1 m deep) at the confluence of the Colorado River and the Little 

Colorado River (LCR). The images use 1°C contours and graduated color, with darkest blue representing 

coldest temperatures (12°C) and red representing the warmest water temperatures (25°C) illustrate the 

warmwater areas associated with three mainstem discharges (227, 368 and 509 m3/s). At discharges of 

227 m3/s (A), warmwater areas are found in the LCR mouth and along the north side of the cobble island in 

association with the general direction of flow from the LCR. At a moderate discharge of 368 m3/s (B), 

available warmwater areas increase and shift toward the lower channel as water from the LCR is directed 

into the lower channel. At a mainstem discharge of 509 m3/s (C), warmwater is limited to the mouth of the 

LCR because colder mainstem water is routed around the confluence and through the lower channel. 

Adapted from Protiva and others, 2010.

Sediment Reworking and Habitat Rejuvenation Associated with a Habitat Maintenance Flow

Historically, high-volume spring floods associated with basin snowmelt transported large 

quantities of sediment through Grand Canyon and reworked existing beaches and shorelines 

(Andrews, 1991; Schmidt and Rubin, 1995; Topping and others, 2003; Schmidt and others, 

2004). The 4-day HMF of 877 m3/s that took place in the middle of Period I (May 3–6, 2000) 

was intended to rework sediment in eddies and along channel margins to create shoreline aquatic 

habitats, particularly backwaters (environments associated with recirculating eddies below debris 

fans) (Rubin and others, 1990; table 1-1). The HMF at 877 m3

/s is a discharge that can be 

released without bypassing the turbines. Management agencies were interested in knowing if 

sandbars and backwaters could be maintained by flows that did not bypass the powerplant. 

Hypothetically, YOY fish would occupy the reworked backwaters and other shoreline habitats 

during the summer. Sandbar building and sediment export are dependent on the balance between 

accumulated sediment volumes in eddies and the channel, and available sediment storage space 

at higher stage elevations (Topping and others, 2000b; Hazel and others, 2006). A sedimentstarved river subjected to a sustained high-discharge event can export more sediment than is 

stored in the river channel (Rubin and others, 2002; Wright and others, 2006). 

Results of sediment input monitoring before the sustained discharge of 481 m3/s in April 

2000 and the May HMF indicated that there were few sediment inputs from tributaries to the 

mainstem in the previous winter months and that accumulated sediment in the eddies was low 

(Schmidt and others, 2007). In the Colorado River downstream from Glen Canyon Dam, 
 27

sustained discharges greater than 282 m3/s increase the amount of sediment exported from the 

system—flows of 424 m3/s transport 2 to 10 times more sediment than flows of 282 m3/s 

(Topping and others, 2000a, 2006). Schmidt and others (2007) identified Period I, with 2 months 

of high discharges, as a sediment-exporting period. Despite the export of sediment, results from 

ground-based surveys of sandbars and eddies in June 2000 indicated that some sediment was 

deposited along higher elevation sandbars and that sediment volumes in submerged eddies 

decreased (Hazel and others, 2006; Schmidt and others, 2007). Because of the low amount of 

sediment inputs in the previous winter months, the area and volume changes observed were 

likely because of reworking of sediment from local sandbar sources rather than from sediment 

that accumulated in the eddies through the winter. 

Although the higher elevation sandbars (elevations equivalent to river stages associated 

with discharges >708 m3/s) do not constitute shoreline habitat, the reduced sediment volumes 

previously scoured from submerged eddy deposits might indicate increased backwater volumes. 

Changes in backwater areas were determined following the September HMF. Sediment inputs 

from tributaries also were low in the summer of 2000 (Schmidt and others, 2007). Results from 

the September analysis indicate that the areas of backwaters located in the vicinity of the LCR 

either increased or remained the same following the September HMF (fig. 2-6; Goeking and 

others, unpub. report, 2003; Schmidt and others, 2007). The changes in backwater area were 

associated with measured declines in sand volume in submerged eddies and increased area in 

mid-elevation sandbars (Goeking and others, unpub. report, 2003; Schmidt and others, 2007). 

Because sand volume deposited in submerged eddies decreased during the May HMF (Hazel and 

others, 2006; Schmidt and others, 2007), just as it did during the September HMF, it can be 

inferred that the spring HMF likely created or enlarged backwater areas before the June steady 

flow period, but the extent of change is unknown. 

 

Figure 2-6. Backwater area in August and September 2000 upstream and downstream from the Little 

Colorado River (LCR). A 4-day 877 m3/s habitat maintenance flow occurred between the two sampling 

events. Data from Goeking and others (unpub. report, 2003).
 28

Period II: Mainstem and Shoreline Warming during Low Steady Discharges 

from June to August 2000

A primary hypothesis associated with Period II of the LSSF experiment was that the low 

steady discharges of 226 m3

/s would provide low velocity, stabilized shoreline environments for 

YOY and juvenile fish from June through September. Lower volumes presumably also would 

promote warmwater (≥16°C) environments (Valdez and others, unpub report, 2000; table 1-1, 

fig. 1-4). Water at least as warm as 16°C was the target temperature for the mainstem because 

optimal spawning temperatures for humpback chub is at least 16°C (Bulkley and others, 1981; 

Valdez and Ryel, unpub. report, 1995; Valdez and Carothers, unpub. report, 1998). The optimum 

rearing temperatures for humpback chub range between 15 and 24°C (Bulkley and others, 1981; 

Valdez and Ryel, unpub. report, 1995; Valdez and Carothers, unpub. report, 1998). Water release 

temperature in the Colorado River from Glen Canyon Dam varies between 9 and 12°C (Vernieu 

and others, 2005; Voichick and Wright, 2007), which limits potential for either spawning or 

rearing of humpback chub in the mainstem. As a result of lower mainstem temperatures, growth 

of fish is reduced (Clarkson and Childs, 2000). Before 2000, release temperatures reached 15°C 

only in the early 1980s when the reservoir was still filling (Vernieu and others, 2005; Voichick 

and Wright, 2007). 

Valdez and others (unpub. report, 2000) anticipated that water in the mainstem would 

reach minimum spawning and rearing temperature requirements for humpback chub by the 

Middle Granite Gorge (RM 126, not shown). Release temperatures from Glen Canyon Dam in 

June through August 2000 were between 9.4 and 9.5°C (Voichick and Wright, 2007). Mainstem 

water temperatures immediately upstream from the LCR (RM 61.0, fig. 2-1) did not meet 

optimal rearing temperatures during any time of Period II. Temperatures further downstream did 

reach rearing temperatures from at least RM 132 (not shown), near the Middle Granite Gorge 

humpback chub aggregation, to RM 225 (fig. 2-7).
 29

Figure 2-7. Mean water temperatures from June through September 2000 from river mile (RM) 62 to 

225. The location of the Middle Granite Gorge humpback chub aggregation (RM 132) is identified by the 

arrow (data adapted from Voichick and Wright, 2007).

Reservoir conditions, time of year, rate of discharge, and distance downstream from the 

dam affect mainstem water temperature (Anderson and Wright, 2007; Wright and others, 2008). 

Reservoir elevation affects release temperatures from Glen Canyon Dam by affecting the 

withdrawal depth of water entering the penstocks (Vernieu and others, 2005; Wright and others, 

2008). The rate at which water is discharged from the dam affects residence time in the river 

channel. Water discharged at lower rates has a longer residence time in the river channel and, if 

air temperatures are high, warming can occur (Korman and Campana, 2009; Wright and others, 

2008). The greatest mainstem warming occurs in July, which is coincident with the greatest 

value recorded for average monthly air temperature (for example, 28.4°C average monthly 

temperature at Page, Ariz. (Wright and others, 2008)). July also is when humpback chub 

frequently enter the mainstem from tributaries in association with tributary freshets from 

monsoon storms (Valdez and Ryel, unpub. report, 1995). 

A general pattern of mainstem warming in Grand Canyon is that initial release 

temperatures from Glen Canyon Dam affect water temperature in the upper reaches of the 

Colorado River (for example, at RM 61 near the LCR). Water temperature in lower reaches of 

the river is affected by three physical properties: discharge rate, which affects residence time 

(Anderson and Wright, 2007; Wright and others, 2008); channel aspect, which affects light 

availability; and air temperature, which is generally greater in the western portion of Grand 

Canyon (Yard and others, 2005). Mainstem water temperatures near the LCR cannot reach 16°C 

in July and August unless release temperatures approach 14°C (Wright and others, 2008). In 

contrast, warmer mainstem temperatures are attainable in the western part of the Colorado River 
 30

in July when releases from Glen Canyon Dam are 12°C because of the longer residence time of 

water in the river channel. 

The rate of river warming from the dam downstream from the LCR was similar in 2000 

to that observed in previous years under Modified Low Fluctuating Flow (MLFF) operations. 

Downstream of the LCR, warming of water is dependent on the discharge rate and air 

temperature, which affects local warming (Korman and others, unpub. report, 2006; Anderson 

and Wright, 2007; Wright and others, 2008). Release temperatures in 2000 were 1°C warmer 

than in previous years, which accounted for the 13°C water temperatures observed in the 

mainstem near the LCR during the experiment (Voichick and Wright, 2007). For mainstem water 

temperatures to reach optimal rearing temperatures for humpback chub at the confluence with 

the LCR, release temperatures needed to be greater than 13°C. Mainstem warming in 2000 was 

greatest in the western reaches of the mainstem (fig. 2-7) because of lower discharges rates, 

compared with previous years, and longer residence time in the channel by the time the water 

traveled 225 miles to Diamond Creek (fig. 2-1) in 2000 compared with the years 1993–99. 

Similar patterns of mainstem water warming observed under steady discharges during the 

summer of 2000 occurred in 2005 during higher discharges and MLFF operations (Voichick and 

Wright, 2007). In 2005, release temperatures during the summer months averaged 13.2°C, 

approximately 4°C warmer than in 2000, and monthly volumes were 141 m3/s greater in June 

through August compared to 2000. Water temperatures at RM 61 were between 15 and 16°C in 

July and August, approximately 1°C warmer than temperatures at Lees Ferry. The pattern 

observed in 2005 illustrates how release temperatures affect downstream warming patterns. Even 

during higher discharges and higher daily ranges of discharge, downstream warming that meets 

optimal rearing temperature requirements of humpback chub is achievable in the mainstem near 

the LCR, if initial release temperatures are greater than 13°C. The mainstem water temperatures 

during 2000 may have benefitted fish in the western parts of Grand Canyon where mainstem 

temperatures reached optimal rearing temperature requirements for humpback chub. 

Period II: Shoreline Warming

The variables of water volume, residence time, and ambient temperatures that affect 

mainstem warming also affect warming of water along the shoreline (Korman and others, unpub. 

report, 2006). Time of year also affects how water temperatures might differ between shoreline 

environments and the mainstem. Geomorphic features associated with low-velocity shorelines 

(for example, talus and backwaters) affect residence time of water by affecting the degree to 

which water is isolated from and mixes with mainstem water. Korman and others (unpub. report, 

2006) collected temperature data from August through October 2004 in low- and high-angle 

shoreline habitats and in backwaters in the mainstem during fluctuating discharges. The greatest 

warming was associated with backwaters, followed by low-angle shorelines. Water temperatures 

at steep-angled talus environments were similar to the mainstem (Korman and others, unpub. 

report, 2006). Individual backwater habitat may warm substantially more than the mainstem 

during parts of the day (Korman and others, unpub. report, 2006) depending on the orientation of 

the backwater to the sun and time of year. Backwater and mainstem temperature data from 

September 2005 (Ralston and others, 2007) indicated no difference in temperature between these 

environments during steady (226 m3/s) and low discharge fluctuating releases, which likely 

reflect the time of year and the effect of local air temperature on warming. 

The depth of warming in the water column reported by Korman and others (unpub. 

report, 2006) indicated that water temperatures measured below daily base-flow stage elevations 
 31

along shorelines varied little from the mainstem. Shoreline water temperature associated with 

vertical stratification was not measured during the LSSF experiment. Korman and others (unpub. 

report, 2006) also indicated that shoreline temperatures reverted to mainstem temperatures at

night or during indirect sunlight conditions. These results corroborate the findings of Ralston and 

others (2007), who reported little difference between mainstem and backwater temperatures 

measured in September and October 2005, when sun angle is low and air temperatures are 

correspondingly lower. Data from 2000 indicated similar warming in a backwater at RM 141

(William Vernieu, U.S. Geological Survey, unpub. data, 2011). At that backwater, the nighttime 

temperatures differed by 3.5°C between the backwater and the mainstem, while the daytime 

temperatures differed by 13.0°C. Ralston and others (2007) also indicate that areas closest to the 

shoreline, which are therefore shallower environments, warmed more than areas farther from the 

shoreline. In 2000, as in other years during MLFF operations, water in backwaters warmed 

during the day and was warmest during the warmest period of the summer. 

Period II: Surface Warming Patterns during Steady Flows

A 1-m2 resolution thermal infrared sensor recorded maximum water-surface temperatures 

along a 42-mile stretch of the river channel (RM 30–72) at 1:30 p.m. on July 21, 2000 (Davis, 

2002). Water-surface thermal data are presented here for a total 44.6 ha of areas that extend 2 m 

from shorelines to quantify the area and pattern of shoreline warming (fig. 2-8). The areas that 

were measured included areas around mid-channel islands, cobble bars, debris fans, eddy bars, 

and associated backwaters. 

Water-surface temperatures along the shorelines varied from 9 to 28°C (Davis, 2002). 

More than 50 percent of the shoreline temperatures were similar to the mainstem temperatures 

during steady flows even in July at maximum light availability (fig. 2-8). Mainstem watersurface temperatures near the LCR averaged 13.5°C. Water-surface temperatures between 13 and 

14°C accounted for the largest proportion of all shoreline areas. Water-surface temperature for 

some areas reached optimal rearing temperatures for humpback chub. 

Fragmented water-surface warming patterns occurred along shorelines in 2000. As 

identified by Korman and others (unpub. report, 2006), shoreline warming is not uniform 

because water in eddies warms more than water along talus slopes. In general, in 2000, 

backwaters had the largest contiguous areas of water with surface temperatures greater than 

16°C. The area near the confluence with the LCR also was a large area of warm surface 

temperatures that equaled or exceeded 16°C. This area could be a transition zone for young 

humpback chub during lower dam releases and could reduce temperature shock to small-sized 

fish. These data complement the subsurface temperature study of Protiva and others (2010). 

Shoreline surface temperatures warmed in a downstream pattern in July 2000 (Davis, 2002), 

similar to what was observed for the mainstem (fig. 2-7). 
 32

Figure 2-8. Area distribution greater than or equal to 0.5 hectares of the water-surface temperatures 

within 2 m from the shoreline, river mile 30–72, 1:30 p.m., July 21, 2000. 

Period II: Low Steady Summer Discharges and Shoreline Habitat for Fish

Persistent suitable habitat is a key variable for early life-stage survival and subsequent 

recruitment of fish into the adult population (Freeman and others, 2001; Korman and others, 

2004; Korman and Campana, 2009). Previous surveys of backwater area for discharges that 

range from 141 to 849 m3/s (McGuinn-Robbins, unpub. report, 1995) and habitat suitability 

studies for humpback chub (Converse and others, 1998) identified that a steady discharge of 226 

m3/s increased low-velocity shoreline habitats compared with higher discharges. Korman and 

others (2004) modeled shoreline habitats for a section of river downstream from the LCR for 

discharges between 141 and 788 m3/s. The habitat study by Converse and others (1998) 

identified the amount and types of shoreline habitats available at the proposed 226 m3/s 

discharge and at other discharges. Korman and others (2004) also evaluated how persistent 

habitat varied with monthly discharge since regulation. 

Available Habitat and Habitat Stability

Korman and others (2004) used velocity (≤0.25 m/s), water depth (<1 m) and shoreline 

types (for example, talus, vegetation, cobbles) to quantify suitable shoreline environments within 

seven geomorphic reaches downstream from the LCR for discharges ranging from 82 to 2,830 

m3/s. The physical constraints they selected for depth and velocity were based on studies of 

swimming performance and shoreline locations of young humpback chub (Bulkley and others, 

1981; Valdez and Ryel, unpub. report, 1995; Converse and others, 1998). Korman and others 

(2004) determined that discharge rates and local morphology affect local velocities and the 
 33

amount and availability of shoreline habitat. Reaches with large debris fans, steep profiles, and 

associated large downstream eddies changed little with increasing discharge except when 

discharges overtopped debris fans (for example, discharges of 2,830 m3/s). Generally, total 

suitable habitat declined with increasing discharge (Korman and others, 2004). 

When discharges were either greater than 424 m3/s or less than 226 m3/s, suitable 

shoreline habitat declined by 50 percent (Korman and others, 2004). Similarly, Protiva and 

others (2010) found discharges greater than 424 m3/s reduced low-velocity habitats at the 

confluence of the LCR (fig. 2-4). Because Korman and others (2004) encountered variability 

between suitable habitat and discharge rates within their short study reach (3.6 km), they 

suggested that general trends about stage elevation and habitat availability could not be 

determined for the whole river system. Further analysis could be done as shoreline mapping is 

completed and sandbar- and reach-based velocity models are developed to explore the variability 

across geomorphic reaches for a range of discharges. 

Comparisons of predam and postdam average monthly discharges were made to 

determine how regulation changed seasonal shoreline habitat availability (Korman and others, 

2004). The authors note that predam seasonal discharges were higher in the May and June 

periods (averaging 1,500 m3/s) than during postdam regulation (averaging 400 m3/s). These high 

predam discharges, which also were warmer, exceed those imposed during Period I of the LSSF 

experiment by 275 percent and would have resulted in extensive ponding at tributaries. Korman 

and others (2004) noted that the annual base discharge increased from 226 to 300 m3/s (based on 

monthly averages), or 32 percent, for the period of record since regulation began in 1963. 

Korman and others (2004) noted that regulation substantially reduced suitable shoreline habitat 

for the months of August through February and increased suitable shoreline habitat for 3 months 

of the year (April–June). YOY native fish, particularly humpback chub, are few in number in the 

mainstem from April through June, but their abundances increase in the later summer months

(Valdez and Ryel, unpub. report, 1995) even though suitable habitat is reduced during MLFF 

operations after June. 

Korman and others (2004) found that the LSSF experiment provided consistent suitable 

habitat that was absent during other regulated discharge patterns. They concluded that the LSSF 

experiment that included 226 m3/s discharges in June and July did not mimic the predam pattern 

of discharge or minimum flows for these same months. The timing of the September HMF 

(September 1–4) served as an interruption in persistent habitat at a time when YOY native fish 

likely are occupying mainstem shoreline habitats, mainstem discharges were historically low, 

and mainstem water was still warm. The results from Korman and others (2004) report suggest 

YOY fish might benefit more from a low steady flow period that started later in the summer 

season, such as August. The delayed timing might benefit YOY fishes entering the mainstem 

from tributaries during monsoon flooding. The resulting reduction in base flow in August 

compared to MLFF could provide maximum shoreline habitats coupled with warmer water 

released from Lake Powell (fig. 2-1; Vernieu and others, 2005) and greater ambient air 

temperatures (Wright and others, 2008). Releasing lower discharges later in the summer season 

also would help conserve sediment inputs associated with late summer and fall monsoonal 

storms. Lower discharges reduce sediment transport (Topping and others, 2000a; Schmidt and 

others, 2007). The sediment could be reworked and deposited in higher elevation sandbars 

during higher discharges (Rubin and others, 2002; Wright and others, 2006; Wright and 

Kennedy, 2011). 

Period II of the LSSF experiment began following a HMF intended to rework eddies and 

rejuvenate backwaters. As mentioned in the description of Period I, the HMF in May mobilized 
 34

sediment resulting in some accumulation of sand at mid-elevation sandbars and a reduction in 

sand volume in eddies, suggesting that backwaters were reworked (Hazel and others, 2006; 

Schmidt and others, 2007). The September HMF also resulted in deposition of sediment in the 

mid-elevation sandbars (Schmidt and others, 2007) and increased backwater area in September 

(Goeking and others, unpub. report, 2003; fig. 2.6). The response was not uniform among all 

eddies or reaches (Goeking and others, unpub. report, 2003), which corresponds to results of 

modeling done by Korman and others (2004). Goeking and others (unpub. report, 2003) 

commented that the total backwater area in 2000 was comparable to that measured in 1984 

following the largest postdam releases (2,747 m3/s) in 1983. 

Conclusions

The high discharges associated with Period I (481 m3/s) were insufficient to create 

extensive ponding at the LCR, and the dynamics between mainstem temperatures and tributary 

mouths created a cold, low-velocity water interface rather than a warmwater interface that could 

be beneficial for larval and YOY fish (Protiva and others, 2010). The low steady flows of Period 

II were associated with average mainstem temperatures for comparable discharges during 

fluctuating flows (Voichick and Wright, 2007)—mainstem temperatures in the eastern Grand 

Canyon were similar to previous years during MLFF operations. Nearshore warming exceeded 

mainstem temperatures in some places by as much as 13°C (Davis, 2002), but less than 50 

percent of the shoreline water-surface area that was measured by thermal infrared imagery was 

warmer than 14°C (fig. 2.8). Warming was not continuous along shorelines, resulting in 

fragmented water-surface warming (Davis, 2002). Korman and others (unpub. report, 2006) 

concluded that backwaters that have a reduced connection with the mainstem and slower 

velocities reached the highest water temperatures measured in 2004 during MLFF operations, 

suggesting that similar warming occurred in 2000. Shoreline warming occurred in 2004 and 2005 

during MLFF operations that were coincident with warmer release temperatures (Johnstone and 

Lauretta, unpub. report, 2006). The warmer mainstem temperatures observed in 2004 and 2005 

may have resulted in more uniform temperatures between the mainstem and shorelines. The daily 

temperature flux between the shoreline and mainstem temperatures within the vertical water 

column may have been less in 2004 and 2005 than in 2000. The basin hydrology that affects 

Lake Powell Reservoir elevations and subsequent release temperatures may be more important 

for providing warm mainstem water than the daily discharge pattern. The linkage between warm 

mainstem water and persistent habitat needs for fish remain unclear and further study would be 

needed to better understand the linkages. 

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K.W., and Wilcock, P.R., eds., Natural and anthropogenic influences in fluvial 

geomorphology, v. 89: Washington, D.C., American Geophysical Union, Geophysical 

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distribution of fine sediment in the Colorado River corridor between Glen Canyon Dam and 

Bright Angel Creek, Arizona—final report: Logan, Utah, submitted to U.S. Geological Survey, 

Grand Canyon Monitoring and Research Center, cooperative agreement no. 1425–98–FC–40–

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 Schmidt, J.C., Topping, D.J., Rubin, D.M., Hazel, J.E., Jr., Kaplinski, M., Wiele, S.M., and 

Goeking, S.A., 2007, Streamflow and sediment data collected to determine the effects of low 

summer steady flows and habitual maintenance flows in 2000 on the Colorado River between 
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1268, 79 p., accessed on March 17, 2010, at http://pubs.usgs.gov/of/2007/1268/.

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Resources Research, v. 36, no. 2, p. 543–570. 

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M., Draut, A.E., and Breedlove, M.J., 2006, Comparison of sediment-transport and barresponse results from the 1996 and 2004 controlled-flood experiments on the Colorado River 

in Grand Canyon, in Federal Interagency Sedimentation Conference, 8th, Reno, Nev., April 2–

6, 2006, Proceedings: p. 8 (CD-ROM).

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instantaneous-discharge record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, 

through September 30, 2000: U.S. Geological Survey Professional Paper 1677, 118 p., 

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panel—Protocols Evaluation Program (PEP - SEDS): Flagstaff, Ariz., U.S. Geological Survey, 

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Canyon—Grand Canyon data integration synthesis final report: Flagstaff, Ariz., SWCA, Inc. 

Environmental Consultants, submitted to Bureau of Reclamation, 250 p. [Available upon 

request by contacting the Center Director, U.S. Geological Survey, Southwest Biological 

Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

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Wegner, D.L., 2000, A program of experimental flows for endangered and native fishes of the 

Colorado River in Grand Canyon—final report: Flagstaff, Ariz., SWCA, Inc. Environmental 

Consultants, submitted to U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center, 68 p. [Available upon request by contacting the Center Director, U.S. Geological 

Survey, Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

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in the Colorado River, Grand Canyon, Arizona—final report: Logan, Utah, BIO/WEST, Inc., 

submitted to Bureau of Reclamation, contract no. 0–CS–40–09110, technical report no. TR–

250–08, 329 p. [Available upon request by contacting the Center Director, U.S. Geological 

Survey, Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

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Colorado River, in Gloss, S.P., Lovich, J.E., and Melis, T.S., eds., The state of the Colorado 

River ecosystem in Grand Canyon: U.S. Geological Survey Circular 1282, 69–85 p., accessed 

on May 10, 2010, at http://pubs.usgs.gov/circ/1282/.

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tributaries between Glen Canyon Dam and Spencer Canyon, Northern Arizona, 1988–2005: 

U.S. Geological Survey Data Survey Series 251, 24 p., accessed on January 14, 2010, at 

http://pubs.usgs.gov/ds/2007/251/.
 38

Wright, S.A., Anderson, C.R., and Voichick, N., 2008, A simplified water temperature model for 

the Colorado River below Glen Canyon Dam: River Research and Applications, v. 25, no. 6, p. 

675–686, accessed on August 19, 2011, at http://dx.doi.org/10.1002/rra.1179.

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experiments at Glen Canyon Dam, in Melis, T.S., ed., Effects of three high-flow experiments 

on the Colorado River ecosystem downstream from Glen Canyon Dam, Arizona: U.S. 

