2005 Knowledge Assessment of the Effects of Glen Canyon 

Dam on the Colorado River Ecosystem: An Experimental 

Planning Support Document 

A report of the USGS Grand Canyon Monitoring and Research Center 

By Theodore S. Melis, Scott A. Wright, Barbara E. Ralston, Helen C. Fairley, Theodore 

A. Kennedy, Matthew E. Andersen, and Lewis G. Coggins, Jr. 

In cooperation with Josh Korman, Ecometric Research, Inc. 

Final Draft, August 30, 2006 
ii

Acknowledgements 

This report documents results from two knowledge assessment workshops conducted in 

May and July, 2005. The content of this report was enhanced by the wisdom of 

participants attending the Knowledge Assessment Workshop I, conducted during the May 

18, 2005, Technical Work Group meeting held in Phoenix, Ariz., and the Knowledge 

Assessment Workshop II, which took place July 5–8, 2005, in Flagstaff, Ariz. 

Andre Potochnik 

Gary Burton 

Jeff English 

Bill Persons 

Rich Valdez 

Mark Steffen 

David Ward 

Glenn Bennett 

Dan Gwinn 

Randy van Haverbeke 

Rick Johnson 

Dennis Kubly 

Dave Garrett 

Matt Lauretta 

Glen Knowles 

Roger Pulwarty 

Wayne Cook 

Paul Higgins 

Mike Bradford 

Scott Rogers 

Dave Speas 

Clayton Palmer 

Stephen Wiele 

Jim Kitchell 

Elizabeth Fuller 

Stephanie Wyse 

Thomas Gushue 

Nick Voichick 

Amy Draut 

Mike Yeatts 

Tim Andrews 

Matt Kaplinski 

Michael Breedlove 

Peter Wilcock 

Larry Stevens 

Bill Davis 

Mary Barger 

Ken McMullen 

Norm Henderson 

David Topping 

Mark Schmeeckle 

Abe Springer 

Lisa Leap 

Wendy Batham 

Mike Yard 

Bill Werner 

Chris Kincaid 

Matthew Anderson 

David Rubin 

Bill Vernieu 

Susan Hueftle 

Lloyd Greiner 

All participants were given a draft version of this report and asked to comment on it. This 

document was revised on the basis of comments received from the participants, as well as 

additional comments provided by the Science Advisors in February 2006. 
ii

Table of Contents 

1.0 Introduction 1 

2.0 Definition of Management Actions and Uncertainty Rankings 6 

3.0 Summary of Decision Matrices 9 

3.1 Physical Resources Matrix 9 

3.1.1 Quality of Water – Downstream Temperature 9 

3.1.2 Fine Sediment Storage throughout Main Channel – 

Suspended-Sediment Flux and related Sand Habitats 11 

3.2 Hydropower Matrix 20 

3.2.1 Power Peaking Capacity and Financial Aspects of 

Replacement Power Costs 21 

3.3 Food Base, Fish, and Lees Ferry Angling Matrices 23 

3.3.1 Food Base 23 

3.3.2 Fish 31 

3.3.2.1 Mainstem Spawning & Incubation 31 

3.3.2.2 Young-of-Year/Juvenile Nearshore Rearing 33 

3.3.2.3 Invasive Fish Species 35 

3.3.2.4 Disease 36 

3.3.2.5 Adult Populations 36 

3.3.3 Angling Opportunity and Quality 37 

3.4 Riparian Habitat 40 

3.5 Recreation 44 

3.5.1 Campable Area 44 

3.5.2 Access to Attraction Sites 46 

 3.5.3 Rafting Navigability 48

3.5.4 Quality of Water and Human Health Issues 50 

3.5.5 Human Safety 51 

3.5.6 Recreational Experience 53 

3.6 Cultural Resources (National Register Historic Properties) 55 

4.0 Science Questions 59 

4.1 Physical Resources 59 

4.2 Hydropower 61 

4.3 Food Base, Fish, and Lees Ferry Angling 61 

4.3.1 Food Base 61 

4.3.2 Native Fish 61 

4.3.3 Rainbow Trout in Glen Canyon 62 

4.3.4 Nonnative Fish in Marble and Eastern Grand Canyons 63 

4.3.5 Lees Ferry Angling 63 

4.4 Riparian Habitat 63 

4.5 Recreation 63 

4.6 Cultural Resources 64 

5.0 References 65 
1

1.0 Introduction 

Background - An active adaptive ecosystem assessment and management approach to 

managing resources below Glen Canyon Dam (GCD) has been applied since 

approximately 1990. At that time, and in conjunction with the preparation of a major 

environmental impact statement on dam operations, a series of test flows were released 

from the dam. These test flows were conducted from June 1990 to August 1991, and 

represented the initial attempt to experiment with alternative operations to promote 

learning about how dam releases influence downstream natural and cultural resources of 

the Colorado River in Grand Canyon. Additional experimental treatments were 

implemented following the test flows of 1990, such as the first test of the beach/habitatbuilding flows concept (BHBF) in March 1996, the first test of the habitat maintenance 

flows (HMF) in 1997, and the low summer steady flow (LSSF) of 2000. A significant 

turning point in the ongoing science planning process occurred during the 1996–97 

period, following formation of the Glen Canyon Dam Adaptive Management Program 

(GCDAMP) and its Federal Advisory Committee, the Adaptive Management Work 

Group (AMWG). Since 1997, the AMWG and its Technical Work Group (TWG) have 

been actively involved in the science planning process and have periodically forwarded 

experimental recommendations to the Department of the Interior for testing Glen Canyon 

Dam operations. 

At the recommendation of the AMWG, longer range experimental planning was 

intensively undertaken by the U.S. Geological Survey’s (USGS) Grand Canyon 

Monitoring and Research Center (GCMRC) in 2002, and the first phase of a long-term 

experimental design was recommended for implementation. Several flow and nonflow 

treatments described in the 2002 experimental plan began at the direction of the Secretary 

of the Interior in January 2003, and the first 4 years of experimental treatments in that 

design are set to continue through at least 2006. Testing of the BHBF concept 

immediately following tributary sand inputs of at least 800,000 metric tons was 

implemented in November 2004 (Topping and others, 2006), which was the primary 

recommendation of sediment researchers. 

Overview of Experimentation - All of the experimental treatments implemented before 

2003 were evaluated as isolated events in time and some were criticized by science 

reviewers as being of such short duration that biological hypotheses tied to them might 

never be resolved. 

Between 1989 and 1995, planning for various flow experiments was undertaken jointly 

by the Bureau of Reclamation (Reclamation) and the U.S. Geological Survey, and 

included numerous and varied science cooperators. Most of this planning focused on the 

question of how various types of flows released from the dam would influence the 

transport and retention of fine sediment throughout Grand Canyon. The planning effort 

culminated in a large-scale field experiment in 1996, termed the controlled flood by 

scientists (Webb and others, 1999). This test focused on the concept of using an artificial 

flood release, termed the beach/habitat-building flows, or BHBF, to restore eroded 

sandbars using newly accumulated sand supplies provided by tributary floods. Following 
2

establishment of the Adaptive Management Work Group (September 1997), several 

attempts were made to deal with the issue of experimental design. For example, some 

design work was undertaken through competitively procured research contracts 

(identification of concepts and elements tied to a test of the seasonally adjusted steady 

flow operating alternative in 1998–99). Following this design study, a steady flow test 

was conducted from June through August 2000, termed the low summer steady flow, or 

LSSF, test. Results from this test are still being evaluated with respect to fishery 

responses, but humpback chub adult population data collected through 2005 suggests that 

this species may have benefited from the 2000 LSSF, which is indicated by a stabilization 

of the humpback chub adult population that began around 2000 (Melis and others, 2006). 

In 2002, the GCMRC proposed implementation of a 16-year long experimental design to 

address uncertainties associated with management of sediment and fisheries resources 

below Glen Canyon Dam. The main treatments within the experimental design included: 

1) additional testing of the BHBF concept to restore and maintain sandbars and related 

habitats, 2) evaluation of expanded diurnal fluctuations intended to limit reproduction and 

recruitment of nonnative fish, and 3) repeated mechanical removal of nonnative 

coldwater fish from the Colorado River ecosystem (CRE) in the vicinity of the Little 

Colorado River confluence. This last treatment was intended to limit competition for food 

between nonnative and native fish, as well as minimize predation of natives by nonnative 

species to promote recruitment of native humpback chub. Mechanical removal was 

proposed to be implemented in 4 consecutive years (2003–6), with six removal trips 

occurring each year. The flow-suppression treatment was intended to overlap with the 

mechanical removal treatment of the first 2 years, and then be suspended in the final 2 

years of the mechanical removal effort. The sediment experiments (implementation of a 

2.5-day BHBF release following introduction of new sediment supplies from the Paria 

River) was proposed to be implemented in the first year in which fine-sediment 

production from the Paria River reached or exceeded 800,000 metric tons. 

In response to the 16-year experimental proposal, the AMWG recommended to the 

Secretary of the Interior that only the first two annual treatments (the BHBF and 

expanded diurnal fluctuations) be undertaken starting in 2003, and that the first of several 

proposed sediment tests be implemented at the first opportunity (agreed upon levels of 

fine-sediment enrichment from Paria River). The sediment test was conducted in 

November 2004, following the required inputs. Mechanical removal of nonnative fish in 

Marble Canyon was conducted from January 2003 to August 2006. Nonnative 

suppression flows were conducted for 3 consecutive years from 2003 to 2005 to be 

followed by a return to the modified low fluctuating flow (MLFF) alternative (the 

“preferred alternative” in the 1995 Operation of Glen Canyon Dam Final Environmental 

Impact Statement (EIS)) in 2006. 