Geological Survey Circular 1366, p. 127–147. 

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sand? Evaluating the fate of Grand Canyon sandbars: Geological Society of America Today, v. 

18, no. 8, p. 4-10, accessed on May 26, 2010, at 

http://www.geosociety.org/gsatoday/archive/18/8/pdf/i1052-5173-18-8-4.pdf. 

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Blinn, D.W., 2005, Influence of topographic complexity on solar insolation estimates for the 

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accessed on July 19, 2011, at 

http://www.sciencedirect.com/science/article/pii/S0304380004004375. 
 39

Chapter 3. Native and Nonnative Fish Response to the 

Steady Summer Flow Experiment in the Mainstem 

Downstream from Lees Ferry

Introduction

The purpose of the Low Steady Summer Flow (LSSF) experiment was to determine if 

reduced discharge and steady flows would improve mainstem spawning success and increase 

growth of young native fish through improved nursery habitats and warmer water. The previous 

section described how the discharge rates and initial release temperatures affect the warming 

potential and water velocities in the mainstem and along shorelines. These variables translate to 

suitable and sustained aquatic environments that young fish can occupy. The young fish use 

these environments in the summer months after migrating from the tributaries, either as drifting 

larvae, or as young-of-year (YOY) fish transported with tributary freshets. Korman and others 

(2004) and Protiva and others (2010) quantified the effects of discharge on shoreline velocities 

and the extent of available and sustained low-velocity areas that young native fish can occupy at 

the confluence with the Little Colorado River and downstream from the confluence along the 

shorelines of the mainstem (fig. 3-1). If mainstem and shoreline water temperatures reached 

optimum rearing temperatures of 15°C or greater, an anticipated response of fish was a shift in 

fish-length frequency toward larger sized YOY fish compared with previous years. An increase 

in the abundance of fish also could be a response to warmer water. 

Critical to understanding the fish response is data collected before, during, and following 

the hydrologic treatment as identified by Valdez and others (unpub. report, 2000). During Period 

I, few biological data related to larval drift and ponding were collected. During Period II, three 

mainstem trips that included seining, trammel netting, and electroshocking were conducted in 

June, August, and September. Trammel and others (unpub. report, 2002) and Speas and others 

(unpub. report, 2004) documented changes in mean total fish length and the relative abundance 

of species during Period II. Data collected before implementation of the experimental 

hydrograph were not focused on long-term monitoring; instead, data were collected in 

association with life-history studies of humpback chub (Valdez and Ryel, unpub. report, 1995) or 

opportunistic sampling of backwater habitats (Arizona Game and Fish, unpub. report, 1996). 

Lacking long-term historical data of mainstem trends for native and nonnative fish hampers 

interpretation of the effects on fish of the 1-year hydrologic treatment. The results reported for 

2000 are discussed in the context of the historical data, hydrology, and sampling effort to assist 

in interpreting the growth and abundance results observed in 2000. Hydrology that benefits 

native fish also was likely to benefit nonnative fish. Efforts to ameliorate the potential positive 

response by nonnative fish to the experiment included the 4-day habitat maintenance flow 

(HMF) at the beginning of September (fig. 1-4), which was intended to displace small-bodied 

nonnative fish (for example, fathead minnows (Pimephales promelas) and red shiners). A 

discussion of the utility of the ponding flows in the spring and the utility and uncertainties that 

the September HMF added to the interpretation of the young fish response is included in this 

section. 
 40

Figure 3-1. Map of the Colorado River downstream from Glen Canyon Dam. River mile (RM) 

designations occur at 25-mile intervals starting from Lees Ferry, designated as RM 0. Large tributaries to 

the mainstem, including the Little Colorado River, are identified. 

Period I: Tributary Ponding 

Tributary ponding was predicted to improve larval survival of all native fish, but tributary 

ponding may benefit native sucker species more than humpback chub (Gila cypha) (Robinson 

and others, 1998). Robinson and others (1998) reported large numbers of drifting bluehead 

sucker (Catostomus discobolus) and flannelmouth sucker (Catostomus latipinnis) larvae, but few

numbers of humpback chub larvae drifting into the mainstem from the Little Colorado River 

(LCR) during the months of May through July. Instead, they report that humpback chub largely 

remain close to spawning sites. YOY humpback chub appear to enter the mainstem in 

association with monsoon storms that occur in July through September (Valdez and Ryel, unpub. 

report, 1995). The small numbers of drifting humpback chub larvae observed by Robinson and 

others (1998) and their observations that chub remain close to spawning sites suggest that 
 41

ponding using high flows in the spring may have had little measurable benefit for larval 

humpback chub survivorship. The negligible benefit of ponding is further called into question 

when the mainstem and LCR flow dynamics under high mainstem volumes reduce low-velocity, 

warmwater areas (Protiva and others, 2010; figs. 2-4, 2-5). In contrast, larvae that drifted toward 

the confluence area of the mainstem in June through August might have benefitted from the 

lower mainstem discharge of 226 m3/s. At this lower discharge, the area of warmer and lowervelocity water at the confluence increases (Protiva and others, 2010; figs. 8 and 9) when LCR 

flows are at base-flow levels. 

Period II: Fish Growth and Abundance during Steady Minimal Discharges

Trammell and others (unpub. report, 2002) compared the 2000 YOY fish abundance and 

mean total lengths of fish caught in seines with previous years (1991–97) to see if these variables 

differed during steady flow and Modified Low Fluctuating Flow (MLFF) operations. Sampling 

with seines, primarily used in backwaters, during the LSSF experiment occurred June 7–23, 

shortly after the spring HMF; August 6–22, about halfway through Period II; and September 14–

29, a week following the fall HMF (Trammell and others, unpub. report, 2002). The data 

collected by Trammel and others (unpub. report, 2002) are presented here with geographic 

separation of sample sites upstream and downstream from river mile (RM) 150 (figs. 3-2 A, B; 3-

3 A, B). Mainstem water temperatures were at or above the optimal rearing temperature for 

humpback chub (15°C) by May 2000 at sites downstream from RM 150 (Voichick and Wright, 

2007; fig. 2.8) and may have affected growth or abundance patterns of YOY fish in 2000. 

Trammell and others (unpub. report, 2002) noted that relative abundance of native fish 

was greater than nonnative fish caught in seines in the eastern and western parts of Grand 

Canyon, except upstream from RM 150 in September (figs. 3-2 A,B). Among native fish species 

sampled in June 2000, flannelmouth suckers and speckled dace (Rhinichthys osculus) were the 

most frequently encountered native fish upstream and downstream from RM 150, respectively 

(figs. 3-3 A, B). The relative abundance of humpback chub was minimal in backwaters upstream 

and downstream from RM 150 throughout the sampling period (figs. 3-3 A, B). Grams and others 

(2010) also documented minimal abundances of humpback chub in a more recent backwater 

study conducted in 2008 and 2009. The abundances Grams and others (2010) indicated for 

humpback chub in 2008 and 2009 were small even in light of increased numbers for this species 

since 2000 (Coggins and Walters, 2009), possibly indicating that humpback chub occupy other 

shoreline environments besides backwaters, as identified by Converse and others (1998). 
 42

Figure 3-2. Bar graphs showing relative abundance of native and native fish species with total lengths

less than 110 mm collected (A) upstream and (B) downstream from river mile 150, June 7–23, August 6–

22, and September 14–29, 2000 (data from Trammell and others, unpub. report, 2002).
 43

Figure 3-3. Bar graphs showing relative abundance of the six most abundant fish species with total 

lengths less than 110 mm collected (A) upstream and (B) downstream from river mile 150, June 7–23, 

August 6–22, and September 14–29, 2000 (data from Trammell and others, unpub. report, 2002).

Abbreviations along horizontal axis are BHS (bluehead sucker), FHM (fathead minnow), FMS (flannelmouth 

sucker), HBC (humpback chub), RSH (red shiner), and SPD (speckled dace).

Mean total length of the six most abundant fish species in 2000 did not differ from 

previous years (Trammell and others, unpub. report, 2002). Fish-length data from 2000 are 

difficult to interpret because the sampling included seining data after the September HMF, and 

mainstem water temperatures met optimal rearing requirements of native fish in the western 

Grand Canyon, but not in the eastern Grand Canyon (the mainstem near the LCR). Trammell and 

others (unpub. report, 2002) reported larger humpback chub caught near the LCR in 2000 

compared to humpback chub located farther downstream (figs. 3-4 A, B). The authors attributed 

the longer lengths of fish caught downstream from the LCR (RM 61–75; fig. 3-4A) to fish 

entering the mainstem from the LCR tributary, where conditions for growth were better. 

Comparisons of mean total length in 2000 with data from 1991–97 for August and September 

indicated that mean lengths for 2000 were less than in previous years (Trammell and others, 

unpub. report, 2002). The smaller fishes observed in 2000 were attributed to protracted spawning 

in the mainstem and the continuous influx of larvae and small fish into backwaters (Trammell 

and others, unpub. report, 2002), particularly in the western Grand Canyon (fig. 3-4B). The 

abundances of fish in backwaters was greater in 2000 compared with previous years. The HMF
 44

likely displaced some fish occupying backwaters and by September fish longer than 100 mm 

may have moved from backwaters. Trammell and others (unpub. report, 2002) concluded that 

steady flows did not appear to result in increased fish growth. This conclusion should be viewed 

cautiously, however, because it is based on only 1 year of data that included the effects of a 

HMF. In addition, the data were collected with a single gear type that does not sample all the 

shoreline environments that YOY may occupy. 

Figure 3-4. Bar graphs showing mean fish length (total length) of the six most common fish species with 

lengths less than 110 mm collected (A) upstream and (B) downstream from river mile 150, June 7–23, 

August 6–22, and September 14–29, 2000. Error bars represent one standard error (data from Trammell 

and others, unpub. report, 2002). Abbreviations along horizontal axis are BHS (bluehead sucker), FHM 

(fathead minnow), FMS (flannelmouth sucker), HBC (humpback chub), RSH (red shiner), and SPD 

(speckled dace).

Nonnative Response to Steady Flows and Warmer Water

Resource competition and native fish predation by nonnative fish are threats to the native 

fish recruitment in the Colorado River in Grand Canyon (Minckley, 1991; Gloss and Coggins, 

2005; Yard and others, 2011). The relative abundance of small-sized nonnative fish remained 

smaller than those of native fish throughout the summer (figs. 3-2 A, B). The greater percentage 

of native small-sized fish was associated with greater abundance of speckled dace earlier in the 
 45

summer followed by increased abundances of flannelmouth and bluehead suckers in August and 

September (figs. 3-3 A, B). Trammell and others (unpub. report, 2002) compared relative 

abundance of nonnative fish captured in seines in 2000 with previous years (1990–97) and 

determined there was no difference among years. Subsequent years, 2002–6, also did not indicate 

substantial differences in YOY abundances for all fish combined (Ackerman, unpub. report, 

2008). 

Three nonnative fish—fathead minnows (Pimephales promelas), plains killifish

(Fundulus zebrinus), and rainbow trout (Oncorhynchus mykiss)—appeared to benefit from either 

the reduced fluctuations, warmer water temperatures, or both. McKinney and others (2001) and 

Korman and others (2011) documented the benefit of reduced flows associated with MLFF

operations on rainbow trout spawning and recruitment in the Lees Ferry fishery (fig. 3-1). Speas 

and others (unpub. report, 2004) reported a large increase in rainbow trout with lengths less than 

120 mm in Lees Ferry in summer 2000, but did not see a corresponding large recruitment into 

the next size class in 2001. Fathead minnows caught in the western Grand Canyon (RM 160–

219) were most abundant in 2000 compared to subsequent years (Johnstone and Lauretta, unpub. 

report, 2004), suggesting that steady flows may benefit this species. Plains killifish catch rates 

increased beginning in 2002 for nearly every reach (Johnstone and Lauretta, unpub. report, 

2004). This species did not constitute a large percentage of the fish captured in 2000. 

A part of the LSSF experiment specifically included developing abundance estimates for 

rainbow and brown trout (Salmo trutta) in the Colorado River mainstem and developing 

monitoring approaches for nonnative fish in the mainstem (Speas and others, unpub. report, 

2004). Data from the Arizona Game and Fish Department (Makinster and others, 2010) indicate 

that abundances of adult rainbow and brown trout during 2000–9 were either at or approached a 

maximum in 2000. No abundance estimates are available for these species before 2000. Because 

of predation by rainbow and brown trout, the large abundance of these fish in the mainstem 

likely affected the abundance of native and small-bodied nonnative fish in 2000, particularly near 

the confluence with the LCR (Yard and others, 2011). 

Habitat Maintenance Flows for the Purpose of Disadvantaging Nonnative 

Fish

The purpose of the second HMF, conducted in September 2000, was to disadvantage 

nonnative fish (Valdez and others, unpub. report, 2000; table 1). Minckley and Meffe (1987) 

documented that flood events affected nonnative fish more negatively than native fish in a small 

stream in the Southwest. More recently, Korman (2009) indicated that the 2004 fall high-flow 

experiment (HFE) displaced young rainbow trout in Glen Canyon, though the fate of these fish is 

unknown. A comparison of the relative abundance of native and nonnative fish in August and 

September 2000 indicated that the relative abundance of nonnative fish was larger in September 

upstream of RM 150 than in August (fig. 3-2; Trammell and others, unpub. report, 2002). This 

suggests that the September HMF did not have the expected effect on small-sized nonnative fish. 

The reasons for the response difference between the small streams studied by Minckley and 

Meffe (1987) and the larger Colorado River are not known. One explanation may be that the 

river scales may not be directly comparable and the expected effects may be unquantifiable in a 

large river. Nonnative fish abundance increased upstream from RM 150 following the HMF (fig. 

3-3A) and Trammell and others (unpub. report, 2002) noted an increase between RM 60 and 80. 

Increased abundances of nonnative fish moving into the mainstem during freshets in the LCR, 

Havasu Creek, and Kanab Creek tributaries (fig. 3-1) in September may explain the observed 
 46

increases in nonnative species. Speas and others (unpub. report, 2004) determined that the HMF 

did not affect the fish-length frequencies of downstream YOY rainbow and brown trout. Because 

of the uncertainty of how the HMF affected native fish and because small-sized nonnative fish 

abundance was less than native fish before the HMF (fig. 3-2), research in conjunction with a fall 

HMF might improve knowledge about native and nonnative fish responses to these below 

powerplant-capacity pulses. 

Factors Affecting Interpretations of Fish Abundance and Growth

Assessing changes in length frequency of small fish based on seining is problematic; 

seining samples backwaters effectively but does not sample other shorelines and captures fish 

that average 51 mm, rather than a range of lengths. Converse and others (1998) found that 

humpback chub less than 200 mm long use shorelines other than backwaters (for example,

vegetated shorelines or talus). Slow electroshocking (for example, shocking along shorelines at a 

slow rate) from a boat may be a sampling method for small fish along shorelines other than 

backwaters. The LSSF data for small-sized fish captured using seining have a limited ability to 

determine the fate of fish that grow beyond 110 mm length or occupy areas other than 

backwaters.

Drawing conclusions about fish growth in 2000 compared to previous years also is 

problematic. Fish abundances, particularly native fish abundances, were small in the 1990s 

(Douglas and Marsh, 1996), and the monthly discharges in the summer, when fish were sampled, 

were variable and generally greater than in 2000. Although Trammell and others (unpub. report, 

2002) correctly point out that daily fluctuations in the 1990s were similar, they failed to note that 

monthly discharges between years varied. The monthly discharges in July and August 1991–93 

were relatively low (<368 m3/s), whereas the discharges were higher (>424 m3/s) for these same 

months in 1994–96. The discharge rate can affect the catchability of small fish (Korman and 

others, 2009) and availability of shoreline habitats (Converse and others, 1998; Korman and 

others, 2004; Korman and others, 2009), thereby affecting abundance and size distribution of fish 

caught. The higher monthly discharges in the late 1990s may have favored larger fish that could 

withstand higher-velocity shorelines. The greater mean lengths reported by Trammell and others 

(unpub. report, 2002) for fish caught in the 1990s may be an indication of these sampling biases 

and merit further exploration. Lastly, the seining locations varied among the years that were 

compared. In some years, seining efforts focused near the LCR, whereas in other years more 

extensive sampling occurred throughout the river corridor (U.S. Geological Survey, unpub. data, 

2009). Trammell and others (unpub. report, 2002) reported larger humpback chub near the LCR 

in 2000 compared to locations farther downstream. The effect of the larger YOY fish entering 

the mainstem from the LCR is another example of how previous sampling efforts can confound 

interpretations of a single-season experiment. In years when sampling focused near the LCR, 

mean total length of native fish may be greater than in years when sampling occurred throughout 

the river corridor. The variability in sampling locations in the 1990s lends further uncertainty to 

the meaning of fish lengths measured under steady-flow conditions in 2000 relative to those 

measured during fluctuating-flow conditions in other years. These findings underscore the need 

for consistent methods and timing of monitoring. 
 47

Conclusions

Variables associated with gear types used to catch fish, fish life history, rarity of fish, and 

the experimental hydrograph contributed to uncertainties associated with fish response to the 

experimental hydrograph of Period II. Ontogenetic shifts (for example, changes in diet and 

habitat use) associated with growth can cause fish to move out of one habitat and into another 

(Robinson and others, 1998; Stone and Gorman, 2006). Gear types that sample one habitat but do 

not effectively sample other habitats result in less than accurate sampling of fish. Korman and 

Campana (2009) demonstrated that as YOY rainbow trout grow in the Glen Canyon reach of the 

Colorado River, they shift away from shallow, low-angle habitats to environments with deeper 

water. Similarly, Stone and Gorman (2006) documented that as they grow, young native fish in 

the LCR shift from shorelines to open channels during daylight hours. Seines used in backwaters 

capture fish that are most often less than 110 mm in length (Ackerman, unpub. report, 2008). The 

limitation of a seine to catch fish longer than 110 mm and fish occupying habitats other than 

backwaters precludes knowing the fate of the fish once they reach a size where they are either 

less vulnerable to seining or have switched habitat from a backwater to another shoreline type. 

Trammell and others (unpub. report, 2002) indicate that the number of fish captured with 

different gear types during the same months in different years were insufficient to document an 

ontogenetic shift. The rarity of native fish, the abundance of shoreline types other than 

backwaters, and the limitations of different gear types (for example, trammel nets only work in 

eddy environments) further obscure information on fish abundance and length frequencies in 

response to the experimental treatment. 

The potential for predation of native fish by the brown and rainbow trout, which were 

abundant in the mainstem in 2000 (Makinster and others, 2010), further confounded assessment 

of the experimental effects on YOY native fish. The large numbers of brown and rainbow trout 

detected in the mainstem in 2000 were sufficient for resource managers to decide to implement 

mechanical removal of these species from the mainstem near the LCR in 2003–6 as an 

experiment to improve native fish recruitment (Gloss and Coggins, 2005; Coggins, 2008; 

Coggins and others, 2011). In association with the mechanical removal experiment, Yard and 

others (2011) demonstrated that greater abundances of brown and rainbow trout result in 

predation of small-sized fish, including native fish, and that water clarity affects rates of 

predation. In 2000, high levels of water clarity existed in the system for much of the summer 

because of a lack of tributary inputs (Schmidt and others, 2007). Yard and others (2011) 

determined that in the absence of fish suppression brown and rainbow trout could consume as 

many as 45,000 native fish per year. The high abundance of adult brown and rainbow trout in the 

mainstem and the high water clarity throughout the river corridor in the summer of 2000 might 

have contributed to higher predation rates on native fish near the LCR and possibly affected 

mean length data collected in 2000 (Trammell and others, unpub. report, 2002). 

Lastly, the HMF in the middle of Period II likely contributed to inconclusive results by 

displacing some small fish, including native fish, through habitat reworking and increased 

shoreline velocities. The HMF changed habitat that had been persistent for 3 months (Schmidt 

and others, 2007). Further, the September HMF was similar to dam operations until the 1990s. 

The Operation of Glen Canyon Dam Final Environmental Impact Statement (U.S. Department of 

the Interior, 1995) concluded that high, fluctuating flows were detrimental to native fish. Korman 

and Campana (2009) determined that young rainbow trout stay in habitats that persist at the daily 

minimum flow levels during MLFF operations instead of moving up and down the water column 
 48

with the daily fluctuations. The young trout remain in place, presumably, to conserve energy and 

avoid predators. The same behavioral pattern might apply to YOY native fish. Warmer mainstem 

temperatures, achievable under lower reservoir elevations, may be more critical to fish growth 

and survival than stable shorelines that only achieve rearing temperatures during part of the day. 

Additional work may clarify how discharge patterns, shoreline warming, the locations of young 

fish within the river channel, and daily variation in temperature along shorelines relate to fish 

growth and movement. 

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Canyon: Flagstaff, Ariz., SWCA, Inc. Environmental Consultants, submitted to Grand Canyon 

Monitoring and Research Center, cooperative agreement 04WRAG0011. [Available upon 

request by contacting the Center Director, U.S. Geological Survey, Southwest Biological 

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Phoenix, Ariz., submitted to Bureau of Reclamation, Glen Canyon Environmental Studies, 

cooperative agreement no. 9–FC–40–07940, 155 p. [Available upon request by contacting the 

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in Grand Canyon, Arizona--an effective program or serendipitous timing?: Transactions of the 

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Community and evolutionary ecology of North American stream fishes, University of 

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of the Colorado and Little Colorado Rivers and implications for habitat for young-of-year 
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Goeking, S.A., 2007, Streamflow and sediment data collected to determine the effects of low 

summer steady flows and habitual maintenance flows in 2000 on the Colorado River between 

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1268, 79 p., accessed on March 17, 2010, at http://pubs.usgs.gov/of/2007/1268/.

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in the Little Colorado River, Arizona: American Midland Naturalist, v. 155, no. 1, p. 123-135, 

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summer flow experiment on native fishes of the Colorado River in Grand Canyon—final 

report: Flagstaff, Ariz., SWCA, Inc. Environmental Consultants, submitted to U.S. Geological 

Survey, Grand Canyon Monitoring and Research Center, cooperative agreement no. 99–FC–

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Impact Statement: Salt Lake City, Utah, Bureau of Reclamation, Upper Colorado Region, 337 

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Colorado River in Grand Canyon—final report: Flagstaff, Ariz., SWCA, Inc. Environmental 

Consultants, submitted to U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center, 68 p. [Available upon request by contacting the Center Director, U.S. Geological 

Survey, Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

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in the Colorado River, Grand Canyon, Arizona—final report: Logan, Utah, BIO/WEST, Inc., 

submitted to Bureau of Reclamation, contract no. 0–CS–40–09110, technical report no. TR–

250–08, 329 p. [Available upon request by contacting the Center Director, U.S. Geological 

Survey, Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

Voichick, N., and Wright, S.A., 2007, Water-temperature data for the Colorado River and 

tributaries between Glen Canyon Dam and Spencer Canyon, Northern Arizona, 1988–2005: 

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Yard, M.D., Coggins, L.G., Baxter, C.V., Bennett, G.E., and Korman, J., 2011, Trout piscivory 

in the Colorado River, Grand Canyon--effects of turbidity, temperature, and fish prey 

availability: Transactions of the American Fisheries Society, v. 140, no. 2, p. 471-486, 

accessed on April 19, 2011, at 

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Chapter 4. Vegetation Response to a Low Steady Summer 

Flow Experiment

Introduction

The effect of river regulation on riparian vegetation includes the removal of the historical 

seasonal hydrologic disturbance patterns (Stromberg, 1997) and the reduction and coarsening of 

sediment inputs (Lowrance and others, 1986; Stevens, 1989a; Schmidt and others, 2004). Both of 

these alterations affect initial colonization dynamics that can promote exotic species 

introductions following disturbance (Naiman and Decamps, 1997; Stromberg, 1997; Sher and 

others, 2002; Cooper and others, 2003). The regulation of the Colorado River imposed by the 

installation of Glen Canyon Dam resulted in reduced annual release volumes, a reduced spring 

runoff hydrograph, and a shift in the peak discharges from spring to the summer and winter 

months. The shift in peak discharges coincided with power demands in the Southwestern United 

States (Turner and Karpiscak, 1980; Topping and others, 2003; Korman and others, 2004). The 

changes in the hydrograph shifted the timing of disturbance for extant riparian vegetation and 

seedlings from spring to early summer. The reduced annual hydrograph and reduced disturbance 

interval resulted in exposure of substrates and subsequent expanded plant colonization (Turner 

and Karpiscak, 1980; Carothers and Brown, 1991; Webb and others, 2002). 

Introduction of tamarisk (Tamarix spp.), a nonnative species, into the Colorado River 

Basin began in the 19th century (Clover and Jotter, 1944). Although tamarisk proliferated in the 

early 20th century before river regulation (Birken and Cooper, 2006), its expansion in the 

Western United States also benefitted from the hydrologic shifts associated with regulation in the 

1050s and 1960s (Stromberg, 1997; Stevens and Waring, 1986; Turner and Karpiscak, 1980). 

Because tamarisk produces copious seeds continuously from late spring through the summer 

months (coincident with the shift in the hydrologic peak) and the seeds germinate quickly, it 

readily occupied the newly exposed shorelines and became a dominant species along the 

Colorado River (Turner and Karpiscak, 1980; Carothers and Brown, 1991; Ralston and others, 

2008; fig. 4.1). Even though tamarisk provides habitat for riparian bird species (Shafroth and 

others, 2005), the Glen Canyon Dam Adaptive Management Program suggests promoting native 

species within the riparian community and notes that hydrologic experiments that promote the 

successful colonization of tamarisk is a concern to resource managers. 
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Figure 4.1. Tamarisk vegetation dominates shoreline vegetation along the Colorado River.

The expansion of tamarisk along the river corridor in Grand Canyon is associated with 

the initial operations of Glen Canyon Dam in 1963 (fig. 4.2; Carothers and Brown, 1991) and 

flood releases in 1983–84 (Stevens and Waring, 1986; Mortenson and Weisberg, 2010). In both 

cases, newly exposed, unvegetated substrates were available for colonization by tamarisk. 

Though other factors contribute to the establishment of tamarisk within the Colorado River 

ecosystem (CRE), a significant factor is that tamarisk can outcompete other species when 

substrates are bare (Sher and others, 2002). The regulation of the Colorado River by Glen 

Canyon Dam reduced the annual flood volume by more than 50 percent (Topping and others, 

2003) and exposed shoreward substrates that previously were subject to annual scour that 

inhibited colonization (Clover and Jotter, 1944; Carothers and Brown, 1991). Flood releases 

from Glen Canyon Dam in the mid-1980s resulted in removal of 50 percent of the postdam 

vegetation and establishment of bare sandbars and channel margins for a secondary expansion of 

tamarisk with little competition (Stevens and Waring, 1986). In both instances, tamarisk 

dominated when bare substrates were made available. 
 55

Figure 4-2. Map of the Colorado River downstream from Glen Canyon Dam. River mile (RM) 

designations occur at 25-mile intervals starting from Lees Ferry, designated as RM 0. Large tributaries to 

the mainstem, including the Little Colorado River, are identified. 

Low Steady Summer Flow Hydrograph and Tamarisk Response

The combination of sustained high releases from the dam in Period I and reduced steady 

discharges in Period II presented an atypical hydrograph for the Colorado River following 

regulation. Under annual operating plans, spring discharges (March through May) from Glen 

Canyon Dam averaged between 271 and 289 m3/s from 1992 to 1995 and then increased to 

average volumes between 403 and 640 m3/s from 1996 to 2000 (U.S. Geological Survey, 2010). 

Average monthly discharges in June during the 1990s were either equal to or greater than the 

previous months’ discharges, as the dam was operated to meet increasing electrical demands 

associated with the summer months (Korman and others, 2004). The discharge in June 2000 

differed from this pattern because the average monthly volume in June 2000 was less than the 

previous months’ volumes. As with the tamarisk colonization events following initial dam 

operations in 1963 and the mid-1980s flooding (Carothers and Brown, 1991; Turner and 
 56

Karpiscak, 1980; Stevens and Waring, 1986), the LSSF hydrograph created conditions favoring a 

tamarisk colonization event. 

The sustained discharges in April and May and the spring habitat maintenance flows 

(HMF) in 2000 resulted in sediment export from eddies and deposition at mid-elevation sandbars 

(Hazel and others, 2006; Schmidt and others 2007). Scour of vegetation occurred below the 437 

m3/s stage elevation and partial burial of vegetation occurred at the mid-elevation stages, which 

are equivalent to discharges between 538 and 877 m3/s (Porter, 2002; Hazel and others, 2006, 

Schmidt and others, 2007). The beaches below stage elevations of 437 m3/s that were exposed in 

June, when discharge volume was reduced to 226 m3/s stage elevations, provided largely 

unoccupied areas for vegetation colonization, principally by tamarisk (Porter, 2002). The buried 

areas associated with mid-elevation sediment deposition above the 437 m3/s stage and to as much 

as the 877 m3/s stage also provided a disturbance event for extant plant response. 

As the summer proceeded, tamarisk seedlings reached their greatest densities in the areas 

closest to the shoreline, which also were the barest areas (Porter, 2002). The profuse seed 

production and quick germination abilities of tamarisk (Stromberg, 1997; Sher and others, 2002) 

were coincident with the exposure of shorelines in June. The expansive growth of tamarisk in 

2000 is quantified by a mean stem density of 196 stems/m2 recorded in July and the 219 percent 

increase in stem densities (626 stems/m2) 30 days later in August (Porter, 2002; fig. 4.3). These 

latter stem densities were comparable to those observed by Stevens and Waring (1986) following 

the mid-1980s floods and to those observed by Stromberg (1997) on the San Pedro River in 

southern Arizona. The availability of water relative to the location of the seeds along the bank 

and the absence of competition supported tamarisk’s successful colonization (Stevens and 

Waring, 1986; Sher and others, 2002). Porter (2002) indicated that the tamarisk establishment 

was greatest at the mid-shore stage elevations (approximately 382 m3/s stage elevation) and 

below. The successful colonization by tamarisk within these stage elevations suggests an optimal 

soil/water relation within those stage elevations compared to higher stage elevations. 

Tamarisk’s successful colonization associated with the LSSF is attributable to the 

antecedent flows in the spring and the timing of reduced flows starting in June. The sustained 

higher discharges in the spring and the May HMF scoured vegetation at low stage elevations. 

The exposure of bare areas in June coincided with the timing of tamarisk seedling production 

and was approximately 1 month later than the seedling production of native riparian species such 

as Goodding’s willow (Salix gooddingii) and Fremont cottonwood (Populus fremontii)

(Stromberg, 1993). The small abundance of these native species in the CRE further limited 

contributing seed sources for either of these species. The newly scoured areas provided a 

substrate for tamarisk colonization in the absence of competition from other woody riparian 

species. 
 57

Figure 4.3. Mean stem density of tamarisk (Tamarix spp.) and native plants below 538 m3/s stage 

elevation in 2000 and 2001. Error bars represent one standard error (data from Porter, 2002).

Low Steady Summer Flow Hydrograph and Native Species Response

Native woody riparian plant species found along the Colorado River in Grand Canyon 

produce seeds in either the spring (for example, coyote willow (Salix exigua), and Goodding’s 

willow) or late summer and fall (for example, arrowweed (Pluchea sericea), mesquite (Prosopis 

glandulosa), and seepwillow (Baccharis spp.)). The timing and duration of flooding affects 

germination success of wetland plant species and the availability of seeds for subsequent 

germination. Some of these species (coyote willow and arrowweed) also reproduce vegetatively. 

Vegetative reproduction, a form of asexual plant reproduction, also is a common form of 

expansion among some wetland species (for example, common reed (Phragmites australis), 

bulrush (Schoenoplectus sp.), and horsetail (Equisetum sp.). 

Vegetative reproduction, rather than seed germination, was the dominant form of 

establishment by and expansion of native plants following the spring HMF (Porter, 2002). A 

seed bank study conducted during the 1996 high-flow experiment (HFE), or artificial flood, in 

Grand Canyon determined that the 7-day sustained flood flows were sufficient to remove seeds 

stored in shoreline sediment deposits (Kearsley and Ayers, 1999). The LSSF experiment Period I 

discharge and HMF that spanned a 2-month period likely removed seeds stored in seed banks. In 

addition, the timing of the reduced steady-flow volumes in June 2000 preceded seed production 

of wetland species, which occurs in late summer and early fall, and was past the viability 

window of most seeds produced by native woody riparian vegetation earlier in the spring (for 

example, seed viability is 1–5 weeks for willows) (Stromberg, 1993). The abrupt change in 

hydrology between May and June also reduced the water table elevation and may have 

compromised any dispersed seeds germinating at the 538 m3/s stage elevations (Stromberg, 

1993). 
 58

Common reed, bulrush, horsetails, and sedges became established in the low-elevation 

sandbar areas through the summer months of 2000, but at a slower rate than tamarisk (Porter, 

2002). By August 2000, stem density of native riparian species averaged 56 stems/m2 compared 

to 626 stems/m2 of tamarisk (fig. 4-3). Densities of native plants followed a progression of 

lowest stem densities near the edge of the water and highest densities near establish plants 

located above the 538 m3/s stage elevation (Porter, 2002). Plants reproducing vegetatively would 

follow this pattern of expansion. 

Plant Response to September Habitat Maintenance Flow 

The September HMF that was intended to disadvantage nonnative fish (Valdez and 

others, unpub. report, 2000) affected other resources. This small, short-duration flood had the 

effect of reducing tamarisk stem densities by 57 percent (Porter, 2002). The resulting stem 

densities of tamarisk were approximately equal to those observed in July (fig. 4-3). Porter (2002) 

reported that the HMF had little effect on native stem densities (fig. 4-3). Instead, stem densities 

of native species continued to increase in September. The vegetative growth observed in native, 

flood-adapted species might have represented a late-season compensatory response to the shortterm flood disturbance (Kozlowski, 1984). Nutrients delivered to the flood-tolerant plants during 

the short flood pulse might have been quickly absorbed (Kozlowski, 1984) and contributed to the 

observed growth. 

Extended inundation from flooding may have resulted in mortality among tamarisk 

seedlings (Koslowski, 1984; Casanova and Brock, 2000). The decline in stem densities observed 

for tamarisk following the HMF was likely associated with the removal of the youngest 

seedlings. Older tamarisk seedlings (12 weeks or older) have greater survival rates and are more 

resistant to uprooting by flooding than younger seedlings (Horton and others, 1960). Levine and 

Stromberg (2001) also identified reduced survival among tamarisk seedlings that were subject to 

burial. The stem densities of tamarisk observed in September 2000 were similar to those 

observed in July 2000 (fig. 4-3). It is likely that the older seedlings that initially established in 

June survived the September HMF, whereas newly established seedlings died. 

Vegetation Response following the Low Summer Steady Flow

It is difficult to evaluate the effects of a single growing season’s hydrology on long-lived 

riparian plant species within a single year. For this reason, Porter (2002) monitored the response 

of vegetation into the following year to evaluate the longer-term response of vegetation (Porter, 

2002). Modified Low Fluctuating Flow (MLFF) operations were released from Glen Canyon 

Dam from October 2000 through the following year. During this time, monthly volumes 

increased and maximum discharges equaled stage elevations of those in April and May 2000. 

The higher discharges in the winter and early spring inundated the previously exposed shorelines 

up to the 538 m3/s stage elevation. Tamarisk stem densities declined by 45 percent in April 2001 

compared with stem densities recorded in September 2000. Native vegetation stem densities 

were similar to those observed in September (fig. 4-3; Porter, 2002). Tamarisk densities 

continued to decline in subsequent months, which had higher monthly discharges than the 

previous summer. The expansion of species through vegetative growth increased. Stem densities 

of tamarisk declined to 71 stems/m2 and native vegetation stem densities increased to 282 

stems/m2 by August 2001 (fig. 4-3). The combination of over-wintering mortality and inundation 

associated with the MLFF operations contributed to the observed decline in tamarisk stem 
 59

densities (Porter, 2002). Inundation frequency determines vegetation composition across stage 

elevation gradients along shorelines of the Colorado River in Grand Canyon (Stevens and others, 

1995). Tamarisk and arrowweed are associated with shoreline elevations that are inundated 16 

percent of days, whereas wetland marsh species are associated with inundation frequencies of 

greater than 50 percent of days (Stevens and others, 1995). The decline in tamarisk seedling 

densities as inundation frequency increased is an expected outcome on the basis of the results of 

Stevens and others (1995). 

MLFF operations and associated inundation frequency in 2001 favored vegetative growth 

and expansion of wetland species instead of tamarisk. The clonal wetland species occupy 

shoreline areas between 300 and 700 m3/s stage elevation and especially shoreline areas below 

the 566 m3/s stage elevation. Shorelines below the 566 m3/s stage elevations are inundated 

greater than 50 percent of days (Stevens and others 1995). The inundation frequency coincides 

with the tolerance to inundation of marsh species. The inundation frequency of the 2001 

hydrograph was similar to the 1991 hydrograph, when Stevens and others (1995) were collecting 

data. The positive vegetative growth response of native wetland species that can withstand a 

greater inundation frequency than tamarisk observed by Porter (2002) was consistent with 

observations by Stevens and others (1995). 

Conclusions

The LSSF experimental hydrograph, which included sustained high discharges in the 

spring, temporarily benefitted tamarisk rather than other woody riparian and wetland species in 

Grand Canyon. Porter (2002) suggests that the steady discharges favor tamarisk establishment, 

whereas fluctuating discharges favor increased densities of native plants. Steady discharges alone 

cannot account for the success of nonnative rather than native riparian species. In the unregulated 

San Pedro River (not shown), Stromberg (1998) indicated that winter flow peaks and high spring 

flows were associated with tamarisk establishment, whereas Fremont cottonwoods commonly 

were associated with large winter floods and high spring and early summer flows along the same 

stretch of river. Along the Colorado River in 2000, shorelines became bare because of the high, 

sustained discharges in Period I. The LSSF hydrograph supported the germination of tamarisk 

seedlings below the 538 m3/s stage elevation because the sustained high discharges in the spring 

scoured vegetation below the 538 m3/s stage elevation and created bare shorelines when 

discharges reached 226 m3/s in June 2000. The timing of the reduced discharges in June that 

coincided with seed production of tamarisk and the open areas for seedling germination created a 

colonization opportunity for tamarisk: the species is successful under high light conditions and in 

the absence of competition from other plants (Sher and others, 2002; Sher and Marshall, 2003). 

The tamarisk seedling success is not solely attributable to the low steady discharge. 

In contrast to the uncertainty associated with steady flows and exotic species expansion, 

the decline in tamarisk seedlings in the fall of 2000 is attributable to the September HMF and the 

return to MLFF operations in October. The HMF buried and removed some of the seedlings that 

germinated later in the summer. Mortality of tamarisk seedlings and seedlings of other woody 

vegetation associated with late summer flooding occurs in other Southwestern river systems 

(Stromberg, 1997). If other woody riparian species seedlings had been present in the plots 

established in 2000, then there would have been mortality among these species also. The higher 

volume MLFF in the winter and summer of 2001 drowned 50 percent of the remaining tamarisk 

seedlings through continuous or partial inundation. The tamarisk seedlings from 2000 associated 
 60

with the lower half of the exposed shorelines died because of extended inundation under the 

MLFF operations. 

In the absence of the spring scouring flows, the persistent clonal wetland species likely 

would have expanded into the open areas in June. Vegetation composition along shorelines 

might have been more homogeneous in association with the steady flows, but the stem densities 

associated with the clonal vegetation might have been a deterrent to extensive tamarisk seedling 

germination. The presence of vegetation along a shoreline under steady operations also may have 

benefitted young fish through increased shoreline warming resulting from decreasing velocities 

and by providing cover for young fish. In the face of tamarisk seedling colonization, high 

discharges that result in extended periods of inundation increase tamarisk seedling mortality. 

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Chapter 5. Effects of the Low Steady Summer Flow 

Experiment on Campsite Area, Rafting Safety and Travel 

Time, and Overall Recreational Experience

Introduction 

Discharge rates, patterns of water release, and available campsite area affect the quality 

of whitewater rafting in Grand Canyon, a high-profile recreational activity (fig. 5.1; Shelby and 

others, 1983; Bishop and others, 1987; Shelby and others, 1992; Stewart and others, unpub. 

report, 2000). Social factors also contribute to the recreational experience, including the number 

of encounters with other groups, the experience of the encounters (Stewart and others, unpub. 

report, 2000; Jonas and others, 2000), and how time is spent while on the river (for example, 

travel time versus off-river experiences (Bishop and others, 1987; Roberts and Bieri, unpub. 

report, 2001). Other factors affecting the rafting experience include perceptions of safety and the 

occurrence and types of accidents (Jalbert, unpub. report, 1992, 2009). Several scientists 

evaluated Period II of the Low Summer Stead Flow (LSSF) experiment to determine how the 

steady flow volume of 226 m3/s affected rafting travel time, time spent at attractions, incidence 

rate and types of accidents, and camping experiences (fig. 5.1). 

Figure 5.1. Passengers and river guide in a raft going through 205-mile rapid, Colorado River in Grand 

Canyon, Arizona. Photograph courtesy of the National Park Service.
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Effects of Steady Discharges on Rafting Travel Time and Available 

Campsite Area

Discharge rates affect the residence time of water within the Colorado River corridor in 

Grand Canyon (Wright and others, 2008). Besides affecting warming of the water, residence

time also affects how quickly objects on the water move downstream. For whitewater rafting, 

discharge rates and patterns affect the amount of time rafting-trip participants spend on the 

water, at attraction sites, and in camp. These variables also affect the excitement level and safety 

of rapids (Bishop and others, 1987; Shelby and others, 1992; Behan, unpub. report, 2000; 

Roberts and Bieri, unpub. report, 2001). The LSSF experimental plan (Valdez and others, unpub 

report, 2000) did not identify negative or positive effects of the experimental hydrograph on 

recreational boating, but previous authors associated with the Operation of Glen Canyon Dam 

Final Environmental Impact Statement (GCDEIS) determined that the 226-m3/s discharge 

represented a minimum discharge for a quality rafting experience (Bishop and others, 1987). 

Steady discharges were a positive variable in these studies because steady discharges made 

management of moored boats easier because river guides did not have to get up in the night to 

move boats (Bishop and others, 1987). Steady discharges made travel times more predictable. 

The studies completed for the GCDEIS associated with recreation (Bishop and others, 1987; 

Shelby and others, 1992) provide baseline information of perceptions about rafting at 226-m3/s 

discharges, and the Period II hydrograph provided an opportunity to verify those perceptions. 

Surveys of river guides from the 1980s about discharges indicated that at flows less than 

283 m3/s oar boats and motorboats operators were more likely to row more frequently or run 

their motors more often to reach destinations than at higher discharges (Bishop and others, 

1987). The guides also indicated they would spend less time at attraction sites and do not visit all 

sites they might with higher discharges. Guides might leave camps earlier in the day in order to 

reach downstream destinations. Surveys completed during the 2000 LSSF experiment by Roberts 

and Bieri (unpub. report, 2001) support the findings of Bishop and others (1987). Roberts and 

Bieri (unpub. report, 2001) surveyed oar boat and motorboat guides to determine how much time 

they spent at attraction sites versus time spent on the water. The researchers also asked if the 226 

m3/s discharges affected campsite selection. Their report indicated that during Period II of the 

LSSF experiment, trips spent approximately 50 percent less time (for example, 3.5 hours versus 

7 hours) at attraction sites or on off-river activities (for example, hiking, swimming, and visiting 

archaeological sites). The slower speed of the water required spending more time on the water 

(Roberts and Bieri, unpub. report, 2001). The shortened period for off-river activities occurred 

with either motor- or oar-powered trips. Trips in which time spent off the river deviated only 

slightly from that of regular summer flows were short oar trips and short motor trips. These 

shorter trips already maximize time on the river and cannot shorten attraction-site stops below a 

certain threshold; there is a point at which a stop is not logistically feasible. These trips may have 

forgone some attraction sites instead of shortening the time spent at them (Roberts and Bieri,

unpub. report, 2001). 

Roberts and Bieri (unpub. report, 2001) compared choice of attraction sites under 

Modified Low Fluctuating Flow (MLFF) operations (before 2000) and the LSSF experiment in 

2000. There were some shifts in the rankings of attraction sites other than the top three sites that 

most river-trip participants visit. In 2000, stops that were ranked higher compared to previous 

years were sites associated with shorter hikes and easier access (table 2-1) (for example, the 

Little Colorado River is a site where hikes are shorter and more manageable compared to Elves 
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Chasm (not shown)). The most frequently visited sites also were sites that are anticipated by 

most passengers on trips. Stopping at these sites, regardless of time spent at them, may be 

considered an essential component of the recreational experience for the participants (Roberts 

and Bieri, unpub. report, 2001; Jonas and Stewart, unpub. report, 2002). 

 

Table 5.1. Comparison of the rank and frequency of visits of selected attraction sites in Grand Canyon, 

Ariz., visited by river-trip participants during the Low Steady Summer Flow (LSSF) experiment in 2000 and 

Modified Low Fluctuating Flow (MLFF) operations in previous years (adapted from Roberts and Bieri, 

unpub. report, 2001).

River mile and side of the 

river (left (L) or right (R)) Site Name

Rank 

(most visited to least visited) 

(MLFF) 

Rank

(most visited to least 

visited) 

(LSSF)

136.2R Deer Creek 1 1 

156.9L Havasu 2 2 

32.8L Redwall Cavern 3 3 

116.5 Elves Chasm 4 5 

61.4L Little Colorado River 5 4 

47.2R Upper Saddle 10 12

31.6R South Canyon 11 19

53.0R Nankoweap Arch site 12 22

212.9L Pumpkin Springs 16 11

Campsite selection was the other variable evaluated by Roberts and Bieri (unpub. report, 

2001). The purpose of the May habitat maintenance flow (HMF) was to rebuild backwaters and 

possibly low-elevation sandbars used for camping (Valdez and others, unpub report, 2000). 

Surveys of campsite area following the May HMF indicated that campsite area below the 708 

m3/s stage elevation increased because of sediment deposition in mid-elevation sandbars 

(Kaplinski and others, 2005; Hazel and others, 2006; Schmidt and others, 2007). The May HMF 

differentially affected sandbar areas at camping sites with some sites in critical reaches (for 

example, Marble Canyon) increasing in area and volume, although some sites in noncritical 

reaches decreased in area and volume (Kaplinski and others, 2005). These changes in sandbar 

area may have affected campsite preferences identified by boaters during Period II. Few of the 

camps surveyed by Kaplinski and others (2005) coincide with those identified by Roberts and 

Bieri (unpub. report, 2001). Two camps that were coincident in the two studies, the Hot Na Na 

Camp Site at RM 16.4L and the Football Field Camp Site at RM 137.5 (not shown), did increase 

in area in 2000 (Kaplinski and others, 2005). 

In general, because discharge during the summer of 2000 was at a base flow of 226 m3/s, 

greater areas of beach were exposed and available for camping, including some campsites that 

are only available during low water. In this case, the recreational experience might have been 

improved by expanded campsite availability and by being able to camp at a place that was “new” 

to an experienced boater. The increased availability of campsites at low water also benefitted trip 

schedules. A good example of this increased availability is the Hot Na Na camp, which is a 

convenient distance downstream from Lees Ferry for oar boats to stop on the first night. 
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Additionally, several campsites downstream from the Little Colorado River that are good lowwater camps increased campsite availability in a reach where campsites are sparse during higher 

discharges (Roberts and Bieri, unpub. report, 2001). The steady discharge pattern also enhanced 

the boating and camping experience. During fluctuating flows, shorelines become inundated or 

dewatered over a matter of hours, which can result in stranding of moored boats. Large motor 

rafts are heavy and difficult to move once stranded and, to prevent the problem, river guides have 

to move their boats through the night. The steady flows meant guides did not have to get up in 

the night to move their boats and people sleeping on shore could sleep close to the edge of the 

water, which is cooler in the summer (Roberts and Bieri, unpub. report, 2001). 

Rafting Safety during Low Steady Flows

Rapids are an important component of rafting trips and represent locations where rafting 

accidents are most likely to occur (Bishop and others, 1987; Brown and Hahn-O’Neill, 1987; 

Underhill and others, 1987; Jalbert, unpub. report, 2009). Factors contributing to a safe, but 

thrilling, ride through a large rapid include prior boating experience and river discharge. River 

guides and private boatmen in the 1980s found that discharges greater than 1,274 m3/s and less 

than 283 m3/s presented greater likelihoods of accidents compared to discharges between these 

extremes (Bishop and others, 1987; Brown and Hahn-O’Neill, 1987; Underhill and others, 1987). 

The minimum discharge that river guides felt was reasonably safe for conducting commercial 

river trips was slightly more than 226 m3/s (238 m3/s for motorboat trips and 260 m3/s for oarboat trips (Bishop and others, 1987)). At lower discharges, 65 percent of oar-boat guides and 35 

percent of motorboat guides were likely to have passengers walk around rapids. The 1980s was a 

period of higher discharges and the guides that were interviewed likely had limited experience 

with discharges lower than 283 m3/s (Bishop and others, 1987; Jalbert, unpub. report, 2009). 

Boating accidents during high discharges in the 1980s and in 1996 were documented in 

association with the high-flow experiment (HFE; Underhill and others, 1987; Harpman and 

Jalbert, unpub. report, 1997). Accidents associated with lower discharges were studied in the 

1980s (Brown and Hahn-O’Neill, 1987) and in 1990–91 during test flows conducted in 

conjunction with the preparation of the GCDEIS (Jalbert, unpub. report, 1992). Both of these 

evaluation periods were associated with fluctuating flows. Period II of the LSSF experiment 

provided an opportunity to evaluate an extended period of a steady, low discharge and its relation 

to boating safety. Assessments of accidents involved making observations at major rapids and 

reviewing National Park Service (NPS) records of boating accidents, including evacuations made 

by the NPS (Jalbert, unpub. report, 2009). The study conducted in 2000 was limited to rapids that 

were accessible to observers by hiking, or through assistance from recreational and 

administrative river trips (Jalbert, unpub. report, 2009). Jalbert (unpub. report, 2009) compared 

the results for 2000 with previous studies to determine if the 226 m3/s discharge resulted in more 

accidents than larger discharges. 

Results from the 2000 study of boating accidents indicated that grounding of motorboats 

was the most common accident earlier in Period II. Injury to passengers, principally arm 

fractures, occurred in greater frequency in the later months of Period II (Jalbert, unpub. report, 

2009). The reduction in groundings as the season progressed may be an indication that riverguide experience reduced these types of accidents. The incidents of grounding as analyzed from 

data collected in the 1999 period were rare during higher discharges (Jalbert, unpub. report, 

2009). However, the single grounding incident in 1999 required an evacuation by the NPS, 

whereas only 50 percent of the groundings in 2000 required evacuation (Jalbert, unpub. report, 
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2009). Groundings and subsequent evacuations may occur more frequently at higher discharges, 

and accidents at specific rapids may usually require evacuation (for example, Hance, Crystal, or 

Horn Creek Rapids; not shown). Differentiating between the inherent difficulty of the rapid and 

discharges that cause grounding was not possible using the available data. 