The 2003 through 2006 treatments are unique in that some treatments were repeated over 

multiple years (alternative fluctuations in winter and nonnative removal) and coincided 

more closely with the early life-history stages and recruitment of higher trophic 

organisms such as native fishes. As a consequence of persistent upper Colorado River 

Basin drought and decreased storage in Lake Powell warmer water releases from Glen 
3

Canyon Dam coincidentally started in the same year as experimental removal of 

nonnative fishes and have continued each summer and fall into 2006. Although 

unplanned, the combination of warm releases and successful reduction in downstream 

nonnative fish abundance has the potential to result in a recruitment signal with respect 

humpback chub response below Glen Canyon Dam. Because of the lag effect in the early 

life history of native fishes, recruitment data for humpback chub entering the system in 

2003 through 2006 cannot be fully assessed through monitoring and modeling efforts 

until at least 2007 through 2010. Although qualitative in nature, catch-per-unit-effort data 

for data for native species of fish (derived from nonnative removal activities) in summer 

2005, and seining data from backwater habitats in September 2005 in lower Marble 

Canyon, show that abundance of juvenile humpback chub increased compared with 2003 

through 2004 data (Melis and others, 2006). While juvenile abundance cannot be 

interpreted to be an indicator for recruitment of native fishes, the data are an encouraging 

sign that may be tied to reduced trout and other nonnative fish abundance, newly 

modified sandbar habitats (resulting from the November 2004 sediment, high-flow test) 

and consistently warmer main channel water temperatures below the dam. 

The process of evaluating new information from past or current experimental actions, as 

well as planning new experiments, is ongoing and can be historically characterized within 

at least four phases of study: (1) Phase I involved early studies by the National Park 

Service and the Glen Canyon Environmental Studies (GCES I, 1972–88), (2) Phase II 

was marked by the preparation of the EIS (GCES II, 1989–95), (3) Phase III included the 

formation of the GCMRC and transition monitoring and flow experiments (Post-EIS, 

1995–2000), and (4) Phase IV as represented by research in support of monitoring design 

and conceptual modeling (Integrated Experimental Phase I, 2001–6). (For more details 

on the use of conceptual modeling of the Colorado River ecosystem in science planning, 

see Walters and others, 2000; and Melis and others, 2005.) Each of the phases represents 

a unique period of advancement in learning about how dam operations are related to 

downstream resources. However, Phase IV is perhaps the most unique era in the program 

because it represents the first period in which the concept of an experimental design was 

implemented for more than a single year. 

To further promote the idea of learning under an ongoing experimental design, in 2005 

managers involved in the Glen Canyon project embraced the idea of continuing the 

experimental process in future years. The goal of the planning process was to maintain an 

active adaptive management effort that focuses experimental research on reducing 

uncertainty about proposed management actions for achieving environmental objectives 

below Glen Canyon Dam. As a milestone in the planning process, which occurred near 

the end of Phase IV, a knowledge assessment, which relied on the participation of 

knowledgeable scientists and managers in a workshop setting, was mandated by the 

AMWG. The knowledge assessment was intended to document learning about the 

various experimental treatments that had been conducted since operations were first 

altered in 1991. In 2005, the scientists complied with this request as a first step in the 

next phase of planning and experimental design (Phase V). 
4

The objectives of the knowledge assessment were to: (1) evaluate the uncertainties that 

persist regarding individual resource attribute responses of the Colorado River ecosystem 

to the various management actions (both flow and nonflow elements) undertaken as 

experiments over the last two decades, (2) develop strategic science questions that would 

need to be addressed to further reduce the uncertainties associated with the various flow 

and nonflow treatments, and (3) identify research and monitoring strategies that might 

need to be undertaken to answer the science questions identified. The 2005 knowledge 

assessment was conducted within two workshops (May and July 2005), both of which 

promoted information transfer between scientists, as well as focused discussion between 

scientists and managers about past research and monitoring relative to various treatments. 

The experimental treatments evaluated during the discussions, included knowledge 

gained about both flow and nonflow actions (artificial floods to restore beaches and 

mechanical removal of exotic coldwater fishes). As a frame of reference for evaluation 

and discussion purposes, particular emphasis was centered on the MLFF operating 

criteria for dam releases, the “preferred alternative” identified in the Record of Decision 

(ROD) that was implemented by the Department of the Interior in 1996. 

One anticipated outcome of the knowledge assessment process was that certain 

treatments might be understood well enough that additional experimentation was not 

required to move the treatment into the category of a fully implemented management 

action. Alternatively, knowledge about a particular treatment might also be so complete 

and the benefit of the treatment with respect to downstream resources so small (or 

possibly even detrimental relative to stated environmental goals) that the treatment option 

might be abandoned completely. If an action was deemed beneficial and transferred to 

“management status,” it would be routinely implemented and only dealt with in the 

science program in the context of conceptual modeling and monitoring for status and 

trends of the targeted resources. It is important to note that classification of an action to 

“management status” is a decision that depends not only on scientific certainty, but also 

on values concerning restoration and financial impacts. For example, there is 

considerable scientific uncertainty about whether mechanical removal of nonnative 

rainbow trout and other nonnative fishes will result in a long-term increase in the 

abundance of native humpback chub. Nevertheless, mechanical removal could be given 

“management status” if the perceived potential restoration benefits outweighs the 

financial costs and impacts on other resources. We stress that our classification of 

certainty of various management actions is purely based on a scientific assessment using 

the available data. We make no attempt to integrate this assessment with our perception 

of stakeholder values to classify actions into “management” and “experimental” status. 

This report documents and summarizes the results from the GCMRC’s 2005 knowledge 

assessment workshops. The report is not intended to provide a state of knowledge about 

the Colorado River ecosystem. This would require an extensive literature review and a 

level of effort that is well beyond the original scope of the knowledge assessment. This 

summary report documents what was discussed at the two knowledge assessment 

workshops. Citations are provided when they apply to particular discussions. This level 

of documentation is more than sufficient for experimental planning and framing of key 

science questions, which are the primary objectives of the knowledge assessment. During 
5

summer and fall 2006, the information contained in this summary report provided the 

basis or current knowledge for evaluating the biophysical and sociocultural resource 

implications associated with three experimental options (Options A, B, and C) proposed 

by members of the Science Planning Group (an ad hoc of the Technical Work Group). As 

in the knowledge assessment workshops of 2005, the MLFF operating policy for Glen 

Canyon Dam was used in those experimental evaluations as a frame of reference for 

comparing the three experimental operations and management alternatives. A 

recommendation from the Technical Work Group for one of the three options is 

anticipated on the basis of the resource evaluation in winter 2006. 

The knowledge assessment report is divided into four sections, including this 

introduction. Section 2 defines the management actions that were considered in the 

exercise and the general approach taken in uncertainty classification. Section 3 

summarizes the results in matrices for physical science (3.1), hydropower (3.2), aquatic 

food base/fish (3.3), riparian habitat (3.4); recreation (3.5), and cultural (3.6) resource 

groups. In Section 4, the uncertainties defined in the matrices are used to develop a series 

of scientific questions for each major resource group. 
6

2.0 Definition of Management Actions and Uncertainty Rankings 

A consequence table or decision matrix (hereafter, referred to as the ‘matrix’) predicting 

the response of key performance measures (rows) to particular experimental management 

treatments (columns) was populated at the Knowledge Assessment Workshop (KAW). 

This section defines the actions considered in the exercise. These were derived mainly 

from those considered as part of the Glen Canyon Dam Environmental Impact Statement, 

but also include new flow and nonflow actions that are currently being implemented but 

were not defined in the EIS. Definition of performance measures is provided in Section 3. 

Definitions of uncertainty rankings for each prediction are provided at the end of this 

section (Table 2.1). 

The experimental management options considered in the KAW were evaluated relative to 

the MLFF operations, which for the purpose of the evaluation were considered to be the 

experimental control. Treatments discussed and evaluated during the KAW’s included: 

Increases in GCD Release Water Temperature: The increased downstream 

temperatures could result from naturally occurring conditions, dam operations or a 

combination of both factors related to: 1) the construction of a Selective Withdrawal 

Structure (SWS) or Temperature Control Device (TCD) at the dam, 2) reduced lake 

elevations associated with diminished storage resulting from prolonged drought or 

increases in water use by Upper Basin states or 3) release of stable versus fluctuating 

flows from the dam. Until recently, release temperatures from Glen Canyon Dam (GCD) 

ranged from 9–11°C. The increased temperatures that were considered ranged from 14–

17°C between June and November. 