During the LSSF experiment, Jalbert (unpub. report, 2009) indicated that the low steady 

discharges affected the entire river in Grand Canyon with respect to the incidents of accidents. 

Among rapids where observations occurred, all rapids were associated with accidents instead of 

a few rapids. The greatest number of accidents was associated with Crystal Rapid followed by 

Hance Rapid. These are two of the more technical rapids in Grand Canyon, requiring quick 

maneuvering by guides to avoid large hydraulic holes and rocks. In most cases, accidents were 

associated with hitting exposed rocks (Jalbert, unpub. report, 2009), which affected motorboats 

the most. Despite the fact that twice as many accidents occurred in 2000 compared to the 

previous year, Jalbert (unpub. report, 2009) concluded that there was not a significant relation 

between the 226 m3/s discharge and increased number of boating accidents. A higher incidence 

of accidents cannot exclusively be assigned to lower discharges because several other factors 

may contribute to boating accidents. 

Recreational Experience Relative to the Low Summer Flows 

The factors previously discussed, the relationships between river guides, relationships 

with guides and passengers, time between trips, and the participant’s level of rafting experience 

come together to define a participant’s experience as enjoyable or disappointing (Shelby and 

others, 1992; Jonas and Stewart, unpub. report, 2002). A Colorado River rafting trip is often the 

trip of a lifetime for a participant and is unlikely to be repeated (Jonas and others, 2000; Jonas 

and Stewart, unpub. report, 2002). In this case, the participant has preconceived notions of what 

to expect, but his/her experience also can be affected by attitudes of the people encountered on 

the trip, including those viewed in a leadership role such as a commercial guide, or more 

experienced participants (Shelby and others, 1992; Jonas and others, 2000; Jonas and Stewart, 

unpub. report, 2002). Jonas and Stewart (unpub. report, 2002) argue that a participant’s 

recreational experience is affected by these encounters and how factors including trip type, 

number of participants, and physical factors of river discharge and weather are interpreted or 

expressed among participants. Participants on trips during the summer of 2000, in which trips 

were scheduled with less time at attraction sites, but participants were able to utilize camp sites 

that were potentially larger and closer to shore (Roberts and Bieri, unpub. report, 2001; 

Kaplinski and others, 2005), had a different experience than trips in other years. Because they 

only had their trip as a reference point, the participants’ experience was not reduced (Jonas and 

Stewart, unpub. report, 2002). 

 The context of the steady discharges may have added uniqueness to the trips as they took 

place at different discharge patterns than previous years (Jonas and Stewart, unpub. report, 

2002), and most boaters would view the experience as excellent or perfect because that trip was 

the only reference for a passenger’s viewpoint. Crowding at attractions, which can detract from 

the “wilderness experience,” may not have been an issue during the LSSF experiment. Shorter 

hikes that better fit the trip’s time constraints might have been completed instead of longer hikes 

(Roberts and Bieri, unpub. report, 2001). The lower discharges that were initially unfamiliar to 

some guides possibly added to the excitement of the whitewater rafting for the participant (Jonas 

and Stewart, unpub. report, 2002). A guide’s perception and his/her reaction to a situation 

convey information to the participant about danger and excitement. For example, getting through 
 68

a rapid like Horn Creek without incident, but still experiencing a big ride in the rapid can be a 

relief to all and be remembered in positive terms. Because the discharges were not extremely 

low, less thrilling rides in rapids and safety concerns because of exposed rocks may have been 

detractors. The 226 m3/s discharge minimally affected the overall multidimensional recreational 

experience (Jonas and Stewart, unpub. report, 2002). 

Conclusions

Multiple studies evaluated single factors associated with Period II of the LSSF 

experiment and concluded that river guides generally adapt to the changing boating conditions, 

so as to ensure to an “excellent” rafting experience for guests. The aspects that were measured 

(for example, travel time, accidents, camping choice) were easily quantified, but they did not 

provide a complete evaluation of the effect the LSSF hydrograph had on the rafting experience 

(Jonas and Stewart, unpub. report, 2002). Each individual aspect that was studied suggests that 

more time was spent on the water than at attraction sites located away from the river; campsite 

availability increased, and the steady discharges improved overnight boat mooring and sleeping 

conditions. Lastly, boating accidents declined with time as guides became accustomed to the 

reduced discharges. Because of the multidimensional aspect of the recreational experience, 

evaluating discharge releases and recreational use is difficult without first understanding the 

basic questions about linkages between experience and resource characteristics within the river 

corridor (Stewart and others, unpub. report, 2000). Embedding a researcher with the 

recreationists (see for example, Jonas and others, 2000) is an approach that can address these 

questions. 

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Colorado River in Grand Canyon National Park—final report: Flagstaff, Ariz., submitted to 

Bureau of Reclamation and U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center, cooperative agreement no. CA8210–99–002, interagency acquisition no. 00–AA–40–

4330, 18 p., accessed on August 23, 2011, at 

http://www.gcmrc.gov/library/reports/cultural/Recreation/roberts2001.pdf. 

Schmidt, J.C., Topping, D.J., Rubin, D.M., Hazel, J.E., Jr., Kaplinski, M., Wiele, S.M., and 

Goeking, S.A., 2007, Streamflow and sediment data collected to determine the effects of low 

summer steady flows and habitual maintenance flows in 2000 on the Colorado River between 

Lees Ferry and Bright Angel Creek, Arizona: U.S. Geological Survey Open-File Report 2007–

1268, 79 p., accessed on March 17, 2010, at http://pubs.usgs.gov/of/2007/1268/.

Shelby, B., Heberlein, T.A., Vaske, J.J., and Alfano, G., 1983, Expectations, preferences and 

feeling crowded in recreation activities: Leisure Sciences, v. 6, no. 1, p. 1–14. 

Shelby, B., Brown, T.C., and Taylor, J.G., 1992, Streamflow and recreation: Fort Collins, Colo., 

U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range 

Experiment Station, general technical report RM-209, 27 p., accessed on August 23, 2011, at 

http://www.gcmrc.gov/library/reports/other/Recreation/Shelby1992b.pdf. 
 70

Stewart, W., Larkin, K., Orland, B., Anderson, D., Manning, R., Cole, D., Taylor, J., and Tomar, 

N., 2000, Preferences of recreation user groups of the Colorado River in Grand Canyon—final 

report: Flagstaff, Ariz., submitted to U.S. Geological Survey, Grand Canyon Monitoring and 

Research Center, cooperative agreement no. 98–FG–40–0190, 232 p., accessed on August 23, 

2011, at http://www.gcmrc.gov/library/reports/cultural/Recreation/Stewart2000.pdf. 

Underhill, A.H., Hoffman, M.H., and Borkan, R., 1987, An analysis of recorded Colorado River 

boating accidents in Glen Canyon for 1980, 1982, and 1984, and in Grand Canyon for 1981 

through 1983: University of Arizona, Cooperative Park Studies Unit, contribution no. 053/02, 

submitted to U.S. Bureau of Reclamation, Glen Canyon Environmental Studies, technical 

report no. 18, interagency agreement no. 4–AA–40–01930, 32 p. 

Valdez, R.A., Carothers, S.W., House, D.A., Douglas, M.E., Ryel, R.J., Bestgen, K.R., and 

Wegner, D.L., 2000, A program of experimental flows for endangered and native fishes of the 

Colorado River in Grand Canyon—final report: Flagstaff, Ariz., SWCA, Inc., Environmental 

Consultants, submitted to U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center, 68 p. [Available upon request by contacting the Center Director, U.S. Geological 

Survey, Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

Wright, S.A., Anderson, C.R., and Voichick, N., 2008, A simplified water temperature model for 

the Colorado River below Glen Canyon Dam: River Research and Applications, v. 25, no. 6, p. 

675–686. 

 
 71

Chapter 6. Effect of the Low Steady Summer Flow 

Experiment on Angling Quality in the Lees Ferry Trout 

Fishery

Introduction

Accessibility to fishing locations and the quantity and size of fish available for capture 

affect fishing quality in trout fisheries, including in Lees Ferry, Arizona (fig. 6.1; Bishop and 

others, 1987; Petering and others, 1995; Fisher, 1997; Radomski and others, 2001). Minimum 

discharge, magnitude of fluctuations, fish density, and food availability affect the abundance and 

condition of rainbow trout size classes in the fishery (McKinney and others, 2001; Speas and 

others, unpub. report, 2004; Korman and others, 2009; Korman and Campana, 2009). Surveys in 

the 1980s of preferences of Lees Ferry anglers indicated a preference for discharges between 226 

and 424 m3/s. Anglers identified that lower discharges increased the likelihood of catching large 

fish and being able to fish in preferred areas (Bishop and others, 1987). When surveyed about 

specific discharge patterns, respondents preferred steady to fluctuating discharges, and they 

preferred lower to higher steady discharges (Bishop and others, 1987). Anglers reiterated their

preferences for reduced fluctuations and lower discharges in a 2006 knowledge assessment 

conducted by the U.S. Geological Survey (Melis and others, unpub. report, 2006b).

Discharge rates and patterns of water release affect the densities, composition, and drift 

concentration of filamentous green algae (for example, Cladophora glomerata; hereafter 

Cladophora), macrophytes (emergent or submerged aquatic plants), and invertebrates 

(Gammarus lacustris (scuds) and chironomids (midges)), all food sources for rainbow trout 

(Oncorhynchus mykiss) (Stevens and others, 1997; McKinney and others, 1999; Shannon and 

others, 2001; Melis and others, unpub. report, 2006b). Higher discharges expand the permanently 

wetted area along shorelines that host algae, macrophytes, and associated invertebrates. An 

abrupt decrease in discharge, like that between May and June 2000, can strand previously 

submerged periphyton (a collection of algae and microbes), macrophyte beds, associated 

invertebrates, and young fish (Hardwick and others, 1992; Blinn and others, 1995; McKinney 

and others, 1999; Korman and others, 2010). The Low Steady Summer Flow (LSSF) experiment

in 2000 represented the first time since the Record of Decision (U.S. Department of the Interior, 

1996) that the Lees Ferry fishery was subject to an extended period of lower discharges. An 

unanticipated effect of the LSSF experiment was enhanced survival of young-of-year (YOY) 

rainbow trout (Speas and others, unpub. report, 2004). The stabilized habitat in the summer 

months may have offset negative effects of the abrupt change in discharge between Periods I and 

II of the LSSF experiment on the aquatic food base, YOY rainbow trout, and angling 

satisfaction. 
 72

Figure 6-1. Map of the Colorado River downstream from Glen Canyon Dam. River mile (RM) 

designations occur at 25-mile intervals starting from Lees Ferry, designated as RM 0. Large tributaries to 

the mainstem, including the Little Colorado River, are identified. 

Patterns of Standing Mass and Drift Concentrations of Periphyton, 

Macrophytes, and Benthic Organisms during the Low Steady Summer 

Flow Experiment

Periphyton and macrophyte standing mass (grams ash-free dry mass per square meter

(AFDM/m2)) did not differ significantly between June 2000 and previous years (Rogers and 

others, unpub. report, 2003). The change in discharge between Periods I and II did not affect 

periphyton and macrophytes because they were established at depths below the 226 m3/s stage 

elevations. Densities and distributions of macrophytes and periphyton increased through Period 

II, reaching peaks in standing mass in July and August (figs. 6-2 and 6-3). Rogers and others 

(unpub. report, 2003) noted that the period during which periphyton reached its greatest standing 

mass was similar to previous years, whereas the pattern for macrophytes differed (July in 2000
 73

versus fall in the previous years). Rogers and others (unpub. report, 2003) attributed the shift in 

the peak for macrophyte density to a shift in the species composition from Chara sp. (a green 

alga) to pondweed (Potamogeton sp.) and leafy elodea (Egeria densa) that began following the 

1996 HFE (McKinney and others, 1999; Benenati and others, 2001).

Figure 6-2. Mean standing mass of periphyton in the Lees Ferry reach of the Colorado River collected 

during the Low Steady Summer Flow Experiment at the 226 m3/s stage elevation in April, June, July, 

August, and September 2000. Sampling did not occur in May 2000. Error bars represent 95-percent 

confidence intervals. There were no statistically significant differences in biomass among sample periods. 

Data from Arizona Game and Fish Department (Rogers and others, unpub. report, 2003).
 74

Figure 6-3. Mean standing mass of macrophytes in Lees Ferry collected at 226 m3/s stage elevation in 

April, June, July, August, and September 2000, during the Low Steady Summer Flow Experiment. 

Sampling did not occur in May 2000. Error bars represent 95-percent confidence intervals. Biomass was 

not statistically significantly different among sample periods. Data from Arizona Game and Fish Department 

(Rogers and others, unpub. report, 2003). 

Rogers (unpub. report, 2003) compared the values for periphyton and macrophyte 

biomass from August 2000 during steady discharges with data from previous years during 

Modified Low Fluctuating Flow (MLFF) operations (McKinney and others, 1999). Comparisons 

for periphyton were limited to the years 1991 to 1993. Only a single year, 1997, was available 

for macrophyte comparisons. Ash-free dry mass values of periphyton collected in August 2000 

using an integrated depth sample were approximately twice the average values recorded in 

August, 1991–93 (approximately 264.2 g AFDM/m2 in August 2000 versus 112.9 g AFDM/m2 in 

August, 1991–93; fig. 6-4). Macrophyte biomass values were similar between years (130.7 g 

AFDM/m2 in August 2000 versus 114.2 g AFDM/m2 in August 1997; fig. 6-5). Increased 

discharge during 1994–96 also increased the wetted perimeter in the Glen Canyon reach and 

contributed to a greater standing mass for macrophytes (Rogers and others, unpub. report, 2003). 

The higher discharges in the late 1990s that cropped submerged algae and macrophytes may 

have contributed to comparable values of standing mass to those recorded in 2000. The stabilized 

habitat associated with the prolonged steady flows in June 2000 may have expanded growth and 

increased standing mass in periphyton and macrophytes (Rogers and others, unpub. report, 2003; 

Melis and others, unpub. report, 2006b). 
 75

Figure 6-4. Comparison of mean standing mass of periphyton collected in the Lees Ferry reach of the 

Colorado River at the 226 m3/s stage elevation in August 1991–93 and 2000. Biomass was greater in 

August 2000 (statistically significant at p<0.05). Error bars represent 95-percent confidence intervals about 

the mean. Data from Arizona Game and Fish Department (Rogers and others, unpub. report, 2003).
 76

Figure 6-5. Comparison of mean standing mass of macrophytes in the Lees Ferry reach of the Colorado 

River collected at the 226 m3/s stage elevation in August 1997 and 2000. There is no statistically significant 

difference in standing mass between the two periods. Error bars represent 95-percent confidence intervals. 

Data from Arizona Game and Fish Department (Rogers and others, unpub. report, 2003).

Comparison of benthic invertebrate densities of scuds, midges, and New Zealand 

mudsnails (Potamopyrgus antipodarum) during the LSSF experiment with previous years 

revealed that densities were equal to or greater than densities in previous years (Rogers and 

others, unpub. report, 2003). The timing of peak standing mass varied for some species 

compared with previous years. Scud densities in 2000 peaked in September as in previous years 

(Rogers and others, unpub. report, 2003) and were comparable to densities in 1992–97 

(McKinney and others, 1999; figs. 6-6 and 6-7). The peak of midge densities occurred in July of 

2000 (fig. 6-8), but midges peaked in the springtime in previous years (McKinney and others, 

1999). Midge densities were two to three times greater in 2000 compared to 1992–97 (fig. 6-9; 

5,000–9,000 midges/m2 in July 2000 versus 1,800–2,200 midges/m2 in the spring of 1992–97; 

Rogers and others, unpub. report, 2003). Lastly, gastropod densities were the greatest recorded in 

2000 compared to previous years (4,000–16,000 gastropods/m2 in summer 2000 versus 200–400 

gastropods/m2 in summer and fall of 1992–97 (Rogers and others, unpub. report, 2003). The 

historical values include the period before the first observation of New Zealand mudsnail in the 

Lees Ferry reach in 1995 (Shannon and others, unpub. report, 2003). The other year associated 
 77

with greater gastropod densities was 1997, which was associated with a summer of high, steady 

discharges (Pulwarty and Melis, 2001). Gastropod densities in 1997 varied between 400 and 

1,200 gastropods/m2 (Rogers and others, unpub. report, 2003) suggesting steady discharges 

support higher snail densities than fluctuating discharges. 

Figure 6-6. Mean densities of Gammarus in the Lees Ferry reach of the Colorado River in April, June, 

July, August, and September 2000. Sampling did not occur in May 2000. Error bars represent 95-percent 

confidence intervals. Data from Arizona Game and Fish Department (Rogers and others, unpub. report, 

2003).
 78

Figure 6-7. Mean densities of Gammarus in the Lees Ferry reach of the Colorado River during the 

summer months (June through August) in 1993–97 and 2000. Error bars represent 95-percent confidence 

intervals. Data from Arizona Game and Fish Department (Rogers and others, unpub. report, 2003). 

Figure 6-8. Mean densities of Chironomidea in the Lees Ferry reach of the Colorado River in April, June, 

July, August, and September 2000. Sampling did not occur in May 2000. Error bars represent 95-percent 

confidence intervals. Data from Arizona Game and Fish Department (Rogers and others, unpub. report, 

2003).
 79

Figure 6-9. Mean densities of Chironomidae in the Lees Ferry reach of the Colorado River in April, June, 

July, August, and September 1993–97 and 2000. Error bars represent 95-percent confidence intervals. 

Data from Arizona Game and Fish Department (Rogers and others, unpub. report, 2003).

In 2000, peak drift concentrations of chironomids shifted from June to July compared to 

1994. In 1994, drift concentrations peaked in June and averaged 0.10 g AFDM/m2

 (Rogers and 

others, unpub. report, 2003). In 2000, drift concentrations increased in July and were at values 

that were comparable to or exceeded previous years’ values for the same month (approximately 

0.08 g AFDM/ m2

 in July 2000 versus 0.05 g AFDM/m2 in July 1993 and 1994 (Rogers and 

others, unpub. report, 2003). The differences in drift concentrations between these years may be 

associated with the typically larger discharges in June before 2000 during MLFF operations to 

meet power demands (Korman and others, 2004) compared to April and May 2000 during the 

LSSF experiment. Drift concentrations can increase on the increasing limb of discharges 

(Leibfreid and Blinn, 1987; Melis and others, unpub. report, 2006b) and decline on the 

decreasing limb, which might explain why drift concentrations peaked in June 1994 but reached 

a minimum in June 2000. 

 Shannon and others (2001) studied invertebrate density and drift concentrations during 

high, steady flows in 1997, and suggested that steady flows might produce greater densities, 

which leads to comparably greater drift concentrations. The 1-month lag in drift concentrations 

in 2000 (Rogers, and others, unpub. report, 2003) may lend support to the conclusions reached 

by Shannon and others (2001). The greater density of chironomids in July 2000 compared with 

previous years (Rogers and others, unpub. report, 2003) supports the hypothesis that steady flows 

increase invertebrate density. 

Scud concentrations in the drift were greatest during the increased discharge associated 

with the May habitat maintenance flow (HMF), which supports the findings of Leibfried and 

Blinn (1987), whereas concentrations of midges in the drift were greatest immediately following 

the HMF during a steady release of 495 m3/s. These increased drift concentrations of scuds and 

midges would have been coincident with adult fish moving off redds and actively feeding on 

drift. The relative condition, a measure of the weight-length relation or “plumpness,” of fish in 

the spring of 2000 was greater than or equal to their relative condition in 1991–99, which 
 80

includes years when fish were at their greatest relative condition before 2000 (Speas and others, 

unpub. report, 2004). The increased concentrations of scuds and midges in the spring of 2000 

associated with the high flows and HMF during the same year may have contributed to the 

greater relative condition of fish in the spring of 2000 (McKinney and others, 1999; Speas and 

others, unpub. report, 2004). 

Speas and others (unpub. report, 2004) determined that the mean relative gut volume of 

scuds in all rainbow trout in 2000 was the greatest since 1997, but Cladophora and midge values 

were lower in 2000. In the spring, the mean value of scuds in guts varied by size classes of fish 

such that only fish less than or equal to 305 mm had greater numbers of scuds in their gut 

compared with previous years. By summer, the presence of scuds in fish of all sizes was greater 

than in previous years. The increased presence of scuds among fish through time may correspond 

with the increasing standing mass of scuds in 2000 (Rogers and others, unpub. report, 2003) and 

may indicate a switch in feeding behavior by trout to active foraging for larger prey items 

(Giroux and others, 2005). Additionally, the relative gut contents observed in 2000 may have 

been affected by drift patterns of the prey items. To avoid predation, Gammarus may actively 

drift at night (Cada and others, 1987; Douglas and others, 1994; Flecker, 1992), which coincided 

with sampling of fish but not with sampling of drift, which occurred during the day. In contrast 

to nocturnal drifting behavior of Gammarus, there also may have been more asynchrony 

associated with Cladophora concentrations in the water column during steady flows. The small 

standard error associated with the mean drift samples from the summer months in 2000, which 

was dominated by Cladophora (Rogers and others, unpub. report, 2003), may indicate the more 

consistent concentration of Cladophora in the water column during steady flows compared with 

benthic invertebrates. 

Effect of Low Steady Summer Flow Experiment on the Lees Ferry Trout 

Fishery

Identifying the effects of the LSSF experiment on the rainbow trout fishery is 

complicated by the decline in the fishery that began in 1997 (McKinney and Speas, 2001; 

Makinster and others, 2011). The relative condition of the rainbow trout population peaked in 

1994 and fell 10 percent by 1997 for rainbow trout greater that or equal to 305 mm (total length), 

although, the relative condition of fish less than or equal to 305 mm remained stable until 1997. 

The time period 1997–2000 was characterized by high abundance of small (≤305 mm), juvenilesized fish, with few fish greater than or equal to 406 mm. There was an associated decline in 

catch rates from anglers (McKinney and Speas, 2001). Declines in growth rates and relative

condition were attributed to over-recruitment and density-dependent growth suppression as 

discharges reached a minimum in 2000 (McKinney and Speas, 2001; McKinney and others, 

2001). 

As outlined in the LSSF experimental plan (Valdez and others, unpub report, 2000), the 

timing and magnitude of high discharges affect survivorship of small-sized fishes, including 

early life stages of rainbow trout (Korman and others, 2010). Survivorship in early stages affects 

the subsequent recruitment into the adult population. The April and May period in the life history 

of rainbow trout in the Lees Ferry fishery is a period when spawning is declining, young fish are 

emerging (Korman and Campana, 2009), and adult fish are moving off redds and into riffles to 

feed. In 2000, the discharges before April and May were lower but were not at a discharge less 

than 141 m3/s that would expose redds and negatively affect spawning success (McKinney and 
 81

others, 2001; Korman and Campana, 2009). Because spawning generally peaks in mid-March 

and fish emergence follows approximately 30 days later (Korman and others, 2005; Korman and 

Campana, 2009), fry that emerged in April and May were subject to the higher sustained flows 

during those months. High discharges like those of the May HMF can displace young fish 

(Korman and Campana, 2009; Korman, 2009). 

Speas and others (unpub. report, 2004) determined that the catch per unit effort (CPUE) 

of age-0 fish (≤152 mm) was slightly greater in 2000 than 1999, but not greater than the previous 

5 years. Sampling by electrofishing in the spring of 2000 (Speas and others, unpub. report, 2004) 

indicated that age-0 fish appeared in the catch from April through June, 3 months earlier than in

1991–99. Speas and others (unpub. report, 2004) also reported greater numbers of fish with total 

lengths between 50 and 75 mm in 2000 than in previous years. The higher discharges in the April 

and May period as well as the HMF may have displaced some fish larvae, but there may have 

been a compensatory survival rate among fish that emerged later (Korman, 2009). The fact that 

the CPUE of YOY did not change between the end of June and the end of September 2000 may 

be an indication of compensatory survival. 

The higher spring discharges and the May HMF also may have cleared interstitial spaces 

in the riverbed (Korman, 2009) that can serve as refuges for young fish. The May and September 

2000 HMF moved sediment from eddies to mid-elevation sandbars (Hazel and others, 2006; 

Schmidt and others, 2007). Using side-scan sonar imagery for a section of river 2 miles 

downstream from Lees Ferry, Wong and others (2003) identified erosion of sediment from 

bedrock and coarsening of sand to pebbles and cobbles in association with the September HMF. 

They also identified deposition of sediment in shoreline areas downstream from Lees Ferry. The 

effect of the May HMF on sediment in the Glen Canyon reach, which is identified as a losing 

reach with respect to sediment (Grams and others, 2007), was erosion of sediment rather than 

deposition, including clearing of cobbles and gravels of finer sediment. The increased standing 

mass of Cladophora and benthic invertebrates, organisms that use hard substrates, suggests that 

the HMF evacuated sediment from the Glen Canyon reach.

The increased standing mass of algae macrophytes and benthic invertebrates in 2000 

(Rogers and others, unpub. report, 2003) that provide cover and food for rainbow trout may have 

benefitted survival of YOY fish that emerged later in the season. The effect of the higher spring 

discharges and subsequent summer steady flows on YOY survivorship is speculative, however, 

because YOY survival of rainbow trout was not studied in 2000. Fish sampling conducted in 

2001 in the Lees Ferry fishery (Speas and others, unpub. report, 2004) determined that the CPUE 

of age 2–3-year-old rainbow trout declined, as did the relative condition and proportional stock 

density of the fish. The decline in fish size and relative condition were attributed to reduced 

discharges, which limited food and habitat (McKinney and Speas, 2001). 