Increased Fluctuations Relative to the Modified Low Fluctuating Flow Alternative:

The extent of daily fluctuations in discharge from GCD is determined by the constraints 

on releases as defined in Record of Decision (up and down hourly ramping rates, 

maximum and minimum daily flows, maximum daily flow change). Increased daily 

fluctuations, such as those before 1990 (no action period) or under the recent 

experimental fluctuations (January–March 2003–2005) involved changing all or many of 

the constraints. Making such changes has a confounding influence on scientific attempts 

to determine cause-affect relationships for flow parameters and downstream resource 

responses related to ramping rates versus daily range variations. In an attempt to provide 

informative assessments, we attempted to predict the overall effect of increased 

fluctuations in the resource matrix. For future experimental flow planning, however, it 

will be necessary to identify the individual dam operating constraints that are most 

important in determining performance measure responses downstream. When possible, 

predictions for individual flow constraints were made, but we recognized that hourly to 

daily operating rules were to a large extent interrelated for a given monthly volume 

release. 

Reduced Variation in Monthly Volume: Changes in the monthly release volume from 

GCD can be large. This scenario reduced the variation in release volumes across months, 
7

further flattening the annual pattern of releases, while maintaining ROD peaking 

capabilities over the diurnal timescale tied to the monthly release volume. 

BHBFs and HMFs: Predictions about the effects of beach/habitat-building flows 

(41,000 to 45,000 cfs) and habitat maintenance flows (powerplant capacity ~31,500 cfs) 

with and without an adequate sand supply on the bed were made. Fall and winter/spring 

floods were considered. 

Sustained Low Steady Flow: The effects of low and steady flows were predicted (8,000 

cfs, as was tested in the 2000 low summer steady flows test). The timing of this scenario 

sometimes varied among resources. For example, low and steady flows following the 

monsoon season (later summer through early winter) were considered for sediment 

performance measures, while fish performance measures considered the summer and fall 

period. 

Sustained High Steady Flow: This scenario considers prolonged high and steady 

releases of 20,000 cfs or higher as would occur when inflows are high and Lake Powell is 

relatively full, or to meet a specific management objective such as causing ponding at 

tributary mouths. The timing and duration of high flows varied among performance 

measures but was generally during the spring for 2 weeks to 1 month. 

Mechanical Removal: These scenarios consider mechanical removal of coldwater and 

warmwater nonnative fish species in the mainstem and tributaries; although, only data on 

coldwater species were available for evaluation from recent experimental work in Marble 

and Grand Canyons. 

Humpback Chub Hatchery Supplementation and Tributary Translocation: These 

scenarios consider addition of hatchery-reared juvenile fish into the mainstem, and 

translocation of juveniles or adults from the Lower Colorado River (LCR) to the LCR 

above Atomizer Falls or to other tributaries. 

 

A prediction of the direction of response of each performance measure [decline (-), no 

change (0), improvement (+)] to each action was made along with a ranking of the 

uncertainty of the predictions. If the direction of response could not be determined the 

prediction was defined as highly uncertain and colored RED. If a prediction of the 

direction of response could be made, but was based on limited data and there was a 

relatively low probability that the predicted direction of response was correct (50–70%), 

the prediction was considered uncertain and colored YELLOW. If the prediction was 

based on more data and there was a higher probability that the direction was correct (70–

90%), the prediction was considered relatively certain and was colored LIGHT GREEN. 

If a quantitative prediction about the magnitude of response could be made, which 

required substantial data integrated into a model or stock assessment procedure, the 

prediction was considered very certain and colored GREEN. Table 2.1 summarizes the 

criteria that define the four levels of uncertainty used in this analysis. 
8

Table 2.1. Summary of definitions used to rank uncertainty of predictions in the decision matrix. 

Very Certain Certain Uncertain Very U

Prediction 

Direction and magnitude of response Direction only Direction only Cannot pred

Supported by Data from Colorado River Ecosystem 

Peer reviewed, likely involving a model. Little debate on interpretation of predictions 

Peer-reviewed results, no model 

Limited data, data without peer review, and likely debatable inference No or very l

Data from Other Reference Systems 

Validated prediction in other system that is considered a good model for CRE 

Validated prediction in other system that is a weaker model for CRE 

Weaker prediction from other system that is a weak model for CRE 

No or very l

in other systsystems are model of CR

General Theory / Conventional Wisdom Very Strong Good Moderate Low Probability that Predicted Direction is Correct 90–100% 70–90% 50–70% <50% 
9

3.0 Summary of Decision Matrices 

The decision matrices developed at the KAW are described in the following six sections: 

physical resources (3.1); hydropower (3.2); aquatic food base/fish, and Lees Ferry 

angling (3.3); riparian habitat (3.4); recreation (3.5); and cultural (3.6). 

 

3.1 Physical Resources Matrix 

The decision matrix for physical resources (in this draft, only the ability to predict 

downstream water temperature on the basis of measured or predicted release 

temperatures from GCD, and suspended fine-sediment flux and channel storage) was 

populated at the KAW and is presented in Table 3.1. Owing to time limitations at the July 

KAW, only a subset of the physical resources of interest were discussed: 1) average 

downstream water temperature in the main channel (1-dimensional) and in a variety of 

nearshore habitats (multidimensional), such as eddies and backwaters as related to Goal 

#7 of the GCDAMP Strategic Plan; 2) suspended-sediment flux between sediment 

producing tributaries (influx) versus downstream transport in the main channel (efflux); 

and 3) fine-sediment storage throughout the channel at specific stage elevations described 

in the GCDAMP management objectives under Goal #8. Additional resources related to 

quality of water in Lake Powell and downstream, as well as the flux of coarse-grained 

sediment will be incorporated into future knowledge assessments as experimental and 

monitoring planning activities continue. 

3.1.1 Quality of Water – Downstream Temperature 

Increased Fluctuations – These result in higher flow velocities during part of the diurnal 

pattern of hydropower generation (peak) and lower flow velocity during the trough. 

Increased flow velocities (and water depth) generally equate to decreased warming of 

water released from Glen Canyon Dam owing to increased downstream travel times, and 

the opposite is true for decreased velocities during the trough (Vernieu and others, 2005). 

Thus, the overall effect on mainstem temperature is tied to the balance between decreased 

warming during the peak and increased warming during the trough, and is dependent on 

the specifics of the increased fluctuation. For example, if the peak is increased while the 

low remains unchanged, this would lead to overall decreased warming. On the other 

hand, if the peak is left unchanged but the low is decreased then this would lead to 

increased warming. Wider ranging fluctuations are generally known to promote exchange 

between colder main channel water and warming water in nearshore habitats, such as 

backwaters (Hoffnagle 1996; Korman and others, 2005b; Kaplinski and others, 2004). 

Additional information will be available in the future through development and 

application of mainstem and nearshore water temperature models by Reclamation (Amy 

Cutler, oral communication, August 2005) in collaboration with the USGS (Scott Wright, 

oral communication, August 2005). 

Reduced Variation in Monthly Volume – For any given annual release volume scheduled 

at the dam under the current Record of Decision, months with lower release volumes will 

result in lower average flow velocities. The lower velocities and depths are known to 
10

generally promote warming as water travels downstream from the dam over longer time 

periods and has less volume to warm (Vernieu and others, 2005), but this varies by 

season. Monthly volumes that are lower during portions of the annual release where 

warmer water from Lake Powell is passed through the powerplant (typically late summer 

through fall) will promote maximum warming owing to solar radiation, but the solar 

warming decreases as the annual cycle of release temperature increases in the fall. 

Reducing the variation in the typical pattern of annual monthly volumes would tend to 

decrease the volumes in July and August, but would also likely increase the September 

and October volumes. As warmer releases occur at the dam in late summer, additional 

downstream warming becomes progressively more limited as solar radiation decreases in 

the Canyon into fall. Canyon effects from the steep walls enhance the decrease in 

seasonal solar radiation reaching the river. Lower peaks and slower velocities will delay 

travel time and promote warming for months where the flattening of the annual pattern 

decreases the volume relative to what would be released under the typical ROD operating 

strategy. Additional information will be available in the future through development and 

application of mainstem and nearshore water temperature models by Reclamation (Amy 

Cutler, oral communication, August 2005) in collaboration with the USGS (Scott Wright, 

oral communication, August 2005). 

BHBFs and HMFs – These sustained high flows greatly increase downstream flow 

velocity for relatively short periods of time (2–4 days) and elevate river stage, 

temporarily flooding shoreline habitats. The large volume and faster downstream travel 

time of releases limits warming that can occur owing to solar radiation in the main 

channel (Vernieu and others, 2005). Because nearshore temperatures are linked to 

mainstem temperatures, the decreased mainstem warming would result in cooler 

nearshore temperature at a given site. However, shallow areas along shorelines that are 

not normally inundated might temporarily warm owing to limited flow depth in 

stagnation zones. These types of releases were originally proposed as spring events when 

releases from Lake Powell are not as warm as they are in late summer or fall. The 

transient nature of these high-flow departures suggests that their influence on the 

temperature regime of the river habitats may be of limited concern. Additional 

information will be available in the future through development and application of 

mainstem and nearshore water temperature models by Reclamation (Amy Cutler, oral 

communication, August 2005) in collaboration with the USGS (Scott Wright, oral 

communication, August 2005). 

Sustained Low Steady Flow During Summer – These types of releases promote stability 

in the depth and flow velocities of the main channel and nearshore environments 

downstream of the dam. Reducing the velocity and depth and increasing the travel time 

of releases from the dam is known to have the effect of promoting warming of the water 

as it moves downstream (Vernieu and others, 2005). The stable element of the flow, in 

terms of stage, also decreased water exchange between the mainstem and eddies, thus 

promoting warming in nearshore environments (Kaplinski and others, 2004). Additional 

information will be available in the future through development and application of 

mainstem and nearshore water temperature models by Reclamation (Amy Cutler, oral 
11

communication, August 2005) in collaboration with the USGS (Scott Wright, oral 

communication, August 2005). 