Angler Satisfaction

February to May is a prime season for anglers at the Lees Ferry trout fishery. The effect 

on anglers of the higher discharges in April and May of 2000 and the HMF was a disruption in 

upstream access during the 4 days of the HMF (Hjerpe and Kim, unpub. report, 2003) and a loss 

in revenue for fishing guides. Given the decline of the fishery from 1997 to 2000, it is difficult to 

differentiate between the effects of the experiment and the general decline of the fishery on 

revenue. If the fishery had been in better condition, revenue loss may have been greater. The 

study of Hjerpe and Kim (unpub. report, 2003) did not identify if all available fishing guides 

were unable to work for those days, and the authors did not compare client numbers in 2000 with 
 82

previous years when the fishery was in better condition. The lower steady flow period occurred 

at a time when fewer anglers visit the Lees Ferry fishery. The lower steady discharges should 

have provided easier access to anglers that fish from shorelines. The steady discharges created 

persistent habitats in which fish could feed and anglers could cast. 

Conclusions

The LSSF experiment had little effect on the standing mass of either macrophytes or 

invertebrates in the Lees Ferry trout fishery. The exception was the standing mass of New 

Zealand mudsnail, which significantly expanded its density under steady discharges (Yard and 

Blinn, unpub. report, 2001; Rogers and others, unpub. report, 2003). The timing of the peak 

production for some of the food base was delayed. This delay may be attributed to the HMF in 

May that possibly scoured some benthos (Rogers and others, unpub. report, 2003). The condition 

of the Lees Ferry fishery before the LSSF experiment affects interpretation of the effect of the 

experiment on the resources in the Lees Ferry fishery. The condition of rainbow trout in the 

fishery was declining before the 2000 experiment and continued to decline following the 

experiment (Speas and others, unpub. report, 2004). The steady increase in food availability 

throughout the summer months may have benefitted all fish until the HMF in September 2000. 

The effect of the fall HMF on small fish may have been displacement, as was observed during 

subsequent high flows (Korman and Campana, 2009; Korman and others, 2010), though 

displacement was not studied in 2000. The number of small fish caught in the following year was 

similar to that in previous years (Speas and others, unpub. report, 2004) and suggests that large 

numbers of small fish observed in 2000 were displaced by the fall HMF. The LSSF experiment 

negatively affected angler access during the May and September HMF, but provided stable fish 

habitat in the summer months that provided shoreline anglers with consistent access to cobble 

habitat near Lees Ferry (Hjerpe and Kim, unpub. report, 2003). 

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Korman, J., Wiele, S.M., and Torizzo, M., 2004, Modelling effects of discharge on habitat 

quality and dispersal of juvenile humpback chub (Gila cypha) in the Colorado River, Grand 
 84

Canyon: River Research and Applications, v. 20, no. 4, p. 379-400, accessed on February 22, 

2010, at http://www3.interscience.wiley.com/cgi-bin/fulltext/107614374/PDFSTART.

Korman, J., Yard, M., Walters, C.J., and Coggins, L.G., 2009, Effects of fish size, habitat, flow, 

and density on capture probabilities of age-0 rainbow trout estimated from electrofishing at 

discrete sites in a large river: Transactions of the American Fisheries Society, v. 138, no. 1, p. 

58-75, accessed on June 1, 2011, at http://www.tandfonline.com/doi/abs/10.1577/T08-025.1. 

Korman, J., Kaplinski, M., and Melis, T.S., 2010, Effects of high-flow experiments from Glen 

Canyon Dam on abundance, growth, and survival rates of early life stages of rainbow trout in 

the Lees Ferry reach of the Colorado River: U.S. Geological Survey Open-File Report 2010-

1034, 31 p., accessed on November 1, 2010, at http://pubs.usgs.gov/of/2010/1034/. 

Leibfried, W.C., and Blinn, D.W., 1987, The effects of steady versus fluctuating flows on 

aquatic macroinvertebrates in Colorado River below Glen Canyon Dam, Arizona: Salt Lake 

City, Utah, Bureau of Reclamation, Glen Canyon Environmental Studies, Arizona Game and 

Fish Department contract no. 6400042, GCES/15/87. [Available from National 

Technical Information Service, Springfield, Va. as NTIS Report PB88–206362/AS.] 

Makinster, A.S., Persons, W.R., and Avery, L.A., 2011, Status and trends of the rainbow trout 

population in the Lees Ferry Reach of the Colorado River downstream from Glen Canyon 

Dam, Arizona, 1991–2009: U.S. Geological Survey Scientific Investigations Report 2011-

5015, 17 p., accessed on June 13, 2011, at http://pubs.usgs.gov/sir/2011/5015/. 

McKinney, T., Rogers, R.S., and Persons, W.R., 1999, Effects of flow reductions on aquatic 

biota of the Colorado River below Glen Canyon Dam, Arizona: North American Journal of 

Fisheries Management, v. 19, no. 4, p. 984–991, accessed on August 19, 2011, at 

http://www.tandfonline.com/doi/abs/10.1577/1548-8675(1999)019%3C0984:EOFROA%3E2.0.CO;2

McKinney, T., and Speas, D.W., 2001, Observations of size-related asymmetries in diet and 

energy intake of rainbow trout in a regulated river: Environmental Biology of Fishes, v. 61, no. 

4, p. 435-444, accessed on November 1, 2010, at 

http://www.springerlink.com/content/u177356554v07546/. 

McKinney, T., Speas, D.W., Rogers, R.S., and Persons, W.R., 2001, Rainbow trout in a 

regulated river below Glen Canyon Dam, Ariz., following increased minimum flows and 

reduced discharge variability: North American Journal of Fisheries Management, v. 21, p. 

216–222. 

Melis, T.S., Wright, S.A., Ralston, B.E., Fairley, H.C., Kennedy, T.A., Andersen, M.E., and 

Coggins, L.G., Jr., 2006, 2005 knowledge assessment of the effects of Glen Canyon Dam on 

the Colorado River ecosystem—an experimental planning support document—final draft: 

Flagstaff, Ariz., U.S. Geological Survey, Grand Canyon Monitoring and Research Center, 88 

p. [Available upon request by contacting the Center Director, U.S. Geological Survey, 

Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

Petering, R.W., Isbell, G.L., and Miller, R.L., 1995, A survey method for determining angler 

preference for catches of various fish length and number combinations: North American 

Journal of Fisheries Management, v. 15, no. 4, p. 732–735, accessed on August 23, 2011, at 

http://www.tandfonline.com/doi/abs/10.1577/1548-

8675(1995)015%3C0732%3AASMFDA%3E2.3.CO%3B2. 

Pulwarty, R.S. and Melis, T.S., 2001, Climate extremes and adaptive management on the 

Colorado River—lessons from the 1997–1998 ENSO event: Journal of Environmental 

Management, v. 63, no. 3, p. 307– 324, accessed on August 23, 2011, at 

http://www.sciencedirect.com/science/article/pii/S0301479701904943. 
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Radomski, P.J., Grant, G.C., Jacobson, P.C., and Cook, M.F., 2001, Visions for recreational 

fishing regulations: Fisheries, v. 26, no. 5, p. 7–18, accessed on August 23, 2011, at 

http://www.tandfonline.com/doi/abs/10.1577/1548-

8446(2001)026%3C0007%3AVFRFR%3E2.0.CO%3B2.

Rogers, R.S., Persons, W.R., and McKinney, T., 2003, Effects of a 31,000-cfs spike flow and 

low steady flows on benthic biomass and drift composition in the Lees Ferry tailwater: Arizona 

Game and Fish Department, cooperative agreement no. 1425–98–FC–22390, mod. 3, 26 p., 

accessed on August 23, 2011, at http://www.gcmrc.gov/library/reports/biological/Foodbase/

Rogers2003b.pdf. 

Schmidt, J.C., Topping, D.J., Rubin, D.M., Hazel, J.E., Jr., Kaplinski, M., Wiele, S.M., and 

Goeking, S.A., 2007, Streamflow and sediment data collected to determine the effects of low 

summer steady flows and habitual maintenance flows in 2000 on the Colorado River between 

Lees Ferry and Bright Angel Creek, Arizona: U.S. Geological Survey Open-File Report 2007–

1268, 79 p., accessed on March 17, 2010, at http://pubs.usgs.gov/of/2007/1268/.

Shannon, J.P., Benenati, E.P., Kloeppel, H., Richards, D., 2003, Monitoring the aquatic food 

base in the Colorado River, Arizona during June and October 2002—annual report: Flagstaff, 

Ariz., U.S. Geological Survey, Grand Canyon Monitoring and Research Center, cooperative 

agreement no. 02WRAG0028, 50 p. [Available upon request by contacting the Center 

Director, U.S. Geological Survey, Southwest Biological Science Center, 2255 N. Gemini 

Drive, Flagstaff, Ariz. 86001.] 

Shannon, J.P., Blinn, D.W., McKinney, T., Benenati, E.P., Wilson, K.P., and O'Brien, C., 2001, 

Aquatic food base response to the 1996 test flood below Glen Canyon Dam, Colorado River, 

Arizona: Ecological Applications, v. 11, no. 3, p. 672-685, accessed on January 11, 2010, at 

http://www.jstor.org/stable/3061109.

Speas, D.W., Persons, W.R., Rogers, R.S., Ward, D.L., Makinster, A.S., and Slaughter, J.E., IV, 

2004, Effects of low steady summer flows on rainbow trout in the Lee's Ferry tailwater, 2000: 

Phoenix, Arizona Game and Fish, submitted to U.S. Geological Survey, Grand Canyon 

Monitoring and Research Center, cooperative agreement no. 1425–98–FC–40–22690, 38 p., 

accessed on August 23, 2011, at http://www.gcmrc.gov/library/reports/biological/Fish_studies/ 

AZGame&Fish/2004/Speas2004.pdf. 

Stevens, L.E., Shannon, J.P., and Blinn, D.W., 1997, Colorado River benthic ecology in Grand 

Canyon, Arizona, USA--dam, tributary, and geomorphological influences: Regulated Rivers--

Research and Management, v. 13, no. 2, p. 129-149, accessed on December 1, 2010, at 

http://www3.interscience.wiley.com/cgi-bin/fulltext/11832/PDFSTART.

U.S. Department of the Interior, 1996, Record of Decision, Operation of Glen Canyon Dam Final 

Environmental Impact Statement: Washington, D.C., Office of the Secretary of Interior, 15 p. 

Valdez, R.A., Carothers, S.W., House, D.A., Douglas, M.E., Ryel, R.J., Bestgen, K.R., and 

Wegner, D.L., 2000, A program of experimental flows for endangered and native fishes of the 

Colorado River in Grand Canyon—final report: Flagstaff, Ariz., SWCA, Inc., Environmental 

Consultants, submitted to U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center, 68 p. [Available upon request by contacting the Center Director, U.S. Geological 

Survey, Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

Wong, F.L., Anima, R.J., Galanis, P., Codianne, J., Xia, Y., Bucciarelli, R., and Hamer, M., 

2003, Grand Canyon riverbed sediment changes—experimental release of September 2000—a 

sample data set: U.S. Geological Survey Open-File Report 03–0265, 7 p., accessed on March 

25, 2010, at http://geopubs.wr.usgs.gov/open-file/of03-265/.
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Yard, M.D., Blinn, D.W., 2001, Algal colonization and recolonization response rates during 

experimental low summer steady flows—final report: Flagstaff, Ariz., Northern Arizona 

University, submitted to U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center, 60 p. [Available upon request by contacting the Center Director, U.S. Geological 

Survey, Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 
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Chapter 7. Financial Costs Associated with the Low Steady 

Summer Flow Experiment

Introduction

Economic studies associated with the Low Steady Summer Flow (LSSF) experiment 

were limited to the financial costs of the experiment to Federal power users (Palmer and others, 

unpub. report, 2004; Harpman and Douglas, 2005; Veselka and others, unpub. report, 2011), 

recreational businesses, and other businesses that support recreational businesses (for example, 

restaurants, motels) (Hjerpe and Kim, unpub. report, 2003). Financial cost associated with the 

Glen Canyon Dam and the Colorado River include the cost of purchasing power to meet daily 

electrical demands and revenues and jobs associated with the recreational and tourism industries 

(Harpman and Douglas, 2005). Several elements of the LSSF hydrograph contributed to the 

financial costs to power users and recreational businesses. 

When and how much water was released, how water was reallocated to accommodate the 

experiment, the steady release pattern, and uncontrollable nature of the power market all 

contributed to the financial costs of the experiment. The movement of monthly water allocations 

into early March and later into November 2000 to accommodate the lower monthly volumes in 

the summer affected water volumes available to generate power when it is in greatest demand 

(summer months). The sustained high flows in April and May and the habitat maintenance flow 

in May and September affected power production and recreational access in Glen Canyon. The 

reduced discharge in Period II reduced power-generating capacity, increased damage to boats 

traveling downstream, and initially increased rafting accidents (Jalbert, unpub. report, 2009). The 

drought and the associated energy crisis in California and the Western United States during the 

summer of 2000 (Federal Energy Regulatory Commission, 2003; Harpman and Douglas, 2005; 

Hjerpe and Kim, unpub. report, 2003; Korman and others, 2004) increased the demand for power 

and increased the cost of power. Lastly, energy market manipulation by some energy suppliers in 

2000 (Federal Energy Regulatory Commission, 2003) also contributed to the inflated financial 

costs associated with the LSSF experiment.

Financial Costs to Federal Power Users

The Western Area Power Administration (Western) is a power marketing administration 

within the U.S. Department of Energy that sells power to preferred and non-preferred customers. 

Western is responsible for selling and delivering electricity that is in excess of Colorado River 

Storage Project (CRSP) use (power required to operate irrigation projects). Western sells power 

first to municipalities and public corporations or other organizations associated with the Rural 

Electrification Act of 1936 (Palmer and others, unpub. report, 2004; Harpman and Douglas, 

2005). These entities constitute preferred customers. Non-preferred customers can purchase any 

excess power supply. Western purchases power on the daily market to meet demand of their 

preferred customers when power demands exceed Western’s power supplies (Palmer and others, 

unpub. report, 2004). Because of the long-term relationship that exists between the Bureau of 

Reclamation and Western with respect to water delivery and power generation, the power 
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generated from the CRSP hydroelectric dams is predictable. Western can identify when the 

purchase of supplemental power is required on the basis of the long-term average consumption 

of preferred customers. The predictability associated with water delivery, power generation, and 

preferred customer demand allows Western to purchase power in seasonal blocks at a firm price 

rather than at the daily price, which can vary depending on demand. 

Western’s costs began to increase for the experiment with the reallocation of water 

releases to alternate months to accommodate the 2000 LSSF experimental hydrograph. Monthly 

water allocations needed to be reapportioned by the Bureau of Reclamation, the agency 

responsible for water delivery along the Colorado River. To meet water-delivery requirements 

associated with the Colorado River Compact signed in 1922, which guarantees delivery of 8.23 

maf of water to the lower basin states (Arizona, California, Nevada, New Mexico) and Mexico, 

the Bureau of Reclamation released an extra 604,000 acre-ft of water from Glen Canyon Dam in 

early March 2000. This release occurred during a time of reduced power demand and contributed 

to the financial costs of the experiment because the water was not available to generate power in 

months of higher demand (Harpman and Douglas, 2005). The loss of potential power generation 

in 2000 presumably would be recovered in water year (WY) 2001 when inflows would return to 

those observed in the 1990s. The higher releases in WY 2001, particularly if the releases 

occurred in the summer months would compensate for the excess water released in March 2000 

during lower power demand. Unfortunately, the low snowpack and associated runoff forecast 

that began in WY 2000 that initiated LSSF experiment continued through the next decade and 

equalization did not occur until the spring of 2011. The extra water releases in March 2000 and 

the reallocation of water to November 2000 (another month associated with low power demand) 

are a financial cost that Western did not recover (Palmer and others, unpub. report, 2004). 

The steady discharges during Period II reduced power-generating capacity, and Western 

was required to purchase power on the daily market. At the same time, the drought and extreme 

heat in the Southwest and West increased power demand in the region and caused the cost of 

power to increase substantially. Because of the short notice associated with implementing the 

LSSF experiment, Western could not anticipate the need to purchase additional power beyond 

the usual seasonal block approach traditionally used (Palmer and others, unpub. report, 2004). 

The short notice associated with the implementation of the experimental plan affected planning 

for power purchases and the drought increased power demands, which, in turn, increased the spot 

market price; these factors contributed to the financial cost of the experiment. 

The combined effect of shifting water releases to alternate months and reduced powergeneration capacity in the summer of 2000 for the LSSF experiment resulted in estimated 

financial costs to Federal power users of approximately $32 million (2000 dollars; Palmer and 

others, unpub. report, 2004; Harpman and Douglas, 2005). Revised financial analysis (Veselka 

and others, unpub. report, 2011) reduced the final costs to $26.4 million from $32 million. The 

cost estimate was reduced because the experimental releases in WY 2000 (HMFs) resulted in a 

$410,000 in increased power revenue, which reduced the overall cost of the LSSF experiment 

(Veselka and others, unpub. report, 2011). 

Financial Costs to Recreation

The LSSF experiment incurred some financial costs to whitewater rafting companies 

operating downstream from Lees Ferry and fishing guides operating in Glen Canyon (Hjerpe and 

Kim, unpub. report, 2003), but the two groups were affected by different parts of the hydrograph. 

The LSSF hydrograph did not affect day rafting in Glen Canyon because discharge rates do not 
 89

affect this recreational industry (Bishop and others, 1987). The May and September habitat 

maintenance flows (HMFs) affected fishing guides in Glen Canyon by limiting upstream access. 

The fishing guides did not operate for 4 days in May and September (Hjerpe and Kim, unpub. 

report, 2003). Fishing guides reported a combined loss of $33,000 because of an inability to 

conduct trips (Hjerpe and Kim, unpub. report, 2003). Because the fall HMF occurred during a 

holiday weekend in September, the loss of business may have been greater than if the HMF had 

occurred during the week or later in the month. Throughout the summer months, the fishing 

guides reported improved fishing; however, the increase in business was proportional to 

increases in the previous 5 years and could not be attributed conclusively to the LSSF 

experimental hydrograph (Hjerpe and Kim, unpub. report, 2003). 

Financial costs to whitewater rafting companies occurred during Period II, which 

included lower discharges. Jalbert (unpub. report, 2009) identified several boating trips that 

experienced stranding in rapids and required rescue early in Period II. These boating incidents 

contributed to the loss or damage of rafting equipment and injury to passengers. Most equipment 

damage resulted in motor damage when propellers and motor units hit exposed rocks. Hjerpe and 

Kim (unpub. report, 2003) attribute some of the loss to equipment inflexibility: large commercial 

boats have difficulty maneuvering through some rapids at low water. However, these boats were 

used before changes in minimum discharges that occurred in 1996 (U.S. Department of the 

Interior, 1996), when discharge volumes fluctuated between 84 and 877 m3/s daily. It is more 

likely that boat operators were unaccustomed to the lower discharges and their inexperience 

contributed to equipment damage. As time passed, boat stranding decreased, suggesting that boat 

operators learned to avoid exposed rocks (Jalbert, unpub. report, 2009). 

Increased fuel costs associated with boats moving slower during low discharges and 

requiring more motor support (Roberts and Bieri, unpub. report, 2001) also contributed to costs. 

The cost of personal injury and the reimbursement of trip costs to commercial passengers 

because of evacuation were other costs incurred by whitewater rafting companies. Hjerpe and 

Kim (unpub. report, 2003) estimated that the total regional cost of the LSSF was $124,000 (1998 

dollars), including $70,000 for the combined costs to whitewater rafting companies (for example, 

damaged equipment and customer refunds) and a total expenditure of $30,000 by the National 

Park Service to evacuate stranded rafting trips. This estimate does not include the financial 

impacts to businesses such as lodging, health care, and restaurants that benefit from recreational 

economic activity that may have been affected by the experiment. 

Conclusions

The LSSF experiment resulted in substantial financial costs to Federal power users and 

local businesses directly related to Colorado River resources (Hjerpe and Kim, unpub. report, 

2003; Palmer and others, unpub. report, 2004; Harpman and Douglas, 2005; Veselka and others, 

unpub. report, 2011). The Bureau of Reclamation’s need to implement the LSSF experiment to 

meet the requirement’s of the Biological Opinion issued by the U.S. Fish and Wildlife Service in 

a short period of time reduced Western’s ability to purchase additional power in seasonal blocks 

that might have reduced the financial costs of the experiment. Instead, Western had to purchase 

power on the daily market, which was subject to unforeseen price manipulations by power 

suppliers (Federal Energy Regulatory Commission, 2003). The timing of the HMF for a holiday 

weekend may have had a greater effect on fishing guides than if it occurred in the middle of the 

week. On the basis of the responses by fishing guides to queries during the summer of 2000, it 

appeared that trout in the Lees Ferry reach were not affected by the high spring flows (Hjerpe 
 90

and Kim, unpub. report, 2003). Evaluating only the financial costs of the LSSF experiment limits 

the understanding of the total economic costs and benefits of the experiment. 

References

Bishop, R.C., Boyle, K.J., Welsh, M.P., Baumgartner, R.M., and Rathbun, P.R., 1987, Glen 

Canyon Dam releases and downstream recreation—an analysis of user preferences and 

economic values: Washington, D.C., Huberlein-Baumgartner Research Service, submitted to 

Bureau of Reclamation, Upper Colorado Region, Glen Canyon Environmental Studies, 

contract no. 4–CS–40–01780, technical report no. GCES/27/87, 398 p. 

Federal Energy Regulatory Commission, 2003, Final report on price manipulation in western 

markets, fact-finding investigation of potential manipulation of electric and natural gas prices: 

Department of Energy, Federal Energy Regulatory Commission, docket no. PA02–2–000, 189 

p., accessed on August 23, 2011, at http://www.ferc.gov/legal/maj-ord-reg/land-docs/PART-I3-26-03.pdf. 

Harpman, D.A., and Douglas, A.J., 2005, Status and trends of hydropower at Glen Canyon Dam, 

in Gloss, S.P., Lovich, J.E., and Melis, T.S., eds., The state of the Colorado River ecosystem in 

Grand Canyon: U.S. Geological Survey Circular 1282, 165–176 p., accessed on May 10, 2010, 

at http://pubs.usgs.gov/circ/1282/.

Hjerpe, E., and Kim, Y.-S. 2003, Economic impacts of summer steady flows on recreation: 

Flagstaff, Northern Arizona University, School of Forestry, 21 p. [Available upon request by 

contacting the Center Director, U.S. Geological Survey, Southwest Biological Science Center, 

2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

Jalbert, L.M., 2009, The effects of the 2000 Low Steady Summer Flows on observed and 

reported boating accidents on the Colorado River in Grand Canyon National Park. Final report 

from Grand Canyon National Park to U.S. Geological Survey, Grand Canyon Monitoring and 

Research Center, Flagstaff, Ariz., 25 p. [Available upon request by contacting the Center 

Director, U.S. Geological Survey, Southwest Biological Science Center, 2255 N. Gemini 

Drive, Flagstaff, Ariz. 86001.] 

Korman, J., Wiele, S.M., and Torizzo, M., 2004, Modelling effects of discharge on habitat 

quality and dispersal of juvenile humpback chub (Gila cypha) in the Colorado River, Grand 

Canyon: River Research and Applications, v. 20, no. 4, p. 379-400, accessed on February 22, 

2010, at http://www3.interscience.wiley.com/cgi-bin/fulltext/107614374/PDFSTART.

Palmer, S.C., Burbidge, C., and Patno, H., 2004, The financial impacts of the low summer steady 

flow experiment at Glen Canyon Dam: Salt Lake City, Utah, U.S. Department of Energy, 

Western Area Power Administration, CRSP Management Center, 32 p., accessed on August 

23, 2011, at http://www.gcmrc.gov/library/reports/Other/GlenCanyonDam/Palmer2004b.pdf. 

Roberts, C.A., and Bieri, J.A., 2001, Impacts of low flows on recreational rafting traffic on the 

Colorado River in Grand Canyon National Park—final report: Flagstaff, Ariz., submitted to 

Bureau of Reclamation and U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center, cooperative agreement no. CA8210–99–002, interagency acquisition no. 00–AA–40–

4330, 18 p., accessed on August 23, 2011, at 

http://www.gcmrc.gov/library/reports/cultural/Recreation/roberts2001.pdf. 

U.S. Department of the Interior, 1996, Record of Decision, Operation of Glen Canyon Dam Final 

Environmental Impact Statement: Washington, D.C., Office of the Secretary of Interior, 15 p. 
 91

Veselka, T.D., Poch, L.A., Palmer, C.S., Loftin, S., and Osiek, B., 2011, Revised financial 

analysis of experimental releases conducted at Glen Canyon Dam during water years 1997 

through 2005: U.S. Department of Energy, Western Area Power Administration, ANL/DIS-11-

1, 83 p., accessed on August 10, 2011, at www.ipd.anl.gov/anlpubs/2011/01/68980.pdf. 
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Chapter 8. Management Implications Associated with the 

Low Steady Summer Flow Experiment

Introduction

The purpose of the Low Steady Summer Flow (LSSF) experiment was to evaluate factors 

that limit the growth and survival of young native fish (for example, habitat stability and water 

temperatures). Spring and summer operations of Glen Canyon Dam were altered in 2000 to 

mimic seasonal hydrology that might benefit native fish more than Modified Low Fluctuating 

Flow (MLFF) operations. It was presumed that mimicking elements of the predam hydrograph 

(for example, high releases in the spring and low releases in the summer that did not fluctuate) 

would present more hospitable habitats for native fish than MLFF operations and increase young 

fish survival, growth, and subsequent recruitment. 