High Sustained Flow During Spring – Owing to higher flow velocities and depths, and 

decreased downstream travel times, these releases will limit warming in the main channel 

derived from solar radiation. Because nearshore temperatures are linked to mainstem 

temperatures, the decreased mainstem warming would result in cooler nearshore 

temperature at a given site. However, shallow areas of nearshore habitat that are 

temporarily inundated during these sustained periods of high flow may warm, especially 

in areas where flow stagnation occurs within eddies. This type of effect will be less than 

when flows are low steady, but the area of warming might be larger. Additional 

information will be available in the future through development and application of 

mainstem and nearshore water temperature models by Reclamation (Amy Cutler, oral 

communication, August 2005) in collaboration with the USGS (Scott Wright, oral 

communication, August 2005). 

Non-GCD Actions – Nonflow treatments, such as mechanical removal, do not influence 

downstream water temperature. However, other nonflow elements of interest mostly 

include the annual pattern of solar radiation in combination with the physical aspects of 

the river channel within Glen, Marble, and Grand Canyons (i.e., canyon orientation and 

geometry, reach morphology). Tributary spates versus base flow discharges also 

influence the temperature of the main channel and related nearshore habitats. Additional 

information on these factors will be available in the future through development and 

application of mainstem and nearshore water temperature models by Reclamation (Amy 

Cutler, oral communication, August 2005) in collaboration with the USGS (Scott Wright, 

oral communication, August 2005). 

3.1.2 Fine Sediment Storage throughout Main Channel – Suspended-Sediment Flux and 

related Sand Habitats 

Increases in GCD Release Water Temperature – Elevated temperature of dam releases 

has a limiting influence on suspended-sediment transport of fine sand in the river (e.g., 

Lane and others, 1949; Hubbell and Ali, 1961; ASCE, 1975). This is the result of 

viscosity effects in the water column that enhance settling rates for certain grain sizes of 

sand (sand particles spend less time in suspension and more time on the channel bed, 

hence slowing downstream migration or export from the ecosystem for a given daily dam 

release). Suspended transport may also be influenced by the types of bed forms that will 

persist along the channel bottom. This phenomenon is thought to be most significant over 

the annual range of river temperatures that naturally occurred in the predam era (1–28°C) 

and is now relatively limited due to the anticipated range of natural warming associated 

with Lake Powell under drought conditions or with operation of a Selective Withdrawal 

Structure designed and operated to release warmer water from the reservoir (~9–15°C). 

The size range of particles that are known to be most influenced by water temperature 

effects is centered around fine sand, a size range that coincides closely with the median 

grain size of sand produced by the Paria and Little Colorado Rivers (Colby and Scott, 

1964; Lane and others, 1949). 
12

Increased Fluctuations Relative to the Modified Low Fluctuating Flow Alternative– 

On the basis of suspended-sediment transport theory and data, increased diurnal 

fluctuating flows from Glen Canyon Dam result in increased downstream transport and 

export of both newly input and background sand from areas of the river channel within 

the range allowed under the current ROD (DOI, 1995). Laursen and others (1976) 

estimated that the Colorado River below Glen Canyon Dam would persist in a sand 

deficit state under the range of unconstrained fluctuating-flow operations associated with 

the legal mandates for downstream water transfers, the powerplant design, and historical 

Upper Colorado River basin hydrology. This conclusion was made on the basis of the 

limited sand supply available from tributaries below the dam as well as the fine-grained 

nature of those sand inputs. In fact, Laursen and others’ (1976) over estimated the level 

of sand transport below the dam for operations that occurred during the so called “noaction” period from 1966 to 1990. However, more recent, high-resolution suspendedsediment measurements made over a range of sediment supply conditions below the dam 

(Topping and others, 2004; Topping and others, 2006) still support Laursen and others 

(1976) conclusion that the river system persists in a state of sand deficit, even under 

Modified Low Fluctuating Flow operations during consecutive years (2000 through 

2004) of minimum hydrology and associated release operations required under the 1996 

ROD (Rubin and others, 2002; Topping and others, 2000a; 2000b). The only documented 

departure from this persistent state of sand deficit occurred in association with the 

November 2004 high-flow sediment test which resulted in a net positive sand mass 

balance for the ecosystem (Topping and others, 2006) when a short-duration high flow of 

41,000 cfs (60-hour duration) was released following a period of Paria River sand inputs 

(equal to about the long-term historical median annual input). The sand mass balance of 

the ecosystem also benefited during Water Year 2004–5, from intermittent sand inputs 

from the Little Colorado River following the 2004 test, throughout winter 2005–6. 

Daily Peak Flows - Increasing the daily range of peaking power generation may result in: 

1) higher peaks than would otherwise occur, 2) daily peaks that are of longer duration 

than would otherwise occur, or 3) a combination of both diurnal operational elements. 

Both the MLFF operation and any other operations that promote higher or longerduration diurnal peak flows are known to promote: 1) export of new tributary sand 

introduced to the ecosystem that resides in the main channel, 2) export of pre-existing 

sand stored in similar environments and 3) sandbar erosion of deposits that exist both 

above and below the allowable daily fluctuating flow peak of 25,000 cfs (DOI, 1995; 

Topping and others, 2004; Rubin and others, 2002; Schmidt and others, 2004; Topping 

and others, 2006). Changes in the daily pattern of fluctuating releases that do not result in 

increased daily peak flow, but lower daily minimum releases will also result in higher 

sand export for a given monthly release volume as such patterns result in longer duration 

peak flows each day. The elevated sand transport occurs owing to the fact that sand 

conservation associated with suspended transport during the lower daily minimum 

releases is more than equaled by higher transport rates during the extended duration of 

the daily peak flow. This phenomenon is related to the exponential relationship that exists 

between suspended-sand transport and discharge in general. In the Colorado River 
13

ecosystem, high-resolution suspended-sand monitoring indicates that this exponential 

relationship is to the 4th or 5th power. 

The sand transport dynamics of the Colorado River below Glen Canyon Dam have been 

related to geomorphic changes of sand deposits through detailed studies and recent 

synthesis of those research results (Schmidt and others, 2004). Historical sandbar 

mapping data (Schmidt and Graf, 1990) showed rapid erosion rates between 1986 and 

1990 (period of unconstrained fluctuations) following deposition of large sandbars 

created during the 1983–84 period of high-flow releases. Schmidt and others (2004) 

determined that the area of sandbars was decreased by about 25 percent between 1984 

and 2002 despite the constraints imposed upon the daily fluctuating operations of the 

ROD. Ongoing erosion of these sandbars and related fine-sediment habitats during the 

period of MLFF operations parallels the documented sand mass-balance deficit reported 

by Rubin and others (2002) and Topping and others (2006) between 1999 and first half of 

Water Year 2004. 

As reviewed by Wright and others (2005) conventional theory and field experiences have 

historically identified discharge as the primary factor governing suspended-sediment 

transport in most rivers and streams characterized as having alluvial channels. The 

bedrock controlled Colorado River in Grand Canyon is atypical in this respect, as was 

reported by Rubin and others (2002). Earlier studies by Rubin and Topping (2001) 

reported that the degree to which the Colorado River ecosystem has recently been 

enriched with fine sediment by tributary floods, or has been depleted of fine sediment by 

clear water releases between enrichment episodes, have as much or more influence on the 

suspended-sediment export rates as does discharge from Glen Canyon Dam. Topping and 

others (2005) also demonstrated that during most of the postdam era, main channel 

suspended sand transport has responded to the level of sand storage within eddies, during 

the periods that main channel sand supply conditions have been depleted or winnowed. 

Further, Melis and others, (2003), suggested that new methods of suspended-sediment 

monitoring in the Colorado River ecosystem [combined use of beta (also, see Rubin and 

Topping, 2001) and laser-acoustics technologies] might be used at discrete points below 

the dam to monitor the grain-size condition (level of fine-sediment enrichment with 

respect to sub-aerial distribution of the channel bed in real time) following periods of 

tributary enrichment. 

Ramping Rates – There are two elements to this topic relative to sand conservation and 

sandbar stability: 1) influence on sandbar stability with respect to seepage forces within 

sandbars (destabilizing existing sandbars along shorelines through bank failures, etc. 

during and immediately following daily periods of river stage decrease) and 2) influence 

on sandbar erosion with respect to “tractive” or entrainment forces related to flow 

turbulence that promotes re-suspension and transport of sand away from eddies and 

downstream (promoting sand export). 

Previous field studies undertaken during the Glen Canyon Dam EIS, and model 

simulations from those studies, led directly to current down ramping rates associated with 

MLFF and the Record of Decision (Budhu, 1992, 1995a, 1995b, 1997; Budhu and Gobin, 
14

1994, 1995a, 1995b; Budhu and others, 1994). Only recently, as part of the November 

2004 high-flow test, were additional field studies proposed to identify whether or not the 

ROD ramping rates are appropriate (or perhaps overly conservative) with respect to 

sandbar stability under MLFF operations. Additional studies are currently underway to 

better understand the groundwater dynamics that occur within sand banks under 

fluctuating flows with various rates of stage change (M. Schmeeckle, oral 

communication, GCMRC Science Symposium, October 2005). 

While we may not fully understand the influence of the seepage processes related to 

sandbar erosion and ramping rates, the daily allowable fluctuating range is also an 

important part of the overall stability of sandbars. It is also clear that any change in the 

daily range and ramping rates directly influences the duration of the daily peak discharge 

(for a given monthly volume). Both sediment transport theory and recent high-resolution 

suspended-sediment transport data collected below Glen Canyon Dam indicate that 

longer periods at peak discharge increase downstream export of new sand supplies and 

increase erosion of existing sandbars within the allowable MLFF fluctuating range (up to 

25,000 cfs) associated with the 1996 Record of Decision. These results are best 

documented for the experimental fluctuations (5,000 to 20,000 cfs) that occurred in 

January through March 2003, 2004 and 2005 (Topping and others, 2004; Topping and 

others, 2006). 

Reduced Variation in Monthly Volume – If the pattern of monthly release volumes at 

Glen Canyon Dam was more evenly distributed throughout the annual release period, the 

daily peaks under the MLFF (or other non-ROD peaking strategies) would be more 

limited in magnitude and duration than those associated with historical patterns of 

monthly releases. This operational influence on sediment transport is most significant 

when annual release volumes are minimal (8.23 million acre feet); owing to the fact that 

daily peaks for equal monthly volumes would likely be limited to about 13,000 to 14,000 

cfs (equalized monthly release volumes for each month of 686,000 acre feet). Recent 

daily peaks under the MLFF associated with minimal release hydrology (8.23 million 

acre feet) and historic monthly patterns (see Wright and others, 2005, p. 25), have ranged 

between 17,000 to 19,000 cfs (typically in June through August) and experimental winter 

fluctuating flows have been at 20,000 cfs (January through March). 

Data reported by Wright and others (2005) and Topping and others (2004; 2006) have 

shown that these peak flow periods have been responsible for export of new tributary 

sand inputs within the same year as the inputs entered the ecosystem during Water Years 

2001 through the summer of 2004. During the spring and fall months of these same years, 

MLFF operations resulted in much lower daily peaks ranging from 10,000 to 14,000 cfs 

and tributary sand inputs were allowed to accumulate in the main channel of the 

ecosystem (especially in the September through December periods), before the release of 

higher winter and summer peaks. The results reported by Wright and others (2005); 

Schmidt and others (in review); Hazel and others (2006); Topping and others (2006), 

indicate that new tributary sand inputs are clearly conserved and accumulated in the main 

channel at stable flows of 8,000 cfs (summer 2000, low summer steady flowtest) and 

under fluctuating flows ranging between 5,000 and 10,000 cfs (fall 2004 period before 
15

high-flow sediment test). Sand transport monitoring data collected since summer 1999 

indicate that from 55% to 78% of tributary sand inputs accumulated within the main 

channel of the Colorado River below Glen Canyon dam during months when MLFF 

operations occurred during months with release volumes in the range of 610,000 acre 

feet. On average, between 82% and 91% of sand inputs were retained during MLFF 

operations in the monitoring reach during months have release volumes in the range of 

490,000 acre feet (S. Wright, oral communication, 2006). 

The 2001 through 2004 data suggest that by limiting daily peaks to 13,000 or 14,000 cfs 

throughout the year tributary sand inputs would tend to be more effectively conserved as 

export rates would be substantially limited relative to typical summer daily peaks. 

However, it is not currently possible to accurately predict whether or not such a flow 

regime would result in sand accumulation within the ecosystem over multiyear periods, 

as was predicted under the MLFF preferred alternative of the 1995 EIS (DOI, 1995). 

Suspended sand transport data collected in September through early November 2004 

indicate that new sand inputs from the Paria River were mostly conserved within the 

upper reaches of Marble Canyon (mostly above river mile 11) under fluctuating flows 

that consistently ranged from 5,000 to 10,000 cfs (Topping and others, 2006). It is not 

possible to accurately predict the fate of similar inputs under flows of 7,000 to 13,000 cfs 

that might occur year around if an 8.23 million acre foot release volume were distributed 

equally throughout the water year under MLFF operating constraints. Hence, such an 

operation would be useful as a test to acquire the transport data needed to fully evaluate 

such a modification in operating strategies. 

Both theory and data indicate that suspended-sediment transport and downstream export 

are exponentially related to discharge for a given fine-sediment supply (to the 4th or 5th

power). As a result, reduced daily peaks of limited duration throughout the year will 

result in reduced downstream transport and export of limited sand supplies delivered to 

the ecosystem by the tributaries below the dam. This limitation of the sand transport rates 

would also limit erosion of sandbar habitats along the river banks. A test of annual peaks 

constrained by 8.23 MAF hydrology, equal monthly volume releases, and the Record of 

Decision (MLFF) operation would provide valuable new information to determine 

whether or not new tributary sand inputs can be accumulated over an entire year and 

across multiyear periods in the main channel, before release of beach/habitat-building 

flows intended to restore and maintain sandbar habitats. 

BHBFs and HMFs – As recently described by Rubin and others (2002), these dam 

operations are a two-edged sword in that sandbars can only be deposited and maintained 

under conditions of elevated river stage, but those same elevated flows also promote 

much higher suspended-sediment transport rates. High flow velocities and sand transport 

rates therefore, can also result in large sand export volumes to Lake Mead, as well as 

sandbar erosion at sites where suspended-sediment concentrations are lower when new 

sand supplies are not abundantly available (Rubin and others, 2002; Rubin and Topping, 

2001). Sand bars must be inundated by main channel river flow carrying sufficient 

suspended sand concentrations where the sand will rain out of suspension when flow 

velocity decreases abruptly, as in eddies and along stagnant shorelines. Hence, new 
16

sandbar deposition requires that there be abundant sand supply in the river during a 

period when river flows are elevated, in the case of the beach/habitat-building flows 

(BHBF), typically to somewhere in the range of 41,000 to 45,000 cfs. During such highflow departures sand transport rates are increased to much higher levels than normally 

occurs under the typical MLFF operation (fluctuating diurnal flows not exceeding 25,000 

cfs stages). The basic goal then for such operations is to optimize for sandbar deposition 

(highest stage under the most fine-sediment enriched supply condition) while limiting 

downstream export of new sand and erosion of existing sandbars (minimal export to Lake 

Mead). 

Data from the 1996 controlled flood test (see Webb and others, 1999) indicated that the 

7-day long duration of that BHBF experiment was far too long for the available sand 

supply in the main channel (Topping and others, 1999; Schmidt, 1999). Most sandbar 

responses that were measured during the 1996 test suggested that the bar geometry was 

altered (the higher, but not wider results documented in the NAU sandbar sub-sample), 

but sandbars were not increased in overall size through incorporation of new, tributary 

sand supplies that were assumed to have accumulated between 1991 and 1996, (as was 

hoped for to support sandbar restoration). Data from the three HMF tests conducted in 

1997 and 2000 indicate that the sustained flows of 31,500 cfs (3–4 days) were very 

efficient in exporting sand downstream, but had what was interpreted as a stage-limited 

result with respect to depositing main channel sand within eddies to restore sandbars 

(Topping, oral communication, July 2005, Wiele and others, 2002; Hazel and others, 

2006). Preliminary results from the November 2004, high-flow test (60-hours of 

sustained flow at about 41,000 cfs) suggest that the reduction in peak flow duration (168 

vs. 60 hours) in the recent test was warranted under the more highly enriched antecedent 

sand supply conditions in the main channel, but that the uneven longitudinal distribution 

of the new tributary sand delivered by the Paria River throughout Marble Canyon may 

have actually limited sandbar restoration response to eddies immediately downstream 

from where new sand inputs were deposited in the main channel under low, diurnal, 

fluctuating flow operations (5,000 to 10,000 cfs) between the time of tributary inputs and 

release of the high-flow test in November (Topping and others, 2006). 

Although data analysis is still underway for the 2004 test, future BHBF implementation 

might more effectively consist of even shorter duration peak flows (24 to 48 hours) under 

enriched sand supplies that are more uniformly distributed downstream throughout entire 

reaches where sandbar restoration is desired (for instance, Paria River sand inputs from 

summer and fall are allowed to be partially winnowed and distributed downstream by 

limited dam operations between 10,000 and 15,000 cfs from November through 

December or January, before release of the BHBF). An experimental test of such a “preconditioning” operation on new sand supplies might be useful as a means of learning how 

to further optimize sandbar restoration while limiting sand export and bar erosion. 

Despite two tests of the BHBF concept (1996 and 2004), it is still not clear whether a 

“flow-only” prescription exists operationally that can achieve the objective of restoring 

and sustaining sandbar habitats below Glen Canyon Dam using only 6 percent (Marble 

Canyon) to 16 percent (Marble and Grand Canyons) of the Colorado River’s predam 

fine-sediment supplied by the Paria and Little Colorado Rivers (Wright and others, 2005). 
17

The utility of HMF’s as a tool to “maintain” nearshore habitats is still unclear and 

additional analyses of data and future studies are needed. However, results to date on 

such operations suggest very limited potential for them to effectively promote sandbar 

restoration objectives (Wiele and others, 2002; Hazel and others, 2006; Schmidt and 

others, in review). One proposed purpose for the spring-timed HMF in the final EIS 

(DOI, 1995) was to rejuvenate and condition nearshore habitats, such as backwaters, 

before summer and fall when Young of Year (YoY) native humpback chub typically 

enter the main channel from spawning areas in the Little Colorado River. 