Knowledge gained by fishery and physical resource studies executed during the LSSF 

experiment are reviewed in the context of the hypotheses that were generated by Valdez and 

others (unpub report, 2000; table 8-1). The responses of other resources not identified by Valdez 

and others (unpub. report, 2000) to the LSSF experiment appear in table 8-2. The last column in 

tables 8-1 and 8-2 provides a rating (for example, good, fair, poor, and inconclusive) of the 

effectiveness of a study to assess the resource response to the LSSF hydrograph. In general, the 

responses of physical resources to the LSSF experimental hydrograph were adequately 

understood, but the biological and cultural resource responses were less definitive. The studies 

during Period I are often ranked “poor” because the resource response was not studied (tables 8-

1 and 8-2). Studies conducted during Period II for fish resources are ranked “poor” because 

monitoring data before the experiment were absent (table 8-1). The immediate response of 

physical resources to discharges makes it easier to link the effect of a flow treatment to the 

resources (Melis and others, 2006). In contrast, the response of biological resources to a change 

in the hydrology of a single season cannot be assessed immediately and may require several 

years to detect a recruitment response, as in the case of native fish (Coggins and Walters, 2009). 

These delayed responses by biological resources make it more difficult to link the effect of a 

flow treatment to the resource response. 

The variability of flow within the LSSF hydrograph hampered the effectiveness of studies 

to assess resources responses. The hydrograph may have been classified as a steady flow 

discharge, but it was composed of periods with different steady discharges that varied in 

discharge and duration (for example, individual steady flows ranged from 4 days to 12 weeks) 

(fig. 8-1). During the LSSF experiment, the longest uninterrupted period of persistent habitat for 

young-of-year (YOY) fish was 3 months. YOY fish that entered the mainstem in mid-July (for 

example, humpback chub (Gila cypha) had a shorter exposure to persistent habitat. The 

variability in the duration and magnitude of the discharge complicated drawing conclusions 

about resource responses to “steady discharges.” Uncertainty in resource response to the 

variable discharges is particularly true for fish populations that have been documented to 

improve under persistent habitat conditions (Travnichek and others, 1995; Freeman and others, 

2001; Korman, 2009). The LSSF experimental hydrograph might not have provided persistent 

habitat for the target resource: YOY humpback chub in the mainstem. 
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Table 8.1. Responses of physical and aquatic resources of the Colorado River in Grand Canyon, Arizona, to the Low Steady Summer Flow 

experimental flows in 2000, and an evaluation of the effectiveness of studies to provide information about resource responses. 

Potential benefits to 

physical resources

Potential risks to 

physical resources

Resource response Effectiveness of 

studies in 

measuring 

physical-resource 

response 

Potential benefits to 

aquatic resources

Potential risks to 

biological 

resources

Resource response Effectiveness of 

studies in 

measuring 

biological-resource 

response

Period I: March to May

Scour, backwaters, 

eddies, and 

increase in area for 

shoreline fish 

habitat

No hypotheses 

offered

Inferred backwater 

scoured by 

increased sand 

volumes in midelevation sandbars 

(Goeking and 

others, unpub. 

report, 2003; Hazel 

and others, 2006; 

Schmidt and 

others, 2007). 

FAIR 

Backwaters 

measured only 

following 

September habitat 

maintenance flow 

during somewhat 

enriched sediment 

conditions 

compared with 

May habitat

maintenance flow

Ponding as thermal 

refuges for drifting 

larvae and young 

fish

Warm, ponded 

tributaries attract 

nonnative fish 

predators/

competitors

Not studied POOR 

Not studied

Mobilize and store 

sand and sediment 

in 

campsites/increase 

campsite area

Export sediment, 

reduce campsite 

areas

Sediment was 

exported and 

habitat 

maintenance flow 

executed during 

low sediment 

inputs (similar to 

1996 high flow 

experiment) 

(Schmidt and 

others, 2007)

GOOD

Sediment response 

is instantaneous 

compared with 

biological 

resources (Melis 

and others, 2006). 

Sediment 

monitoring 

measured 

deposition and 

export of sediment

Destabilize habitats 

to disadvantage 

nonnatives

No hypotheses 

offered

Not studied POOR 

Not studied
 96

Table 8.1. Responses of physical and aquatic resources of the Colorado River in Grand Canyon, Arizona, to the Low Steady Summer Flow 

experimental flows in 2000, and an evaluation of the effectiveness of studies to provide information about resource responses.—Continued

Potential benefits to 

physical resources

Potential risks to 

physical resources

Resource response Effectiveness of 

studies in 

measuring 

physical-resource 

response 

Potential benefits to 

aquatic resources

Potential risks to 

biological 

resources

Resource response Effectiveness of 

studies in 

measuring 

biological-resource 

response

Create warm, lowvelocity areas at 

tributary mouths

No hypotheses 

offered

Increasing 

mainstem 

discharge 

decreases lowvelocity, warm 

water areas at 

Little Colorado 

River mouth 

(Protiva and others, 

2010)

GOOD

Steady discharges 

permitted 

measurement of 

velocities at 

multiple stage 

elevations

Redistribute 

nutrients

No hypotheses 

offered

Not studied POOR 

Not studied

No hypotheses 

offered

No hypotheses 

offered

Not applicable Not applicable Reset community 

primary production

No hypotheses 

offered

Primary production 

was affected 

minimally in Lees 

Ferry fishery 

(Rogers and others, 

unpub. report, 

2003)

FAIR

Studies conducted 

in Lees Ferry 

fishery, but no 

studies associated 

with downstream 

shoreline habitats

May Habitat 

Maintenance Flow 

flush nonnative 

fish from nearshore 

habitats

More native fish in 

June backwaters 

(Trammell and 

others, unpub. 

report, 2002)

POOR 

inconclusive; data 

not collected 

before Habitat 

Maintenance Flow 

to determine if 

abundance 

decreased. 
 97

Table 8.1. Responses of physical and aquatic resources of the Colorado River in Grand Canyon, Arizona, to the Low Steady Summer Flow 

experimental flows in 2000, and an evaluation of the effectiveness of studies to provide information about resource responses.—Continued

Potential benefits to 

physical resources

Potential risks to 

physical resources

Resource response Effectiveness of 

experiment to 

measure response

Potential benefits to 

aquatic resources

Potential risks to 

biological 

resources

Resource response Effectiveness of 

experiment to 

measure response

Period II: June to September

Store sediment in 

river channel

No hypothesis 

offered

Few sediment 

inputs in Period II, 

but some sediment 

stored with Period 

II flows (Schmidt 

and others, 2007). 

habitat 

maintenance flow 

moved sediment to 

mid-elevation 

sandbars

GOOD

Sediment response 

is instantaneous 

compared with 

biological 

resources (Melis 

and others, 2006). 

Identifies trade-off 

between habitat 

maintenance flow 

and a high flow 

experiment with 

respect to 

magnitude and 

amount of 

sediment 

conservation.

Increase growth 

and survival of 

young native fishes

Increase growth 

and survival of 

nonnative fishes 

No difference with 

previous years, 

though comparing 

growth with data 

from previous 

years may be 

problematic

POOR Duration of 

steady flows was 

interrupted by 

habitat 

maintenance flow. 

Expand campable 

beach area

No hypothesis 

offered

Low stage 

elevation expanded 

campsites and 

provided stable 

campsite area 

(Roberts and Bieri, 

unpub. report, 

2001).

GOOD

Sediment 

area/volume 

monitoring 

identified changes 

in mid-elevation 

sandbars. 

Recreation studies 

identified benefits 

of low steady 

discharges on 

camping 

experience

Increase 

autotrophic algal 

and macro 

invertebrate 

production

Decrease drift of 

food for fish

Chironomid 

production was 

greater in July 

2000 than previous 

years; other 

invertebrate 

standing mass was 

similar (Rogers 

and others, unpub. 

report, 2003)

POOR

Data for 

comparisons were 

limited by 

frequency of 

sampling in 

previous years and 

discharges in 

previous years
 98

Table 8.1. Responses of physical and aquatic resources of the Colorado River in Grand Canyon, Arizona, to the Low Steady Summer Flow 

experimental flows in 2000, and an evaluation of the effectiveness of studies to provide information about resource responses.—Continued

Potential benefits to 

physical resources

Potential risks to 

physical resources

Resource response Effectiveness of 

experiment to 

measure response

Potential benefits to 

aquatic resources

Potential risks to 

biological 

resources

Resource response Effectiveness of 

experiment to 

measure response

September habitat 

maintenance flow 

resuspend and 

store sand from 

summer tributary 

inputs

September habitat 

maintenance flow 

export sand and 

sediment instead of 

storing it

Sediment stored as 

well as exported 

(Hazel and others, 

2006; Schmidt and 

others, 2007).

GOOD

Sediment 

area/volume 

monitoring 

identified changes 

in mid-elevation 

sandbars. Trade-off 

between discharge 

and sediment 

storage.

Possible mainstem 

hatching success

Mainstem 

reproduction by 

nonnative fishes

Some indication of 

protracted 

spawning though 

source of fish less 

than 30 millimeters 

long unknown.

Mainstem adult 

rainbow trout 

numbers were 

greatest in 2000 

and 2001 

(Makinster and 

others, 2010). 

POOR 

Methods to study 

larval origin and 

monitoring 

abundance are not 

well developed.

POOR

Population 

estimates or catch 

per unit data were 

unavailable for 

rainbow and brown 

trout before Low 

Steady Summer 

Flows experiment 

to determine 

downstream 

effects.

Create stable 

shorelines for fish 

habitat

No hypothesis 

offered

No data POOR

Not studied

September habitat 

maintenance flow 

may flush 

nonnative fishes 

from nearshore 

habitats

Minimizing 

thermal plume at 

springs located at 

river mile 30 may 

reduce survival of 

young humpback 

chub

Habitat 

Maintenance Flow 

effect on 

nonnatives 

inconclusive

POOR

Abundances of 

nonnative fish 

were the same or 

less than native 

fishes (Trammell 

and others, unpub. 

report, 2002) 
 99

Table 8.1. Responses of physical and aquatic resources of the Colorado River in Grand Canyon, Arizona, to the Low Steady Summer Flow 

experimental flows in 2000, and an evaluation of the effectiveness of studies to provide information about resource responses.—Continued

Potential benefits to 

physical resources

Potential risks to 

physical resources

Resource response Effectiveness of 

experiment to 

measure response

Potential benefits to 

aquatic resources

Potential risks to 

biological 

resources

Resource response Effectiveness of 

experiment to 

measure response

No other 

hypotheses

No other 

hypotheses

Not applicable Not applicable No other 

hypotheses

Increased water 

clarity leading to 

increased predation 

of native fish by 

sight predators

POOR Stomach 

analysis of 

predators was 

limited, smallsized fishes leave 

few parts for 

identification. 

Subsequent data 

collection and 

correlation with 

population 

estimates of 

rainbow and brown 

trout 2000 (Speas 

and others, unpub. 

report, 2004; Yard 

and others, 2011) 

could suggest a 

high probability of 

predation occurred 

in 2000 and 

contributed to a 

lack of response by 

native fish.

No other 

hypotheses

No other 

hypotheses

Not applicable Not applicable No other 

hypotheses

Increased 

infestation of 

parasites and 

diseases

not studied POOR

No data were 

collected
 100

Table 8.1. Responses of physical and aquatic resources of the Colorado River in Grand Canyon, Arizona, to the Low Steady Summer Flow 

experimental flows in 2000, and an evaluation of the effectiveness of studies to provide information about resource responses.—Continued

Potential benefits to 

physical resources

Potential risks to

physical resources

Resource response Effectiveness of 

experiment to 

measure response

Potential benefits to 

aquatic resources

Potential risks to 

biological 

resources

Resource response Effectiveness of 

experiment to 

measure response

Period III: October to February 

Retain sediment 

with low-volume 

discharge

No data Not studied and 

returned to 

Modified Low 

Fluctuating Flow 

operations

POOR 

Not implemented

Increased survival 

of young native 

fishes

Possible 

overwinter survival 

and expansion of 

nonnative fishes

Not studied and 

returned to 

Modified Low 

Fluctuating Flow 

operations

POOR 

Not implemented

No other 

hypotheses

No other 

hypotheses

Not applicable Not applicable Maintain stable 

winter conditions 

to minimize energy 

expenditure

Possible greater 

spawning success 

of downstream 

populations of 

trout

Not studied and 

returned to 

Modified Low 

Fluctuating Flow 

operations

POOR 

Not implemented 

Returned to 

Modified Low 

Fluctuating Flow 

operations

No other 

hypotheses

No other 

hypotheses

Not applicable Not applicable Maintain 

overwinter 

autotrophic 

production in 

mainstem, 

shorelines, and 

backwaters

Decreased drift of 

food for fish

Not studied and 

returned to 

Modified Low 

Fluctuating Flow 

operations

POOR 

Not implemented 

Returned to 

Modified Low 

Fluctuating Flow 

operations

No other 

hypotheses

No other 

hypotheses

Not applicable Not applicable No other 

hypotheses

Increased predation 

by sight feeders

Not studied and 

returned to 

Modified Low 

Fluctuating Flow 

operations

POOR 

Not implemented 

Returned to 

Modified Low 

Fluctuating Flow 

operations
 101

Table 8-2. Responses of other than physical and aquatic resources of the Colorado River in Grand 

Canyon, Arizona, to the Low Steady Summer Flow experimental flows in 2000 and an evaluation of the 

effectiveness of studies to provide information about resource responses. 

Resource Anticipated resource 

response Observed resource response Effectiveness of studies in measuring resource response

Period I: March to May

Riparian Vegetation Reworking of 

shoreline habitats

Scour of vegetation until 

sustained high-flow stage 

elevation

POOR

No data collection before 

sustained flows. Observation 

began at beginning of Period 

II. Insufficient time available 

to collect Period I data

Recreation

Safety

Accidents associated 

with May habitat 

maintenance flow

Unknown with respect to safety POOR

Not studied. Insufficient 

time available to collect 

Period I data

Recreation

Perceptions/Satisfaction 

(Angling/Whitewater 

Rafting)

No response 

anticipated

Unknown with respect to rafting 

experience.

Fishing trips did not occur 

during habitat maintenance flow

POOR

Not studied. Insufficient 

time available to collect 

Period I data

Economics 

Angling/Whitewater 

Rafting

No response 

anticipated

Angling—Access limited during 

habitat maintenance flow and 

revenues reduced.

Whitewater Rafting—not 

studied

FAIR

Angling; contacted fishing 

services but did not evaluate 

in context of fish population 

condition.

POOR

Whitewater Rafting; not 

studied. 

Economics 

Power Revenues

Shift in extra power 

generation to lower 

power demand 

months—no preanalysis available

Revenues increased with May 

habitat maintenance flow, but 

extra water released in March 

resulted in financial loss because 

of subsequent minimum delivery 

years associated with ongoing 

drought.

FAIR

Only post-hoc analysis 

available and multiple 

factors (climate, market 

manipulation) affected 

financial outcome

Period II: June to September

Riparian Vegetation No effects anticipated Extensive tamarisk seedling 

establishment. September HMF 

reduced stem densities 

GOOD

Steady flows timed in June 

before other plant flowering 

insured a positive response 

by tamarisk
 102

Table 8-2. Responses of other than physical and aquatic resources of the Colorado River in Grand 

Canyon, Arizona, to the Low Steady Summer Flow experimental flows in 2000 and an evaluation of the 

effectiveness of studies to provide information about resource responses.—Continued

Resource Anticipated resource 

response Observed resource response Effectiveness of studies in measuring resource response

Recreation

Safety

No effects anticipated Some accidents early in season 

because of inexperience with 

lower discharges

 

FAIR

Unpublished report from 

National Park Service 

referencing other 

unpublished reports

Recreation

Perceptions/Satisfaction 

(Angling/Whitewater 

Rafting)

No effects anticipated Angling; Steady discharge 

provided more access to anglers 

without boats. Drift was at lower 

concentrations and may have 

affected angler satisfaction 

Whitewater rafting; Perceptions 

affected by multiple inputs with 

flows a minor component.

Hiking options changed.

Camps available at low 

discharge used, which reduced 

crowding issues. 

POOR

Angling; minor studies 

evaluated angling 

satisfaction. Difficult to 

assess because of continued 

decline of fishery that began 

before Low steady summer 

flow experiment

FAIR

Whitewater rafting; Multiple 

studies assess single aspects 

of low discharge and 

whitewater rafting. Did not 

provide a multidimensional 

evaluation of the effect the 

LSSF hydrograph had on the 

rafting experience

Economics 

Whitewater 

Rafting/Angling

No effects anticipated Whitewater rafting;

Financial costs with equipment 

replacement and trip refunds

Angling; Loss of revenue during 

September habitat maintenance 

flow

FAIR

Analysis of longer-term 

economic effect would be 

useful – How does LSSF 

experiment affect financial 

costs in subsequent years? 

Economics 

Power Revenues

Reduction in power 

generation

Reduced power generation and 

spot market costs to meet 

consumer needs were inflated 

because of California energy 

crisis and price manipulation in 

summer 2000 –an unanticipated 

cost.

FAIR

Post-hoc analysis was good. 

Sufficient time for a preexperiment analysis to 

estimate costs and possibly 

refine/refocus experimental 

efforts, which would have 

been helpful.

Implementing flows intended to mimic natural conditions is challenging in river systems 

that are constrained by water-delivery requirements and the needs of multiple resources that can 

be in conflict. The magnitude and duration of discharges required for a desired response by one 

resource may be in direct conflict with the desired responses by other resources. Yet approaching 

experimentation in a manner that mitigates for effects to resources can produce a weak 

experimental design and a poor or undetectable response by the target resource (Webb and 

others, 2010). The resulting information can be inconclusive for some resources (tables 8-1 and 

8-2), but costly to others, and can promote the perception that ecological experiments are costly, 
 103

yield little benefit to the target resources, and thus fail to demonstrate the benefits of an 

experimental flow. Follow-up cost/benefit evaluations might suggest that the experiment does 

not merit repeating. Success of an ecosystem-scale experiment is limited further if time for 

planning an experiment and associated monitoring is reduced, as occurred with the LSSF 

experiment in 2000. 

Figure 8-1. Actual release pattern of the experimental flow period from March to October 2000 (dotted 

line) compared with the proposed experimental hydrograph (solid line) from Valdez and others (unpub. 

report, 2000). 

The LSSF experimental hydrograph proposed by Valdez and others (unpub. report, 

2000), and subsequently modified and implemented in 2000, is an example of an experiment that 

was modified to ameliorate potential negative resource effects and resulted in compromising the 

effectiveness of the experiment to test response of the target resource (humpback chub). The 

spring discharges, limited by water allocations, were only 25 percent greater (U.S. Geological 

Survey, 2010) than the average MLFF discharges in the 1990s and 78-percent smaller than the 

average predam spring discharge of approximately 2,407 m3/s (Topping and others, 2003). The 

changes in discharge associated with the experimental hydrograph were likely too small 

compared to standard operations to observe a response that differed from previous years. Though 

overall discharges in Period I were steady, they varied in magnitude between April and May and 

failed to provide a consistent, elevated discharge that was greater than average discharges 

observed in the 1990s. Further, the sustained high flows occurred in a year of few sediment 
 104

inputs in the winter, resulting in sediment export from the Grand Canyon reach (Schmidt and 

others, 2007), a negative resource response. 

For the native fishes that persist in the Colorado River downstream from Glen Canyon 

Dam, the YOY life stage is affected by the Colorado River mainstem hydrology, whereas earlier 

life stages (for example, larvae and eggs) are affected by tributary hydrology (Gorman and 

Stone, 1999; Robinson and others, 1998). The bulk of YOY fish enter the mainstem from 

tributaries in the summer months, with humpback chub YOY entering the mainstem primarily in 

association with monsoons that begin in July (Valdez and Ryel, unpub. report, 1995; Valdez and 

Carothers, unpub. report, 1998; Robinson and others, 1998). It is more likely that dam operations 

can be used to affect YOY and later life stages of native fish because of their mainstem habits, as 

opposed to larval fish that are found more often in tributaries. 

Although Period I might be characterized by a failure to reach discharge magnitude 

sufficiently different from average discharges, Period II might be characterized by a failure to 

reach a duration of sufficient length to measure a response. The duration of the low steady flow 

part of the experiment was interrupted by a September HMF and was suspended at the end of 

September. The September HMF was implemented to reduce the negative effects of exotic fishes 

in the system. The suspension of steady discharges in October was to mitigate for financial costs 

to power resources incurred during the summer months (Palmer and others, unpub. report, 2004). 

Because humpback chub first enter the Colorado River in July from tributaries, it is 

difficult to expect that measurements of growth or abundance would have been noticeably 

different from previous years. In 2000, most YOY humpback chub likely entered the mainstem 

in July and August, just as in other years (Valdez and Ryel, unpub. report, 1995; Valdez and 

Carothers, unpub. report, 1998; Robinson and others, 1998). The humpback chub length data 

collected near the Little Colorado River (LCR) lends support to this observation (Trammell and 

others, unpub. report, 2002). These fish would have been under a steady-flow discharge pattern 

for a month or 6 weeks before an HMF. The HMF disrupted the habitat stability intent of the 

steady flows. A month following the September HMF, the fish were exposed to MLFF 

operations that were similar to those implemented since 1990. Further, the greater predation 

threat to native fish is adult salmonids (brown (Salmo trutta) and rainbow trout (Oncorhynchus 

mykiss)) that are affected minimally by discharges that reach 1,412 m3/s (Hoffnagle and others, 

1999; McKinney and others, 1999; Hilwig and Makinster, 2010). The timing and duration of 

steady flows for the target species may have been too early (Korman and others, 2004) and too 

brief a period to be effective. The implementation of the late season HMF, which was 

incorporated to disadvantage nonnative species (Valdez and others, unpub report, 2000), when 

young fish are using the mainstem shoreline, disrupted stable habitats and closely mimicked noaction discharges that brought about the changes to dam operations in the 1990s (U.S. 

Department of the Interior, 1995). The return to fluctuating flows to help alleviate the negative 

financial effects of Period II (Palmer and others, unpub. report, 2004; Harpman and Douglas, 

2005) shortened the duration of stable habitats for YOY fishes. Both measures diluted the ability 

to observe a response by native fish to stabilized habitats. 

Planning for effective high-magnitude discharges for ecological experiments is 

challenging in a regulated system (Webb and others, 1999; Patten and Stevens, 2001; Webb and 

others, 2010). The presence of a dam restricts flow magnitude, and a reservoir system designed 

for water storage further restricts annual discharges. However, a change in discharge magnitude 

is the most common element associated with regulation, and fish appear to be most sensitive to 

this variable (Poff and others, 2009). The life-history stages of fish are integrally tied to seasonal 
 105

discharges and associated thermal conditions and, as a result, are sensitive to changes in the 

timing rates of discharge (Humphries and others, 1999; Marchetti and Moyle, 2001). Identifying 

a single life stage that is associated with a particular discharge and that limits recruitment in an 

endangered fish species is difficult. Colorado River native fishes are long-lived and the year-toyear recruitment variability may be affected by conditions in tributaries and the mainstem. 

Identifying the linkage between discharge and recruitment limitations after a single-season flow 

treatment is unrealistic. Determining the effectiveness of a single-year experimental hydrograph 

becomes particularly difficult when the variables intended to be changed (optimum water 

temperatures and long-duration steady discharges) are not achievable at key locations within the 

river system at the time of the experiment. 

Lessons Learned and Considerations for Experimental Planning

The results associated with the LSSF experiment were inconclusive for the target 

resource: humpback chub. In some cases, the responses of ancillary resources were more 

definitive (for example, sediment, recreational whitewater rafting), but the responses of these 

resources are more immediate and potentially more easily quantifiable than early life stages of 

fish. For informative flow experiments, the duration and magnitude of the discharge and the life 

history or response time of a targeted resource need to be considered. Furthermore, altering only 

discharge rates may be insufficient for some resources. The LSSF hydrograph was insufficient in 

duration to elicit a measurable response by YOY fishes. Multiple years of a single treatment 

may be required to observe a response by native fish because they are long-lived, and 

distinguishing between the effects of a single year’s hydrologic treatment from year-to-year 

variability in YOY numbers and growth is difficult. Mainstem water temperatures did not warm 

sufficiently during Period II of the LSSF experiment to meet optimal growth requirements of 

humpback chub, except in the western parts of Grand Canyon. The greatest numbers of 

humpback chub occur near the LCR (Douglas and Marsh, 1996; Coggins and Walters, 2009). 

Targeting low discharges when release temperatures from Glen Canyon Dam are greater than 

average may be more informative than simply reducing discharges when water temperatures at 

the dam are average. Native fish monitoring data associated with multiple years of warmwater 

discharges for Glen Canyon Dam between 2003 and 2006 suggest that some native fish 

benefitted from the warmer water even during fluctuating discharges. However, predator control 

measures were implemented at the same time, which confounds knowing if increased 

temperatures or decreased predator loads affected the recruitment response by humpback chub 

and other native fishes (Coggins, 2008). Given the constraints in water delivery, experimental 

hydrographs that focus on life stages that are affected directly by mainstem operations may be 

more informative experiments (for example, lower mid-summer and fall flows and YOY 

habitat).