Goeking and others (2003), assessed the historical abundance and distribution of sandbars 

associated with return-current channels, typically referred to as “backwaters”, from a rich 

series of decadal-to-annual scale air photos taken throughout Marble and Eastern Grand 

Canyon between 1935 and 2000. These sandbar related habitats have been identified as 

being potentially important for native fishes in promoting elevated growth rates in their 

early life history, particularly for YoY fish that spend their early life stages in the main 

channel of the Colorado River below Glen Canyon Dam after emerging from the Little 

Colorado River. The potential critical element of such main channel habitats is that they 

are known to be relatively shallow, low-velocity aquatic, nearshore settings where 

substantial warming of river water occurs in summer and fall. The time series analysis of 

Goeking and others (2003), concluded that the abundance and distribution of backwaters 

in the upper one-third of the Colorado River ecosystem was greatest in the 1984 images 

(on the basis of two-dimensional return-current channel measurements made between 

about 5,000 and 8,000 cfs under steady flows) compared to any air photo record that 

recorded such features either before or after dam closure. Goeking and others (2003) also 

concluded that to achieve and/or sustain the 1984 level of abundance in backwaters in the 

postdam era, that periodic floods (perhaps as high as the bypass releases of 1983–84, of 

45,000 to 96,000 cfs) must occur at some frequency under conditions of sufficient sand 

supply. The basis for suggesting this strategy concurs with the strategy of the final EIS 

(DOI, 1995) and is tied to data showing that sandbars typically experience a progressive 

pattern of erosion in between higher flows, while backwater return current channels tend 

to become filled in with fine sediment and become progressively colonized by plants that 

evolve into marsh habitats that do not promote access by young, native fish. Rubin and 

others (2002), reported that the concept of the BHBF as a means of restoring and 

maintaining sandbars was partially supported by data and experimental results, but that 

the sand mass flux data collected in the main channel below the dam indicated that MLFF 

operations, even under minimum hydrology and required releases, does not accumulate 

new sand supplies throughout the main channel over multiple years. 

Sustained Low Steady Flow During Summer – Flows of 8,000 cfs during summer 

2000, related to the LSSF test, were effective in limiting suspended-sediment of both 

sand, and to a lesser degree, finer sediment in the main channel of the Colorado River 

ecosystem (Rubin and others, 2002; Wiele and others, 2002; Hazel and others, 2006; and 

Schmidt and others, in review). Topping and others (2000a; 2000b) concluded that 

periods of flow in the predam river that roughly equated to the mean daily discharge 

(9,000 to 10,000) corresponded with periods in which sand accumulated in the channel 

bed of the river. On this basis, sediment scientists hypothesized that sustained flows of 
18

8,000 cfs during the LSSF test should allow tributary sand inputs from the Paria and 

Little Colorado Rivers to accumulate throughout the bed of the main channel and lower 

eddies. Despite the fact that tributary sand inputs were limited during the June through 

August period of 2000, the sand mass balance measurements for the Marble Canyon 

reach of the ecosystem indicated that sand inputs were accumulated below the Paria 

River when flows remained constant at 8,000 cfs. The accumulated sand supply was 

quickly exported downstream during the HMF release that followed in September 2000. 

Sustained low flows also generally limit the degree to which existing sandbars are 

actively eroded through tractive forces owing to turbulent flow, although such periods 

may also promote adjustments of sandbars through bank collapse owing to seepage 

forces. Sand bars may be eroded by rainfall runoff events during periods of low, 

sustained flow, and may be subjected to reworking by wind. 

Camping areas are also increased significantly during periods of sustained low flows. 

More recent information reported by Topping and others (2006) on the fate of Paria River 

sand inputs in the main channel under fluctuations of 5,000 to 10,000 cfs in September 

through mid-November 2004 indicate that such diurnal fluctuating flows are also highly 

effective at limiting downstream export of new sand supplies once they enter Upper 

Marble Canyon. The results from the LSSF test of 2000 (Schmidt and others, in review; 

Hazel and others, 2006; and Wiele and others, 2002), and the fall 2004 sediment transport 

data (Topping and others, 2006) support the idea of testing higher steady flows (above 

8,000 cfs) or additional fluctuating flows with peaks between 10,000 and 14,000 cfs 

relative to questions concerning operations that promote accumulation of new sand 

supplies before future BHBF releases. Equalizing monthly volumes to meet annual 

minimum release would provide such a sediment test. 

High Sustained Flow During Spring – See above section on HMF. The LSSF test of 2000 

also included a multi-week period of sustained high flow (about 17,000 cfs) in April and 

May that was intended to cause ponding of the Little Colorado River inflow area, located 

near that river’s confluence with the Colorado River. It is unclear what role this flow may 

play in the spawning activity of adult humpback chub or the early life history of YoY 

native fishes, but this level of stable flow is definitely effective at transporting high sand 

loads downstream with little benefit to conservation of high-elevation sandbars and 

related habitats (Hazel and others, 2006; Schmidt and others, in review). 

Non-GCD Actions – Treatments such as mechanical removal of nonnative fish are not 

thought to be an influence on sediment transport as there are no data suggesting that 

changes in abundance of fish (and their related behaviors, such as spawning, etc.) 

significantly influences suspended-sediment transport. Drought in the Upper Colorado 

River Basin and major tributaries below Glen Canyon Dam can have a significant 

influence on the abundance of fine sediment delivered to the ecosystem (diminished 

tributary flooding and sand production of Paria and Little Colorado Rivers). Minimal 

annual release volumes from the dam caused by reduced upper basin hydrology and Lake 

Powell storage has a limiting influence on suspended-sediment transport and export of 

sand from tributaries, as well as reduced erosion of existing sandbars. 
19

Summary of Physical Matrix Assessment – 

Fine Sediment – Following is a summary of what sediment scientists concluded during 

workshop discussions relating flows to fine-sediment and sandbar resources. These 

conclusions were derived on the basis of the Physical Matrix cells relating to fine 

sediment (see Table 3.1). The overall effect of increased fluctuations relative to the 

MLFF operation has the influence of increasing sand transport downstream and thereby 

does not promote retention of new sand inputs from tributaries. The result is opposite for 

less widely fluctuating flows or stable flows for a given month’s release volume. 

Reduced variation in monthly release volumes, during at least minimal hydrology release 

years (8.23 million acre foot releases), is predicted to increase retention of new tributary 

sand inputs by limiting downstream export of new sand under high peaks in winter and 

summer months when volumes are typically increased compared with spring and fall 

months. This reduction in export for a given input of new sand is related to the fact that 

lower monthly peaks would occur as monthly volumes are equalized and reduced in 

winter and summer months. Sand transport data collected between 1999 and 2006 

provide the basis for this conclusion. A summary of the sand transport data indicate that 

monthly release volumes associated with MLFF operations under monthly releases in the 

range of 610,000 acre feet retained from 50% to 78% of tributary sand inputs during the 

months and seasons when such flows occurred. 

Test flow data from summer 2000, indicate that sustained low steady flows in summer 

and fall (flows of 8,000 cfs) can significantly promote accumulation of tributary sand 

inputs that occur in association with warm season rainfall runoff events. This is 

particularly important with respect to the Paria River inputs which occur predominantly 

in these seasons. 

As a result of the 1996 controlled flood experiment, scientists have concluded that 

Beach/habitat-building flows (BHBFs) implemented under conditions of insufficient new 

sand supply result in erosion of sandbars in areas of the eddies and channel below 25,000 

cfs stage (as determined by the 1996 controlled flood test response). Despite increases in 

sandbars above the 25,000 cfs stage in 1996, the net effect of the high flow under sand 

depleted conditions was a net deficit in the sand mass balance. 

On the basis of previous testing, the concept of using habitat-maintenance flows 

(relatively short duration flows of approximate peak powerplant capacity) to restore 

sandbars does not appear to have a net positive effect on the sand mass balance of the 

river ecosystem during periods when sand supply is not enriched. Under sand enriched 

conditions that occurred during the first such testing in November 1997, some sand 

storage increases occurred within eddies below the 25,000 cfs stage, but these gains were 

relatively small in volume and transient. High, sustained flows, such as the flows that 

occurred in May 2000 to create ponding of tributary confluences (Little Colorado River) 

elevate sand export rates in the main channel and promote sandbar loss. 

The 2004 High Flow test demonstrated a net positive effect on the sand mass balance of 

the river ecosystem and sandbar restoration was significant in the upper reach of Marble 

Canyon. However, it is still not possible to conclude from the 2004 results alone, whether 
20

repeated similar high flows under similarly enriched sand supply conditions will result in 

cumulative, sustainable sandbar restoration over decadal time periods. Additional testing 

in the form of a replicate of the 2004 sediment test (with respect to the level of sand 

supply enrichment) was recommended by sediment scientists as a means for determining 

whether or not such a strategy is viable. This appears to be the only viable science 

alternative owing to the fact that existing sediment models alone cannot determine the 

answer to this critical management question. If a replication of the 2004 test again results 

in net positive outcome to the sand mass balance along with accumulated sandbar 

restoration, then the strategy of implementing repeated BHBFs under sand supply 

enriched conditions might be considered a viable, long-term conservation strategy below 

the dam without additional need to sand supplies from upstream. 