Flexibility in implementing experiments would benefit learning about resource response 

in this system. Basin hydrology and reservoir elevations greatly affect experimental capacity in 

this system. Basin hydrology affects sediment inputs, inflows to Lake Powell, and water delivery 

requirements between the upper and lower Colorado River Basin. Taking advantage of 

unexpected inputs to the system either by way of large sediment inputs or increased release 

temperatures as a result of reduced inflows and associated reservoir elevations can be used to 

further understanding of how best to manage the downstream resources. When reservoir 

elevations allow discharge temperatures to exceed 13oC, it may be informative to implement 

steady discharges to see how YOY fish at the LCR respond to warmer temperatures and steady 
 106

discharges. The results can be compared with data collected during 2003–6 during fluctuating 

discharges and possibly different predator loads, provided sufficient long-term monitoring is in 

place. 

Flexibility in dam releases requires adequate long-term monitoring be in place to measure 

responses. Additionally, stakeholders need to reconcile a need for immediate measures of 

response that may be satisfying in the short term but meaningless in the longer-term, particularly 

for long-lived species. A lack of response observed for 1 year may not mean the treatment was 

ineffective. Multiple years of a treatment may be necessary for a response to be measurable. For 

example, 4 years of predator control between 2003 and 2006 are correlated with an increase in 

the number of native fish, including humpback chub (Coggins and Walters, 2009). However, the 

increased native fish numbers were not an observable response for a year or two following the 

initiation of the treatment. Further, a combination of warmer temperatures and predator control 

likely affected native fish numbers (Coggins, 2008). Because native fish are long-lived, flow 

treatments or other non-flow experimental treatments likely will require multiple-year efforts to 

be informative. Other resources may require shorter-duration treatment (for example, sediment), 

but the duration and seasonal timing still need to be considered (for example, experiments that 

limit tamarisk (Tamarix spp.) expansion or recruitment of rainbow trout in the Lees Ferry 

fishery). 

Experimental flexibility requires the implementation of robust long-term monitoring that 

can measure a response for the long term and supplement data collection that is more intensive 

during the experiment. The variables measured by monitoring are unlikely to be at a detail that 

explains incremental change, but monitoring should be sufficient to measure year-to-year change 

in the resource. A direct mechanistic response to a proposed treatment may first be measured in a 

laboratory setting and then sampled in the field. Having sufficient background data to compare 

against a treatment timeframe is essential to determining an effect. Previous data associated with 

fish in the Colorado River ecosystem were collected in relation to the Operation of Glen Canyon 

Dam Final Environmental Impact Statement and not associated with status and trend monitoring. 

The 2000 LSSF experiment resulted in the initiation of a more consistent, quantitatively robust 

approach to fish sampling in the mainstem and LCR that has been incorporated in long-term 

monitoring. Monitoring needs to include resources beyond the immediately affected resources. 

Evaluating these other resources is an important element in evaluating ecosystem responses to 

experimental flow treatments.

It is important for results from experiments to be published in reports and peer-reviewed 

journal articles, as well as summarized for stakeholder use. Cooperator reports are the first step 

in the process of incorporating knowledge but not the final step. Having citable literature that can 

be incorporated into larger synthetic efforts is critical to building a solid basis for documenting 

results and furthering experimental planning and decisionmaking by resource managers. 

Experimental planning would be most effective if it includes funding and time for these reporting 

efforts. The 10-year delay in the summary of results of the LSSF experiment has allowed 

incorporation of long-term biological responses (for example, Coggins and Walters, 2009), but 

has limited the Glen Canyon Dam Adaptive Management Program with respect to the 

identification of the efficacy of long-duration experiments. The LSSF experiment was the first 

seasonally based experiment using Glen Canyon Dam releases that focused on biological 

resources, primarily native fish. Although some results associated with fish growth were 

inconclusive, the experimental hydrograph did support learning about other fish-related 

resources like shoreline habitat (Korman and others, 2004; Protiva and others, 2010). The 
 107

response by other resources to high and low discharges, particularly sediment (Schmidt and 

others, 2007), riparian vegetation (Porter, 2002), and recreational and economic responses 

(Roberts and Bieri, unpub. report, 2001; Hjerpe and Kim, unpub. report, 2003; Palmer and 

others, unpub. report, 2004) are better known. Incorporating this knowledge into the next event 

that involves measuring a response to long-lived species is critical if the effort is to be useful for 

resource managers.

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Korman, J., 2009, Early life history dynamics of rainbow trout in a large regulated river: 

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Korman, J., Wiele, S.M., and Torizzo, M., 2004, Modelling effects of discharge on habitat 

quality and dispersal of juvenile humpback chub (Gila cypha) in the Colorado River, Grand 

Canyon: River Research and Applications, v. 20, no. 4, p. 379-400, accessed on February 22, 

2010, at http://www3.interscience.wiley.com/cgi-bin/fulltext/107614374/PDFSTART.

Korman, J., Yard, M., Walters, C.J., and Coggins, L.G., 2009, Effects of fish size, habitat, flow, 

and density on capture probabilities of age-0 rainbow trout estimated from electrofishing at 

discrete sites in a large river: Transactions of the American Fisheries Society, v. 138, no. 1, p. 

58-75, accessed on June 1, 2011, at http://www.tandfonline.com/doi/abs/10.1577/T08-025.1.

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monitoring in Grand Canyon, Arizona—2000 to 2009 summary: U.S. Geological Survey 

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http://pubs.usgs.gov/of/2010/1246/.

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regulated California stream. Ecological Applications, v. 11, no. 2, p. 530–539, accessed on 

August 23, 2011, at http://www.jstor.org/stable/3060907. 

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biota of the Colorado River below Glen Canyon Dam, Arizona: North American Journal of 

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management of the Colorado River ecosystem below Glen Canyon Dam, Arizona—using 

science and modeling to resolve uncertainty in river management, in Specialty Summer 

Conference on Adaptive Management of Water Resources, Missoula, Mont., June 26–28, 

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Palmer, S.C., Burbidge, C., and Patno, H., 2004, The financial impacts of the low summer steady 

flow experiment at Glen Canyon Dam: Salt Lake City, Utah, U.S. Department of Energy, 

Western Area Power Administration, CRSP Management Center, 32 p., accessed on August 

23, 2011, at http://www.gcmrc.gov/library/reports/Other/GlenCanyonDam/Palmer2004b.pdf. 

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planned flooding: Ecological Applications, v. 11, no. 3, p. 633-634, accessed on December 6, 
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Apse, C., Bledsoe, B.P., Freeman M.C., Henriksen, J., Jacobson, R.B., Kennen, J.G., Merritt, 

D.M., O'Keeffe, J.H., Olden, J.D., Rogers, K., Tharme, R.E., and Warner, A., 2009, The 

ecological limits of hydrologic alteration (ELOHA)—a new framework for developing 

regional environmental flow standards: Freshwater Biology, v. 55, no. 1, p. 147–170, accessed 

on August 19, 2011, at http://dx.doi.org/10.1111/j.1365-2427.2009.02204.x.

Porter, M.E., 2002, Riparian vegetation responses to contrasting managed flows of the Colorado 

River in Grand Canyon, Arizona: Flagstaff, Northern Arizona University, M.S. thesis, 33 p. 

Protiva, F.R., Ralston, B.E., Stone, D.M., Kohl, K.A., Yard, M.D., and Haden, G.A., 2010, 

Effects of Glen Canyon Dam discharges on water velocity and temperatures at the confluence 

of the Colorado and Little Colorado Rivers and implications for habitat for young-of-year 

humpback chub (Gila cypha): U.S. Geological Survey Open-File Report 2010-1137, 24 p., 

accessed on July 27, 2010, at http://pubs.usgs.gov/of/2010/1137/.

Roberts, C.A., and Bieri, J.A., 2001, Impacts of low flows on recreational rafting traffic on the 

Colorado River in Grand Canyon National Park—final report: Flagstaff, Ariz., submitted to 

Bureau of Reclamation and U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center, cooperative agreement no. CA8210–99–002, interagency acquisition no. 00–AA–40–

4330, 18 p., accessed on August 23, 2011, at 

http://www.gcmrc.gov/library/reports/cultural/Recreation/roberts2001.pdf. 

Robinson, A.T., Clarkson, R.W., and Forrest, R.E., 1998, Dispersal of larval fishes in a regulated 

river tributary: Transactions of the American Fisheries Society, v. 127, no. 5, p. 772-786, 

accessed on June 1, 2011, at http://www.tandfonline.com/doi/abs/10.1577/1548-

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low steady flows on benthic biomass and drift composition in the Lees Ferry tailwater: Arizona 

Game and Fish Department, cooperative agreement no. 1425–98–FC–22390, mod. 3, 26 p., 

accessed on August 23, 2011, at http://www.gcmrc.gov/library/reports/biological/Foodbase/

 Rogers2003b.pdf. 

Schmidt, J.C., Topping, D.J., Rubin, D.M., Hazel, J.E., Jr., Kaplinski, M., Wiele, S.M., and 

Goeking, S.A., 2007, Streamflow and sediment data collected to determine the effects of low 

summer steady flows and habitual maintenance flows in 2000 on the Colorado River between 

Lees Ferry and Bright Angel Creek, Arizona: U.S. Geological Survey Open-File Report 2007–

1268, 79 p., accessed on March 17, 2010, at http://pubs.usgs.gov/of/2007/1268/.

Speas, D.W., Persons, W.R., Rogers, R.S., Ward, D.L., Makinster, A.S., and Slaughter, J.E., IV, 

2004, Effects of low steady summer flows on rainbow trout in the Lee's Ferry tailwater, 2000: 

Phoenix, Arizona Game and Fish, submitted to U.S. Geological Survey, Grand Canyon 

Monitoring and Research Center, cooperative agreement no. 1425–98–FC–40–22690, 38 p., 

accessed on August 23, 2011, at http://www.gcmrc.gov/library/reports/biological/Fish_studies/ 

 AZGame&Fish/2004/Speas2004.pdf. 

Topping, D.J., Schmidt, J.C., and Vierra, L.E., 2003, Computation and analysis of the 

instantaneous-discharge record for the Colorado River at Lees Ferry, Arizona—May 8, 1921, 

through September 30, 2000: U.S. Geological Survey Professional Paper 1677, 118 p., 

accessed on January 28, 2010, at http://pubs.usgs.gov/pp/pp1677/pdf/pp1677.pdf. 
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Trammell, M., Valdez, R.A., Carothers, S.W., and Ryel, R., 2002, Effects of a low steady 

summer flow experiment on native fishes of the Colorado River in Grand Canyon—final 

report: Flagstaff, Ariz., SWCA, Inc. Environmental Consultants, submitted to U.S. Geological 

Survey, Grand Canyon Monitoring and Research Center, cooperative agreement no. 99–FC–

40–2260, 77 p. [Available upon request by contacting the Center Director, U.S. Geological 

Survey, Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

Travnichek, V.H., Bain, M.B., and Maceina, M.J., 1995, Recovery of a warmwater fish 

assemblage after initiation of a minimum-flow release downstream from a hydroelectric dam:

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23, 2011, at http://www.tandfonline.com/doi/abs/10.1577/1548-

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Impact Statement: Salt Lake City, Utah, Bureau of Reclamation, Upper Colorado Region, 337 

p. 

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Dam: Albuquerque, N. Mex., consultation no. 2–21–93–F–167, p. 56. 

U.S. Geological Survey, 2010, National Water Information System (NWISWeb): U.S. 

Geological Survey database, accessed April 27, 2010, at http://waterdata.usgs.gov/nwis.

Valdez, R.A., and Carothers, S.W., 1998, The aquatic ecosystem of the Colorado River in Grand 

Canyon—Grand Canyon data integration synthesis final report: Flagstaff, Ariz., SWCA, Inc. 

Environmental Consultants, submitted to Bureau of Reclamation, 250 p. [Available upon 

request by contacting the Center Director, U.S. Geological Survey, Southwest Biological 

Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

 Valdez, R.A., Carothers, S.W., House, D.A., Douglas, M.E., Ryel, R.J., Bestgen, K.R., and 

Wegner, D.L., 2000, A program of experimental flows for endangered and native fishes of the 

Colorado River in Grand Canyon—final report: Flagstaff, Ariz., SWCA, Inc., Environmental 

Consultants, submitted to U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center, 68 p. [Available upon request by contacting the Center Director, U.S. Geological 

Survey, Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

Valdez, R.A., and Ryel, R.J., 1995, Life history and ecology of the humpback chub (Gila cypha) 

in the Colorado River, Grand Canyon, Arizona—final report.: Logan, Utah, BIO/WEST, Inc., 

submitted to Bureau of Reclamation, contract no. 0–CS–40–09110, technical report no. TR–

250–08, 329 p. [Available upon request by contacting the Center Director, U.S. Geological 

Survey, Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

Webb, J.A., Stewardson, M.J., and Koster, W.M., 2010, Detecting ecological responses to flow 

variations using Bayesian hierarchical models: Freshwater Biology, v. 55, no. 1, p. 108–126, 

accessed on August 23, 2011, at http://onlinelibrary.wiley.com/doi/10.1111/j.1365-

2427.2009.02205.x/full. 

Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., 1999, The 1996 controlled 

flood in Grand Canyon, v. 110: Washington, D.C., American Geophysical Union, Geophysical 

Monograph Series, 367 p. 

Yard, M.D., Coggins, L.G., Baxter, C.V., Bennett, G.E., and Korman, J., 2011, Trout piscivory 

in the Colorado River, Grand Canyon--effects of turbidity, temperature, and fish prey 

availability: Transactions of the American Fisheries Society, v. 140, no. 2, p. 471-486, 

accessed on April 19, 2011, at 

http://www.tandfonline.com/doi/abs/10.1080/00028487.2011.572011. 
 111

Appendix. Annotated Bibliography of Studies Conducted 

During 2000 Low Steady Summer Flow Experiment

Anima, R., Wong, F.L., Hogg, D. and Galanis, P., 2007, Side-scan sonar imaging of the 

Colorado River, Grand Canyon: U.S. Geological Survey Open-File Report 2007–1216, 15 p., 

accessed on August 23, 2011, at http://pubs.usgs.gov/of/2007/1216/. 

Overview

The utility of side-scan sonar was evaluated for the examination of substrate changes 

(boulders, cobbles, pebbles, sand) in the Colorado River from the Glen Canyon Dam tailwaters 

to river mile 88 before (Aug. 28–Sept. 5, 2000) and after (Sept. 10–18, 2000) the September 

Habitat Maintenance Flow (HMF). The HMF was part of the 2000 Low Steady Summer Flow 

(LSSF) experiment and occurred in Period II of the experiment. Imaging focused on pools 

between rapids and smaller rapids where possible. The report presents the methods used and 

discusses how well the technology differentiates bed-material types from a subset of the data.

Side-scan sonar can be used to look at overall changes in bed texture over an extended area, but 

there are caveats. Data density was high, which caused post-processing delays. Extreme canyon 

temperatures also affected instruments. Navigation control did not permit centimeter-scale 

change detection of bed material and required manual registration in the image-processing step. 

Results

• Pixel resolution prohibited individual sand, pebble, and cobble identification, but sand as a 

class can be distinguished from other substrate classes. 

• Pebbles were the predominant bed type in some reaches, particularly in Lees Ferry.

• Imagery showed changes at some sites, including either removal of sand, which exposed 

pebbles and cobbles, or aggradation of sand. 

• Image distortion associated with heave and yaw of the underwater sensor and changes in boat 

speed were present. 

This was a demonstration project and no attempt was made to summarize or quantify findings. 

Problems with the technology included post-processing problems with a large amount of data.

Report includes a DVD subset of processed imagery as well as unprocessed imagery. Efficient 

data processing requires expertise with image processing. These data/methods may be applicable 

to nearshore fish habitat classification. Schmidt and others (2007) questioned the accuracy of this 

technique and state that “the small differences in the area of bed composed of boulders and 

bedrock illustrates the error associated with interpretation of side-scan sonar data and 

incorporation into a GIS.” Schmidt and others (2007) suggest that the technique is more 

applicable to large-scale changes rather than centimeter-scale changes in bed material. 
 112

Benenati, E.P., Shannon, J.P., Haden, G.A., Straka, K., and Blinn, D.W., 2002, Monitoring and 

research—the aquatic food base in the Colorado River, Arizona during 1991–2001—final 

report: Flagstaff, Arizona Game and Fish Department, U.S. Fish and Wildlife Service and 

Northern Arizona University, submitted to U.S. Geological Survey Grand Canyon Monitoring 

and Research Center, cooperative agreement no. 1452–98–FC–225590, 227 p. 

Overview

The authors collected data associated with water quality, benthic biomass, organic drift, 

and a fish habitat selection experiment. Data were collected in June, August, September, and 

October of 2000 and compared with similar data collected in 1997 when discharges were higher 

but also steady. Macroinvertebrate biomass, except snails, was not significantly different from 

previous sample years. There were shifts in phytobenthic composition (aquatic macrophytes 

from Cladophora), which also was noted by Rogers and others (2003) in the Lees Ferry fishery. 

Coarse particulate organic drift values were similar to those recorded under higher sustained 

discharges in 1997. 

Results

• Cladophora biomass increased on average between June and September, though its 

distribution was patchy. 

• Biomas of miscellaneous benthos decreased overall. Comparison was made between June 

and October and cannot distinguish between seasonality versus the effect of the Low 

Steady Summer Flow Experiment.

• Macroinvertebrate biomass increased through the summer, predominantly because of 

snail density increases throughout the system. 

• Simuliid larvae density decreased by 92 percent, but two sample sites were associated 

with Little Colorado River and August–September sampling may be affected by 

monsoonal spates. 

• Drift biomass and density did not vary among sample periods but composition did. 

• Values for organic drift were 14.0 (±0.91) milligrams per cubic meter per second at Lee 

Ferry.
 113

Davis, P.A., 2002, Evaluation of airborne thermal-infrared image data for monitoring aquatic 

habitats and cultural resources within the Grand Canyon: U.S. Geological Survey Open-File 

Report 02–367, 49 p. 

Overview

Thermal-infrared (TIR) image data were acquired using the airborne Advanced Thematic 

Mapper (ATM) in the afternoon of July 25, 2000, over river miles (RM) 30–74. The Daedalus 

ATM 1268 was deployed, and the sensor collected data in 12 channels at a spatial resolution of 

100 cm. The ATM data were provided georectified in a Universal Transverse Mercator map 

projection (WGS84) with a positional accuracy of 2–3 m. The gain setting used for the ATM 

band 12 produced a temperature range of 1 to 83°C. The range could be reduced by a factor of 

three and still capture the temperature range of the water with 0.1°C intervals. Data need to be 

acquired within a 2-hour window of the maximum solar heating (~1:30 p.m.) to provide 

maximum solar heating of the water and to minimize the cooling effects of late-afternoon 

shadows. The cost for acquiring TIR data is high ($620/river kilometer (km)) and does not 

include costs of ground calibration. The benefit of the approach is that it provides an 

instantaneous map of surface water temperatures for a 160 km stretch of the Colorado River. 

Results

• Gradual downstream warming of surface water in the mainstem was visible following 

calibration of TIR data.

• Embayment at RM 34 shows surface temperatures about 4°C warmer than mainstem. 

Only the very nearshore exceeded 18°C because of small area of embayment.

• The debris fan at Tatahatsu Wash creates a low-velocity environment and indicated

gradual increase in temperature reaching temperatures about 10°C greater than the 

mainstem. 

• Cut-off backwaters warmed in excess of 25°C but these would be inaccessible to fish. 

• TIR indicates temperature differential of tributaries and mainstem. 

• Ground-truth calibration that compared TIR temperature data to Thermistor data 

collected on July 24, 2000, at RM 64 indicates airborne temperatures were 2°C greater

than Thermistor temperatures. Cloud cover on the July 24 may account for the observed 

temperature differential.
 114

Hjerpe, E., and Kim, Y.-S. 2003, Economic effects of summer steady flows on recreation: 

Flagstaff, Northern Arizona University, School of Forestry, 21 p. 

Overview

The authors investigated how the variable flows associated with the Low Steady Summer 

Flow (LSSF) experiment affected the economic operations of recreation-based industries. They 

extended their findings to the regional economy. Industries investigated include private 

whitewater boaters, anglers, and river concessionaires. Mail and in-person surveys were 

conducted. Aggregated LSSF-related expenses incurred by rafting concessionaires totaled more 

than $70,000. Angling concessionaires benefitted slightly from improved fishing, though they 

did report a loss of $33,000 because of the inability to conduct trips during the spike flows. An 

analysis of the regional cost of LSSF produced an estimate of $124,000. 

Results

• Angling concessionaires report losses during the May and September habitat maintenance 

flows because of the inability to conduct trips. 

• Rafting concessionaires were more adversely affected by low flows than other groups.

Losses totaled more than $70,000. Volume of trips did not change. 

• The low steady flows are blamed for three major boating accidents, one each at Horn 

Creek, Hance rapids, and Crystal rapids. Search and rescue operations associated with 

these accidents cost the National Park Service approximately $30,000. 

• Interviewed river guides indicate 226 m3

/s (low steady level) is minimum flow level for 

passenger safety at Horn Creek, Hance rapids, and Crystal rapids. 

• Longer “on-river” times because of low flows stressed river crews.

• Day-use rafting companies had minimal effect from the low flows.

• Increased “interest” in low flows among anglers and boaters negated any negative 

economic effects, including a few equipment-damaging incidents.

• Angling concessionaires saw a slight increase in the number of guided trips and revenue 

during the low-flow phase. 
 115

Jonas, L.M., and Stewart, W.P., 2002, An overview of various impacts to Grand Canyon River 

experiences, with a focus on intergroup encounters, flow levels and the 2000 low summer 

steady flow experiment—final report: Flagstaff, Ariz., SWCA, Inc. Environmental 

Consultants, submitted to U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center. [Available upon request by contacting the Center Director, U.S. Geological Survey, 

Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

Overview

The study incorporated previous work associated with recreational attitudes among 

rafters along the Colorado River to identify the physical and social-psychological attributes that 

contribute to Grand Canyon river experiences. To evaluate the effect of flow levels, the authors 

also identified specific attributes affected by flow levels and provided a perspective of the effect 

the Low Steady Summer Flow (LSSF) experiment had on group experiences. Through a 

literature review, the authors concluded that although the LSSF experiment reduced the rate of 

travel, decreased the size of rapids, increased safety concerns, and improved the quality of 

camping beaches. The overall effects on user experience are considered minor because 226 m3

/s 

is not considered an extreme low flow. Furthermore, because the river experience is 

multidimensional, many other factors were minimally or not affected by the experimental flows 

and did not affect river experiences. 

Results

• Flows below 226 m3/s are considered less safe and lead to more boating accidents. 

• Much of the data were collected in late 1980s and early 1990s when boaters were not 

exposed to lower flows, so the point of reference is different. 

• Motorized rafts experienced much higher accident rates than oar-powered boats during 

the low steady flows (Jalbert, 2001). 

• Low flows increase campable area.

• Steady flows allow camping near the edge of the water.

• At flows like the low steady flow, below 283 m3

/s, boatmen had to row more or run the 

motor longer to meet schedules. 

• At flows above 877 cubic meters per second, like the September spike, boatmen can row 

or run motors less 

• Interactions between research and commercial trips can be improved by engaging 

commercial passengers. 
 116

Korman, J., Wiele, S.M., and Torizzo, M., 2004, Modelling effects of discharge on habitat 

quality and dispersal of juvenile humpback chub (Gila cypha) in the Colorado River, Grand 

Canyon: River Research and Applications, v. 20, no. 4, p. 379–400, accessed on February 22, 

2010, at http://www3.interscience.wiley.com/cgi-bin/fulltext/107614374/PDFSTART. 

Overview

The authors used a two-dimensional hydrodynamic model to evaluate how Glen Canyon 

Dam operations affect the availability of shoreline habitats and dispersal of juvenile humpback 

chub (Gila cypha). The study considered seven reaches within a 3.6 km area near the confluence 

of the Colorado and Little Colorado Rivers. Suitable habitat, defined by velocities and depths 

from historical electrofishing data, generally declined at discharges less than 226 m3

/s and 

greater than 425 m3

/s, though this pattern was not consistent among all reaches. 

Results

• Total suitable shoreline habitat generally declined with increasing discharge, but the 

response was variable among reaches and was affected by local morphology.

• Reaches that indicated little response were dominated by large fan-eddy complexes in 

which low-velocity eddies are maintained as discharges increase.

• Highly responsive reaches had low-profile fans that were overtopped at 425 m3/s 

discharges. 

• Availability of habitat when stratified by substrate type varied among reaches. Talus and 

debris fan habitats might stay constant, whereas vegetated shorelines increased with 

increasing discharges to 1,272 m3/s.

• The LSSF flows in June and July presented high suitable shoreline habitat that was not 

available during predam flows (months of when suitable habitat was lowest).

• Operations have generally increased habitat in the spring, but reduced it in the months of 

August–February. 

• Physical retention of randomly placed particles simulating the movement of juvenile 

humpback chub tended to decline with increasing discharge, though this pattern varied 

considerably because of differences in local morphology among reaches. Retention rates 

were greatest in reaches where eddies made up a large proportion of the wetted area. 
 117

Palmer, S.C., Burbidge, C., and Patno, H., 2004, The financial impacts of the low summer steady 

flow experiment at Glen Canyon Dam: Salt Lake City, Utah, U.S. Department of Energy, 

Western Area Power Administration, CRSP Management Center, 32 p., accessed on August 

23, 2011, at http://www.gcmrc.gov/library/reports/Other/GlenCanyonDam/Palmer2004b.pdf. 