Increased water temperature tends to reduce suspended-sand transport by reducing the 

water viscosity in the river, which has the effect of allowing higher settling rates for 

suspended sand in the water column. 

Downstream Water Temperature - Following is a summary of what physical scientists 

concluded during workshop discussion relating flows to downstream water temperatures. 

These points were derived on the basis of the Physical Matrix cells relating to 

downstream water quality, specifically water temperatures (see Table 3.1). Main channel 

water temperatures were documented to warm significantly downstream from the dam 

during summer months when stable flows of 8,000 cfs occurred in 2000. Warming of 

downstream water temperature has also been documented during 2003–6, as a result of 

reduced water storage and Lake Powell reservoir elevations associated with prolonged 

drought in the Upper Colorado River basin. These effects have mainly been notable in 

summer and fall months. Downstream water temperatures are also predicted to warm on a 

seasonally varied basis if water is passed through the dam from higher reservoir 

elevations, as simulated in association with a Selective Withdrawal Structure. 

Operational factors known to limit warming of water released from the dam include 

release of high, sustained flows to create ponding of tributary mouths, as well as shorter 

duration flows of greater magnitude, such as BHBFs and HMFs. These types of releases 

tend to limit downstream water temperatures by reducing the residence time of water in 

the Canyon and thereby limit the amount of warming that can occur by influence of solar 

radiation before the water reaches Upper Lake Mead. 

3.2 Hydropower Matrix 

The decision matrix for HydroPower Resources (load-following capacity and 

replacement power costs) was populated at the KAW and is presented in Table 3.2. Data 

are collected by Federal agencies on both the energy and revenue derived from power 

peaking at Glen Canyon Dam and the costs associated with having to provide 

replacement power to customers during periods when energy capacity at the dam falls 

short of delivery commitments. Simulation modeling is also undertaken by Western Area 

Power Administration to project revenue and replacement power costs related to monthly 
21

volume releases scheduled by the Bureau of Reclamation as derived from its annual 

Colorado River Work Group planning activities. 

During the July 2005 Knowledge Assessment workshop, participants concluded that the 

basic cause/effect relationships for the Hydro Power matrix were well known and 

documented. However, on the basis of later comments and discussions with managers, it 

became clear that such knowledge, while it might either be well known and predictable 

from modeling to some stakeholders or “knowable,” to by some managers who have 

access to the information, that it was not widely available as published data in reports that 

are widely available to the public or other GCDAMP members. 

3.2.1 Power Peaking Capacity and Financial Aspects of Replacement Power Costs 

 

The following summary is made on the basis of discussions with power representatives 

and the flow-treatment evaluations were made relative to the MLFF operating rules. 

While data and related simulations for this resource area are either known or are 

“knowable,” this information is typically only available from the Western Area Power 

Administration by request. 

Increased Water Temperature – Has a relatively small limiting influence on the peaking 

capacity owing to reduced turbine efficiency that results from thermal limits of generator 

performance. Replacement costs may be slightly increased by limitations on the peaking 

capacity related to turbine efficiencies. 

Increased Daily Fluctuations – Increasing the daily range, hourly ramping rates, or daily 

allowable peak, as well as reduced daily minimum, provides a great advantage with 

respect to daily peaking or “load-following” capacity. Higher daily peaks can be achieved 

and maintained for longer periods so as to more optimally follow daily energy demands. 

Replacement costs are limited as a result that peaking capacity is increased. The Glen 

Canyon Dam powerplant is operated in such a way that load following is more efficiently 

achieved than patterns associated with the MLFF operation. 

 

Reduced Variation in Monthly Volume – Flattening of the annual pattern of monthly 

release volumes decreases the benefit of historically banking water in Lake Powell for 

release and energy generation during seasons of the year when replacement energy is 

derived at lower costs from other generating facilities on the Western Grid, such as 

northwest hydropower dams that have abundant water during shoulder seasons of fall and 

spring (Wayne Cook, oral communication, July 2005). Releasing less variable monthly 

volumes for a given annual release volume results in more peaking energy being 

produced in months when it would be more economical to purchase replacement power 

from other facilities. As a result, less water is then available in Lake Powell in summer 

and winter when the value of peaking power from Glen Canyon Dam is higher and 

replacement power costs associated with other producers may also be higher. 
22

BHBFs and HMFs – Currently, any beach/habitat-building flows that is released from 

Glen Canyon Dam (flow at least 10,000 cfs above peak powerplant capacity, as described 

in the 1995 EIS) constitutes a bypass of the hydropowerplant. As such, these operations 

have two-fold influence on peaking capacity and replacement power costs. First, the 

bypass portion of the release has a negative influence on energy generation (no peaking 

or base load energy is generated) since the water never moves through the powerplant 

and once released is not available for energy generation again at the Glen Canyon Dam. 

The portion of the release that moves through the powerplant (sustained release of about 

31,500 cfs) does not follow the daily peak demand for the duration of this operation and 

makes the facility a base loaded generating station. This element of the BHBF is virtually 

identical to the Habitat Maintenance Flow (HMF). The bypass results in both lost energy 

and revenue that increases replacement power costs, while the sustained, high releases 

through the powerplant generate energy, but in a way that does not optimize revenue 

relative to daily energy demand. 

Sustained Low Steady Flow During Summer – Owing to the fact that there is less energy 

generated during this peak-demand season for electric power, this operation has a large 

negative influence on peaking capacity (since no peaking occurs) and greatly increases 

replacement power costs over significant periods of peak demand. Estimated costs from 

the 2000 Low Summer Steady Flow test were presented by Western Area Power 

Administration at the October 2003 Science Symposium convened by the GCMRC in 

Tucson, AZ (Clayton Palmer, oral communication, 2003). Replacement power costs 

associated with that test were reported to be in the range of 20 to 25 million dollars. The 

total cost of the test may not be fully identified for several more years owing to lag 

effects of that test that are propagated into future water years beyond the Water Year 

2000 (David Harpman, oral communication, May 2005). 

High Sustained Flow During Spring – The dam essentially becomes a “base loaded” 

powerplant owing to the fact that such releases are high and steady (assumed to be within 

powerplant capacity) and do not vary on a daily basis. As a result, such operations 

negatively affect peaking capacity and the revenue associated with load following is 

negatively influenced for the duration of this release pattern. Energy that is generated 

during such operations in the non-peak demand period of the day is typically less 

valuable. Revenue generation is not optimized since peaking capacity is compromised 

with respect to the daily pattern of peak energy demand. 

Non-GCD Actions – Experimental treatments such as mechanical removal are conducted 

without impacting normally scheduled flows from the dam. Other experimental support 

activities, such as remote-sensing over flights, are typically conducted following 

sediment experiments and require low-steady flows for a period of days. This activity 

does limit peaking capacity and result in additional replacement power costs. Protracted 

drought in the Upper Colorado River Basin, as well as increased depletion of water 

supply through agreed upon uses, does limit storage in Lake Powell. Reduced storage 

levels in Lake Powell lead to minimal annual release volumes to the Lower Colorado 

River Basin and such minimal annual releases (10.2 Gm3

 or 8.23 million acre feet) limit 

the extent to which peaking capacity can occur under the ROD. Reduced peaking 
23

capacity forced by low hydrology therefore increases replacement power costs to the 

Colorado River Basin Fund. 

Summary of Hydro Power Matrix - 

Some operational changes were identified to have more influence on peaking capacity 

and replacement power costs than others by workshop participants representing 

hydropower resources. For instance, BHBFs can have a significant negative influence on 

replacement power costs (meaning those cost increase significantly) for two reasons 1) 

some volume of water release completely bypasses the powerplant and 2) release 

volumes in one of more months following the BHBF must be reduced to meet the annual 

release schedule. Hence, such reductions have the effect of reducing peaking capacity in 

some months following the BHBF while also increasing replacement power costs in those 

same months. The effects are greatest for the year if the monthly release volumes are 

reduced in summer months in the case where a BHBF might have occurred in the spring 

month of March, as occurred in the 1996 controlled flood test. In the case of reducing 

variability in the annual pattern of monthly volumes for a given annual release schedule, 

peaking power capacity would be limited in winter and summer months (relative to the 

variability in power demand) and replacement power costs in those seasons would be 

increased. Conversely, peaking power capacity would be available in spring and fall 

months when there would likely be reduced demand for it. Low, steady flows during 

summer and fall months do not fit well with energy demand patterns for summer and 

would result in increased replacement power costs. Increased water temperature can also 

have an effect in limiting the turbine efficiency and hence peaking capacity of the 

powerplant, but these types of effects are relatively minor compared with limitations on 

daily range of flows released or reduced ramping rates used to move from daily minimum 

to maximum flows, etc. Owing to the fact that operational models exist for determining 

releases from the dam and for determining economics associated with hydropower 

production, all of the operational influences can be predicted for the most part. As a 

result, the cells in Table 3.2 are all highlighted green. 

3.3 Food Base, Fish, and Lees Ferry Angling Matrices 

The decision matrix populated at the KAW is presented in Table 3.3. In contrast to the 

other sub-models, the effects of individual elements of ROD flow constraints were 

investigated. The overall effect of higher daily fluctuations is described in Table 3.3, 

while the effects of individual elements are presented in Table 3.4. 