Overview 

The authors explain the methods used to estimate the financial cost of the Low Steady 

Summer Flow (LSSF) experiment on Western Area Power Administration (Western) and its 

electrical service customers, describing the change in Glen Canyon Dam operations and the 

power system effects of the LSSF experiment. The report is strictly a financial study pertaining 

to Western and its electrical customers. Costs associated with the LSSF are considered 

nonreimbursable funds to the U.S. Treasury. The authors distinguish between financial and 

economic analyses, stating a financial-impact study focuses on the impact of an action on an 

individual body or institution and that an economic analysis describes the effect of an action on 

the society as a whole. The estimated cost of the LSSF experiment of 2000 to Federal power 

users was $32 million. Though some of the inflow deficit from the experiment was recovered as 

of December 2004, a storage deficit remains due, in part, to ongoing drought. 

Results 

• Cost associated with the LSSF were nonreimbursable and are not repayable to the U.S. 

Treasury, but these funds are credited against reimbursable costs.

• Monthly water budgets were redistributed between March 2000 and November 2000 to 

alleviate the financial costs associated with low water inflows and existing market power 

costs and the LSSF experiment (funds to support the science studies).

• Used two test case scenarios against the base case scenario: (1) actual water releases and 

Glen Canyon Dam constraints in the Hydro LP model for an optimized generation pattern 

and (2) actual generation pattern data from Supervisory Control and Data Acquisition 

(SCADA) to compare the optimized model results against reported values of generation 

from six Colorado River Storage Project units.

• The hourly difference between the test cases and base case generation is multiplied by the 

month’s on- or off-peak price. The difference in purchases and sales is summed by 

month.

• Base case is the monthly water volumes releases in the absence of the LSSF and load 

values were based on historical values.

• Power purchase is usually in seasonal firm blocks rather than on the spot market. 

Additional power when needed is purchased on spot market on day-before basis.

• April and May releases were greater under LSSF than base case, so Western had a net 

financial gain by selling surplus power ($3.6 and $6.8 million in April and May, 

respectively).

• June–August releases were lower than base case and Western had to make purchases to 

meet demand.

• More than normal release of water in water year (WY) 2000 and subsequent years of 

minimum water delivery to the lower basin states because of low inflows add to financial 

costs to Western.

• Total financial cost was slightly less than $32 million. 
 118

Porter, M.E., 2002, Riparian vegetation responses to contrasting managed flows of the Colorado 

River in Grand Canyon: Flagstaff, Northern Arizona University, M.S. thesis, 33 p. 

Overview

The author examined patterns of establishment and survival of native and nonnative 

plants along the shoreline during two contrasting discharge patterns. Greenhouse experiments 

also were used to verify field observations. Sampling plots were established along the Colorado 

River corridor between Lees Ferry and river mile 68.0, which is just downstream from the 

confluence of the Colorado and Little Colorado Rivers. Data collection took place during Periods 

II and III and again in the following year during fluctuating discharges. Seedling (tamarisk

(Tamarix spp.)) and asexual propagule (wetland species) densities increased for native and 

nonnative species, particularly tamarisk during the steady-flow period. The habitat maintenance 

flow (HMF) in September reduced tamarisk seedling densities by 60 percent, although horsetail 

(Equisetum) shoots were not significantly affected. Data from plots sampled under fluctuating 

discharges the following summer indicate that tamarisk densities continued to decline and 

horsetail stem density continued to increase. Water table declines associated with changes in 

discharges between Periods I and II contributed to mortality of plants located above the 226 m3

/s

stage elevation. 

Results

• High densities of tamarisk seedlings were established at mid-shore elevations during the 

low steady flow phase. Stem density averaged 626 stems per square meter by August 

2000. 

• Numbers of Equisetum and other native plants also increased, though to a lesser degree, 

during the same period. 

• Existing horsetails at the spike-flow water line suffered 55 percent mortality during June 

and July 2000. 

• Tamarisk numbers declined by 57 percent following the September HMF (626 stems per 

square meter to 264 stems per square meter). 

• Densities in tamarisk after April 2001 were equivalent to June 2000 levels. 

• Clonal expansion density patterns of horsetails showed greater densities in the upper 

shore region. 
 119

Protiva, F.R., Ralston, B.E., Stone, D.M., Kohl, K.A., Yard, M.D., and Haden, G.A., 2010, 

Effects of Glen Canyon Dam discharges on water velocity and temperatures at the confluence 

of the Colorado and Little Colorado Rivers and implications for habitat for young-of-year 

humpback chub (Gila cypha): U.S. Geological Survey Open-File Report 2010-1137, 24 p., 

accessed on July 27, 2010, at http://pubs.usgs.gov/of/2010/1137/. 

Overview 

This study determined the amount of warmwater, low-velocity environments available to 

young-of-year (30–90 mm) humpback chub (Gila cypha) in the vicinity of the confluence of the 

Colorado and Little Colorado Rivers (LCR) during the 2000 Low Steady Summer Flow (LSSF) 

experiment flow regimes. This study quantified environments that had velocities and 

temperatures identified suitable for young of year humpback chub at discharges of 227 m3/s, 368 

m3/s, 504 m3/s, and 878 m3/s. Discharges as much as 368 m3/s provided the greatest area of 

suitable habitat around the island located at the confluence of the mainstem and LCR. Suitable 

environments that meet temperature and velocity requirements were reduced above this 

discharge. 

Results

• The island and the confluence sets up circulation dynamics that affect how the mainstem

Colorado River travels. Under 227 m3

/s discharges, the mainstem flow is diverted to the 

west of the island and does not interact to a great extent with LCR discharge. 

• At higher discharges, like that during the habitat maintenance flow (1,165 m3/s), the 

mainstem flows around the island and there is no suitable habitat at the confluence area. 

Ponding does occur. 

• The greatest area of suitable habitat within the confluence area occurred at discharges as 

much as 368 m3/s when mainstem volumes pool slightly at the upstream side of the LCR 

mouth/mainstem confluence and force the LCR water to the lower part of the island, 

which increases volume and concentrates the warmer water discharging from the LCR. 
 120

Roberts, C.A., and Bieri, J.A., 2001, Impacts of low flows on recreational rafting traffic on the 

Colorado River in Grand Canyon National Park—final report: Flagstaff, Ariz., submitted to 

Bureau of Reclamation and U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center, cooperative agreement no. CA8210–99–002, interagency acquisition no. 00–AA–40–

4330, 18 p., accessed on August 23, 2011, at http://www.gcmrc.gov/library/reports/cultural/ 

Recreation/roberts2001.pdf. 

Overview

The authors compared trip reports from 600 private and commercial permit holders from 

three rafting seasons that included fluctuating flows and the steady flow of 2000. Distribution of 

data collection was shared between authors and Linda Jalbert of National Park Service Science 

Center at Grand Canyon National Park. Base flow rate affects time spent at attraction sites, 

campsite selection, and time spent on the river. Results indicate 50 percent less time was spent 

on off-river activities during the steady-flow period. 

Results

• All boats were slowed to some extent during the steady-flow period. Oar-powered boat 

speed is affected greatest by discharge. 

• The amount of time boats spent on the river, in campsite selection, and in selection of and 

time spent at attraction sites were all affected by the Low Steady Summer Flow 

experiment. 

• Number of off-river activities did not change, but on average, trips spent 50 percent less 

time engaged in off-river activities during the low summer steady flow period. 

• Attraction site preference did not change. 

• More camps became available during Period II. 
 121

Rogers, R.S., Persons, W.R., and McKinney, T., 2003, Effects of a 31,000-cfs spike flow and 

low steady flows on benthic biomass and drift composition in the Lees Ferry tailwater: Arizona 

Game and Fish Department, cooperative agreement no. 1425–98–FC–22390, mod. 3, 26 p., 

accessed on August 23, 2011, at http://www.gcmrc.gov/library/reports/biological/Fish_studies/ 

AZGame&Fish/2004/Speas2004.pdf. 

Overview

This study measured the effects of the Period II and May and September habitat 

maintenance flow (HMF) on benthic biomass and drift in the Lees Ferry area. The authors 

provide comparisons of biomass with data from 1992–97. The authors attribute differences in 

biomass to compositional shifts in macrophytes that occurred between 1996 and 2000. 

Chironomidae densities were greater in 2000 than in previous years. Previous studies have 

indicated that short-term spike flows can reduce aquatic resources such as macrophytes and 

levels of chlorophyll a in periphyton. Reductions in organic drift are associated with steady 

discharges. 

Results

• Following the May HMF, periphyton ash-free dry mass concentrations from grab samples 

did not differ significantly throughout the experiment with a peak in the July/August 

sample period. 

• Grab samples of macrophyte increase significantly through July with highest relative 

abundance observed in June and September. 

• Macroinvertebrate densities did not decline in association with the HMF. Gammarus

densities increased and were significantly higher in the September 11, 2000, period than 

during previous sampling times. 

• Chironomidae densities increased with time and were significantly higher in July than in 

either April or September. 

• New Zealand mudsnail (Potamopyrgus antipodarum) densities increased throughout the 

experiment. 

• Invertebrate drift did decrease significantly following the May HMF. Gammarus 

disappeared from drift samples after mid-May. 

• Periphyton and macrophyte ash-free dry mass concentrations, chlorophyll a, and benthic 

macroinvertebrates were all higher during the steady flows. 

• Invertebrate drift peaked in July and then decreased subsequently. 

• Invertebrate drift declined after the September HMF, as measured until November 2000. 
 122

Schmidt, J.C., Topping, D.J., Rubin, D.M., Hazel, J.E., Jr., Kaplinski, M., Wiele, S.M., and 

Goeking, S.A., 2007, Streamflow and sediment data collected to determine the effects of low 

summer steady flows and habitual maintenance flows in 2000 on the Colorado River between 

Lees Ferry and Bright Angel Creek, Arizona: U.S. Geological Survey Open-File Report 2007–

1268, 79 p., accessed on March 17, 2010, at http://pubs.usgs.gov/of/2007/1268/. 

Overview

The authors used the experimental hydrograph to test methods to measure influx, efflux, 

and change in fine-grained sediment distribution in Marble Canyon and eastern Grand Canyon to 

Phantom Ranch (river mile (RM) 0– 89). Segments (3–8 kilometers long) within geomorphic 

reaches were used to determine if storage and evacuation varied longitudinally. Changes in finesediment storage were determined using a combination of aerial photography analyses and 

ground surveys. Gaging stations were used to measure discharge and sediment supplied by 

tributaries. Sand accumulated in mid-elevation zones and decreased in low-elevation zones in 

eddies during the habitat maintenance flows (HMF). This pattern reversed during the summer 

low flows. There was substantial site-to-site variation in sand movement and accumulation. 

Because of limited tributary flow in 2000, the predominant source of fine sediment exported 

from Marble Canyon during September HMF was from eddy sandbars. Overall, the Low Steady 

Summer Flow (LSSF) experiment did increase mid-elevation zone volume and areas of fine 

sediment in eddy sandbars, though the increases were smaller than increases measured since 

1984. 

Results

• Sand supply in Marble Canyon was greater during the May HMF than the September 

HMF. Although silt and clay concentrations were comparable between the two high 

flows, sand concentrations were 30 percent lower during September than May. 

• Approximately 80 percent of suspended sand during the September HMF was from 

sources upstream from RM 30. There also was an increase in suspended clay and silt 

downstream from the Little Colorado River. 

• More than half of the sand from RM 30 during September 2000 originated between the 

Paria River and RM 10. 

• Sand accumulated in mid-elevation zones and decreased in low-elevation zones during 

the May HMF. Increases were smaller than any measured since 1984. 

• During summer steady flows, sediment accumulated at low elevations and declined at 

mid-elevations in eddies. The mass balance for sand was positive during the low steady 

summer flow phase. 

• Sand accumulated in mid-elevation zones and decreased in low-elevation zones during 

the September HMF, though not to the same extent as during the May HMF, because of

low sediment input from tributaries during summer 2000. 

• Mid-elevation fine sediment deposits typically were eroded during the steady flow 

period, as average volume decreased from 924 to 698 m3 in upper Marble Canyon and 

from 2,697 to 2,315 m3 in lower Marble Canyon. 

• The total area of backwater habitat increased in nearly all reaches during the September 

HMF. Number and total area of backwaters in each reach in September were comparable 

to those of 1984. 
 123

• During May and September HMFs, upstream sediment supplies were depleted and 

suspended sediments coarsened. The same pattern occurred during the 1996 controlled 

flood and an HMF in 1997. 

• The mean grain size of the fine sediment on the surface of the bed coarsened because of

the September HMF (12 of 15 sites coarsened). Fine sediment coarsened by an average of 

0.16 mm in the upper 93 km of the study area. 

• Accumulation occurred only during the steady flow period. Influxes of tributary fine 

sediment were transported past the lower Marble Canyon gaging station (station number 

09383100) within several months after tributary inflows. Because tributary influxes of 

fine sediment were lower than average in 2000, less sand accumulated during the steady 

flow period than was eroded during the May HMF. 

• The overall effects of the LSSF experiment were not significantly different among upper 

and lower Marble Canyon and upper Grand Canyon. Therefore, no longitudinal 

differences were noted in eddy sandbar response or the relative proportion of fine 

sediment transferred among the bed, eddy sandbars, or channel-margin deposits from 

ground surveys. 

• An increase in sandbar area occurred following the September HMF, as indicated by 

measurements of area and volume in upper and lower Marble Canyon from ground 

surveys. Following the September HMF, the Lees Ferry reach experienced more 

erosion—relative to deposition—than any other reach.
 124

Speas, D.W., Persons, W.R., Rogers, R.S., Ward, D.L., Makinster, A.S., and Slaughter, J.E., IV, 

2004, Effects of low steady summer flows on rainbow trout in the Lee's Ferry tailwater, 2000: 

Phoenix, Arizona Game and Fish, submitted to U.S. Geological Survey, Grand Canyon 

Monitoring and Research Center, cooperative agreement no. 1425–98–FC–40–22690, 38 p., 

accessed on August 23, 2011, at http://www.gcmrc.gov/library/reports/biological/Fish_studies/ 

AZGame&Fish/2004/Speas2004.pdf. 

Overview

Rainbow trout in the Lees Ferry reach were sampled by electrofishing to assess fish 

density, population structure, and relative condition. Seasonal comparisons with previous years 

of data (1991–99) were conducted and stomach contents also were studied. The sampling regime 

encompassed the different periods of the Low Steady Summer Flow (LSSF) experiment, but data 

were interpreted relative to seasonal trends, such that winter (March), spring (June), summer 

(August) and fall (November) seasons were represented. Seasonal variance of catch effort, 

relative condition, and proportional stock density were used in relation to the hydrologic 

components of the LSSF experiment. Null hypotheses and fish metrics (above) were presented 

relative to data from 1991–99 rather than specifically to periods associated with the LSSF 

experiment. The authors concluded that there were minimal immediate effects from the LSSF 

experiment on the trout fishery. Greatest differences were in time of occurrence of fish less than 

152 millimeters (mm) (April in 2000 compared to June in previous years). Authors suggest gains 

in survival of age 1+ year fish associated with stable flows might be offset by reduced habitat at 

226 cubic meters per second stage elevation. 

Results

• Age-0 year trout appeared in catch during April through June, which was 3 months 

earlier than during 1991–99 

• Relative condition of all size categories except fish more than 350 mm peaked during 

spring 2000 (including June for this study), though it peaked in summer in previous years 

(July-September) 

• Large numbers of unusually small Age-0 year trout (50–75 mm total length (TL) were 

collected during June and September, suggesting that the May habitat maintenance flow

did not affect small-sized trout 

• There were more Age-0 year trout (50–75 mm TL) caught in 2000 than in all previous 

years combined 

• Catch per unit effort (CPUE) of age-1 year trout did not decline following the September 

habitat maintenance flow 

• CPUE of age-1 year fish was lowest during autumn and winter, 2000 (nonsignificant 

trend) 

• CPUE of stock-size fish declined in December (nonsignificant trend) 

• Proportional stock density did not vary through seasons, though it peaked significantly in 

winter months during previous years 

• The relative condition of larger trout (greater than 405mm) continued to decline in 2000 

• Instantaneous growth rates in rainbow trout declined by 92 percent since 1991, and 

growth was lower in 2000 than in previous years 
 125

• The number of fish with empty stomachs did not differ from previous years; however, 

there was a large increase in the proportion of larger trout with empty stomachs 

• Stomach contents included a higher volume of Gammarus and gastropods than in 1999 
 126

Trammell, M., Valdez, R.A., Carothers, S.W., and Ryel, R., 2002, Effects of a low steady 

summer flow experiment on native fishes of the Colorado River in Grand Canyon—final 

report: Flagstaff, Ariz., SWCA, Inc. Environmental Consultants, submitted to U.S. Geological 

Survey, Grand Canyon Monitoring and Research Center, cooperative agreement no. 99–FC–

40–2260, 77 p. [Available upon request by contacting the Center Director, U.S. Geological 

Survey, Southwest Biological Science Center, 2255 N. Gemini Drive, Flagstaff, Ariz. 86001.] 

Overview

Species composition, relative abundance, and fish distribution data were collected using 

shoreline electrofishing, mark-recapture methods, seining, hoop nets, and trammel nets. 

Sampling trips occurred in June, August, September, and December 2000. Study sites extended 

from Lees Ferry to Diamond Creek. Mark-recapture population-estimate studies occurred from 

Crash Canyon to Carbon Creek, at Middle Granite Gorge, and at Kanab Creek. Rainbow trout 

(Oncorhynchus mykiss) was the most abundant species in the upper reaches and dominated the 

electrofishing and netting catch effort. Species representation shifted in the lower sections of the 

canyon. Fathead minnows (Pimephales promelas) and speckled dace (Rhinichthys osculsus) 

were the dominant species farther downstream. Catch per unit effort (CPUE) of bluehead suckers 

(Catostomus discobolus) and flannelmouth suckers (C. latipinnis) increased in August compared 

to 1991–97. The September habitat maintenance flow caused a reduction in the CPUE of all 

species, but CPUE for native fishes remained higher than in previous years, whereas CPUE for 

fathead minnows was comparable to previous years. Multiple sampling efforts and gear types 

were employed to evaluate the Low Steady Summer Flow (LSSF) experiment effects, with 

variable results. 

Results

• Relative abundances and composition of fish were not significantly different between 

trips but did vary by reach.

• Most flannelmouth suckers, bluehead suckers, and speckled dace were captured below 

river mile 150.

• Native species were found in greater percentages (49.2 percent of total number) in 

trammel net sets than electrofishing samples.

• More fishes were captured in seine hauls than other gear types.

• 370 seine hauls were made in 243 backwaters collecting a total of 18,309 fish 

representing 11 species.

• The greatest number of fish caught in backwaters (72 percent) was in August 2000. 

• Composition, total abundance, and relative abundance varied seasonally and 

longitudinally. 

• Order of abundance of five most common fish: fathead minnow more than speckled dace

were more than bluehead sucker were more than flannelmouth sucker were more than 

humpback chub. Humpback chub was a small percentage of the total number. 

• The highest proportion of native fish in backwaters was found in June and the lowest in 

August; however, August catch rates of flannelmouth and bluehead suckers were 

significantly higher than in previous years. 

• Mean CPUE of fishes captured by seining compared with previous years did not differ 

markedly, but total abundance increased for all species except humpback chub. 
 127

• Following the September habitat maintenance flow, fathead minnow numbers were 

comparable to previous years values, whereas flannelmouth and bluehead sucker 

numbers were higher than previous years. 

• Length histograms show overlap between age-0 year and age-1 year fish making growth 

rates difficult to assess. 

• Maximum lengths did not appear to increase during the LSSF experiment. 

• Protracted spawn and influx of newly hatched larvae depressed mean total sample length 

in September. 

• Mean total lengths of fish captured in 2000 were similar to August and September lengths 

in 1991–97. 

• There is evidence that nonnative fish including the fathead minnow and largemouth bass 

(Micropterus salmoides) spawned in the mainstem upstream from Diamond Creek. There 

was no record of largemouth bass reproducing upstream from Diamond Creek before

2000. 

• Temperatures were greatest downstream from the Little Colorado River and in August 

2000. LSSF temperatures were warmer than those measured during Modified Low 

Fluctuating Flow operations between 1991 and 1994. Mainstem mean temperature during 

June 2000 reached 16° by RM 130. 
 128

Yard, M., and Blinn, D.W., 2001, Algal colonization and recolonization response rates during 

experimental low summer steady flows—final report: Flagstaff, Ariz., Northern Arizona 

University, submitted to U.S. Geological Survey, Grand Canyon Monitoring and Research 

Center, 54 p., accessed on August 23, 2011, at http://www.gcmrc.gov/library/reports/

biological/foodbase/Yard2001.pdf. 

Overview 

This study measured the effect of different substrate treatments on colonization and 

growth of the phytobenthic and invertebrate community during steady flows (Period II). The

study used experimental treatments to evaluate whether the degree of substrate conditioning and 

atmospheric exposure to desiccation affected the mode of colonization; community composition; 

and accretion rates for the periphyton and invertebrate composition, density, and biomass. The 

primary mode of periphyton colonization was through zoospore propagation and secondarily 

through fragmentation. Rapid periphyton growth occurred for non-desiccated substrate; however, 

biomass did not attain biomass levels that were equivalent to control. The results of the study 

suggested that grazing pressure from New Zealand mudsnail (Potamopyrgus antipodarum) may 

have had effects on periphyton colonization modes, structure, and composition and rates of 

invertebrate colonization, at least under the constant flow conditions of this experiment. 

Results

• Cladophora colonized treatments differentially, which were by composition 3 percent for 

Time 1 (T1), 16 percent for Time 2 (T2), and 85.8 percent for Time 3 (T3) 

measurements. The remaining periphyton were composed of unbranched filamentous 

forms (Ulothrix, Mougeotia, and Spirogyra). 

• There were significant differences in algal growth rates between treatments: T1 (0.008 

grams per square meter per day; g/m2

•d), T2 (0.024 g/m2•d), and T3 (0.93 g/m2•d). 

Growth differences between treatments suggested a positive effect from either residual

holdfasts that remained viable or substrate conditioning. 

• During the sampling period the mean maximum grams of ash-free dry mass (AFDM)

observed for periphyton between the treatments was 2.5 grams per square meter (g/m2

) 

(T1) ±1.3 SE (93 days), 4.8 g/m2 (T2) ±1.6 SE (103 days), and 59.1 g/m2 (T3) ±7.3 SE

(71 days). 

• For treatment T3, detritus accumulated in significant quantities at a rate of 1.1 g/m2

•d and 

reached levels of 49.4 g/m2 ±19 SE by 60 days. Detritus accumulation was insignificant 

for the other two treatments. 

• Total biomass (periphyton, macrophytes, and bryophytes) for the control averaged 176 

g/m2 ±5 SE, and Cladophora comprised 53.4 percent of the algal composition. 

• The control also accumulated detritus throughout the sampling period. Detritus consisted 

of sloughed or senescent algal material and averaged 45.7 g/m2 ±3.8 SE, with a 

maximum biomass of 69 g/m2 ±16 SE attained by 60 days. 

• New Zealand mud snail was the dominant invertebrate, which attained maximum 

densities of 25.8•103 g/m2

 ±8.2•103 SE (92 days), and 35.5•103 g/m2 ± 9.3•103 SE (105 

days), and 129.7•103 g/m2 ± 22•103 SE (49 days), for the respective treatments T1, T2, 

and T3. 
 129

• The control substrate attained the highest New Zealand mud snail densities observed at 

217•103 g/m2 ±19.5•103 SE (60 days). 

• In addition to snails, flatworms (Turbellaria) initially colonized treatments and reached 

mean densities of 214 g/m2 ± 46 SE, 326 g/m2 ± 62, and 1.2•103 g/m2 ±0.14•103 SE for

T1, T2, and T3, respectively.

• For treatment T3, chironomids appeared to have a negative response to increasing snail 

and flatworm densities. Chironomid reached maximum densities of 1.1•103 g/m2

±0.45•103 SE by 60 days and then rapidly decreased to minimal levels. 

• Algal growth differences between treatments suggest that algal susceptibility to snail 

grazing may have been dependent on the colonization modes at earlier stages of 

development. 

References

Jalbert, L., 2001, The effects of low steady summer flows on whitewater boating safety in Grand 

Canyon National Park—presentation to the Glen Canyon Dam Adaptive Management 

Program, in Colorado River Ecosystem Science Symposium, Flagstaff, Ariz., April, 2001. 

Rogers, R.S., Persons, W.R., and McKinney, T., 2003, Effects of a 31,000-cfs spike flow and 

low steady flows on benthic biomass and drift composition in the Lees Ferry tailwater: Arizona 

Game and Fish Department, cooperative agreement no. 1425–98–FC–22390, mod. 3, 26 p., 

accessed on August 23, 2011, at http://www.gcmrc.gov/library/reports/biological/Foodbase/

Rogers2003b.pdf. 

Schmidt, J.C., Topping, D.J., Rubin, D.M., Hazel, J.E., Jr., Kaplinski, M., Wiele, S.M., and 

Goeking, S.A., 2007, Streamflow and sediment data collected to determine the effects of low 

summer steady flows and habitual maintenance flows in 2000 on the Colorado River between 

Lees Ferry and Bright Angel Creek, Arizona: U.S. Geological Survey Open-File Report 2007–

1268, 79 p., accessed on March 17, 2010, at http://pubs.usgs.gov/of/2007/1268/.
Ralston—Summary Report of Responses of Key Resources to the 2000 Low Steady Summer Flow Experiment, along the Colorado River downstream 

from Glen Canyon Dam, Arizona—Open-File Report 2011–1220