3.3.1 Food Base 

The food base performance measure represents the flux of drifting invertebrates in the 

water column, referred to as drift rate. We selected drift rate as our metric of food base 

performance because humpback chub and rainbow trout, the fish for which food base 

research and monitoring are principally conducted, are generally drift feeders; drift rate is 
24

a better metric of food base performance than the standing stock of benthic invertebrates 

because it is a direct measure of the food items that rainbow trout and humpback chub 

most often encounter and consume. Drift rate will depend on the biomass or density of 

benthic invertebrates (g/m2

) that provides the source for drift, as well as the flow regime 

that determines the extent of disturbance to and distribution of the benthos; the effect of 

operations on both biomass and disturbance were considered when determining drift rate. 

Water temperature from Glen Canyon Dam was hypothesized to be an important driver 

of food base performance in both Glen and Grand Canyons because it has an overriding 

influence on invertebrate growth rates in stream and river ecosystems throughout the 

world (Benke 1993; Huryn and Wallace 2000). The daily variation in flow and minimum 

daily flow were hypothesized to be the most important flow constraints affecting the food 

base. 

Water Temperature—Increases in water temperature will increase the flux of 

invertebrates in both Glen and Grand Canyon (Garrett and others, 2003). (Note: July 

2005 Knowledge Assessment Workshop participants coded this cell yellow +. After 

further research on the subject, Kennedy decided there was sufficient evidence to 

code this light green +. See discussion below and in Garrett and others, 2003). 

Secondary production of invertebrates (i.e., biomass of invertebrates produced per area 

per time—g/m2

/yr) are a function of benthic biomass and growth rates (Production = 

Biomass*GrowthRate; Benke 1993). Across the range of water temperature increases that 

are likely with natural warming or installation of a TCD (i.e., release temperatures of up 

to ~16°C with higher temperatures possible downstream), growth rates of aquatic 

invertebrates that are important food items in the CRE (i.e., amphipods such as

Gammarus lacustris, chironomids, and simuliids) are all strongly and positively related to 

water temperature (Sutcliffe et al 1981; Hauer and Benke 1987; Benke and others, 1988; 

Pockl, 1992; Benke, 1993; Huryn and Wallace, 2000). Vinson (2001) reported no change 

in the biomass of benthic invertebrates following installation of a TCD on Flaming Gorge 

Dam. The standing biomass of benthic invertebrates is not likely to increase with 

installation of a TCD on GCD, but it is almost certain that there will be more invertebrate 

biomass produced annually because of temperature driven increases in invertebrate 

growth rates. In fact, temperature-mediated increases in growth rates will probably lead 

to an additional 1–2 cohorts per year for invertebrates with short generation times (i.e., 

simuliids and chironomids with generation times of ~20 days; Benke 1993, Huryn and 

Wallace 2000). 

If aquatic invertebrates were exhausting their food supply in the CRE, increases in 

temperature might actually have a negative impact on invertebrate flux because 

invertebrate metabolic and respiration requirements could increase without a concomitant 

increase in food supply. However, algae biomass and production in Glen Canyon will 

increase with temperature because light is not limiting (Phinney, 1965; Brock, 1970; 

DeNicola, 1996; Garrett and others, 2003; Yard, 2003), thereby increasing available food 

for invertebrates. In fact, the range of temperature increases that are possible fall within 

the temperature optima for Cladophora glomerata (maximum growth occurs between 

13–16°C with upper limit of 24°C; Graham, 1982). 
25

It seems likely that increasing water temperatures will lead to shifts in the species 

composition of algae, aquatic macrophytes, and invertebrates. Blinn and others (1989) 

found that epiphytic diatom composition shifted with increasing temperature, with the 

abundance of upright forms declining (combined relative abundance of Diatoma vulgare

and Rhoicosphenia curvata went from 58% to 30%) and adnate or small forms increasing 

(combined relative abundance of Cocconeis pediculus, Achnanthes minutissima, and 

Cymbella affinis increased from 33% to 59%) with an incubation temperature of 18°C 

relative to the control of 12°C. Pinney (1991) found that Gammarus lacustris in the CRE 

preferentially consume the upright forms listed above and also R. curvata, a relatively 

small diatom, and tended to avoid C. pediculus, an adnate form. However, it is unclear 

how large a shift in diatom composition would occur over the range of warming that is 

likely with installation of a TCD (up to ~16°) or whether the shift in diatom composition 

that Blinn and others (1989) observed would actually affect invertebrate biomass or 

growth rates. Vinson (2001) documented shifts in the species composition of aquatic 

invertebrates following installation of a temperature control device on Flaming Gorge 

Dam. However, it is unclear if this shift in species composition affected food availability 

for fish. Even if there are shifts in the species composition of invertebrates and algae, the 

vast literature on invertebrate production provides compelling evidence that warming will 

increase invertebrate growth rates, and hence invertebrate flux, in Glen Canyon. In fact, 

there are robust models available that allow us to predict how much simuliid, chironomid, 

and Gammarus growth rates will increase with increasing temperature (Benke, 1993). 

Increases in water temperature could also lead to no net benefit to the food base in Glen 

Canyon if growth rates for New Zealand mudsnails were to increase dramatically at the 

expense of other invertebrates. Hall and others (2006) measured growth rates for New 

Zealand mudsnails in streams of Yellowstone National Park that spanned a range of 

water temperatures and found most of the variation in growth rates was explained by 

differences in snail size—small snails grow fastest. Mudsnail growth rates were 

positively correlated with water temperature, but the temperature coefficient (0.0024, the 

increase in growth rate that will occur for a 1o C increase in water temperature) is 

considerably lower than for other macroinvertebrates in Glen Canyon (i.e., growth rate 

for amphipods such as Gammarus have a temperature coefficient of 0.111, the 

temperature coefficient for chironomids is 0.05, and for simuliids it is 0.031; Benke, 

1993). Benke (1993) compiled secondary production data for a wide variety of 

invertebrate taxa and found that the growth rate of mollusks is actually negatively related 

to temperature. For individual species of mollusks it seems unlikely that growth rate is 

negatively related to temperature (unless temperatures begin to exceed their thermal 

optima), but averaging across all species of snails for which secondary production data 

were available (n=16) Benke (1993) found that growth rates were negatively related to 

temperature. We believe that growth rates for New Zealand mudsnails will in fact 

increase with temperature in Glen Canyon, as reported by Hall and others (2006). 

However, data from Hall and others (2006) and Benke (1993) suggests that growth rates 

for mudsnails may actually increase less with rising temperatures than growth rates of 

other invertebrates in the Lees Ferry reach. 
26

Increasing water temperatures will also increase invertebrate growth rates in Grand 

Canyon. (FootNote: July 2005 Knowledge Assessment Workshop participants coded 

this cell red. Main reason for uncertainty was because the relative importance of 

allochthonous and autochthonous carbon to secondary production is unknown. 

Algae production is unlikely to increase with increasing temperatures because it is 

strongly light limited. Participants felt that if invertebrate growth rates increase 

without a concomitant increase in food availability, invertebrates may exhaust their 

food supply. After further research on the subject, Kennedy decided there was 

sufficient evidence to code this light green +. See discussion below and in Garrett 

and others, 2003) Increasing water temperatures will probably not increase algae 

production in Grand Canyon because it is strongly light limited (DeNicola, 1996). 

However, the most common invertebrates in Grand Canyon are filter feeders (simuliids) 

that capture small food particles from the water column and chironomids that tend to feed 

mainly on detritus (Stevens and others, 1997a); these types of invertebrates consume a 

miniscule fraction of available food (Edwards and Meyer, 1987) and thus are incapable of 

depleting their food supply in a large river such as the CRE. Thus, higher temperatures, 

even in the absence of increased food availability, will also increase invertebrate growth 

rates (Benke 1993, Huryn and Edwards 2000), and hence invertebrate drift rates, in 

Grand Canyon (Garrett and others, 2003). 

It should be noted that temperature mediated increases in food availability will probably 

not lead to increases in fish condition or density because the energetic demands for fish 

will also increase with temperature (Peterson and Paukert, 2005). 

Increased Daily Fluctuations - Flux of invertebrates was hypothesized to exhibit a domeshaped response to increased daily fluctuations in flow from GCD. Very stable flows 

would likely maximize benthic biomass of both algae and invertebrates, but benthic algae 

production would be lower and reduced disturbance would limit the transfer of 

invertebrates from the benthos to the water column. At the other extreme, high 

fluctuations would limit benthic biomass and production to the point where flux is 

reduced. 

In Grand Canyon, where light is limiting autotrophic production (Yard 2003), higher 

daily variation in flow will further reduce algae production to the point where it would 

reduce benthic invertebrate standing stock, and hence invertebrate flux. Further, 

simuliids, the dominant food item consumed by rainbow trout and humpback chub at 

downstream locations (Valdez and Ryel, 1995, Yard and Coggins unpublished data), 

have a limited range of optimum water velocities and preferentially occupy habitats with 

fast water velocities because these habitats provide a continual supply of food particles 

that are consumed via filter feeding (Ross and Merritt 1987). Increasing daily fluctuations 

in flow would likely reduce the standing stock or productivity of simuliids because water 

velocities would become more variable. 

The direction of response of flux to increased flow fluctuations is unknown in Glen 

Canyon because the dome-shaped relationship between flux and fluctuating flows has not 

been adequately quantified. However, if increasing fluctuations are coupled with a 
27

decrease in the minimum daily flow it seems likely that there will be a decrease in 

invertebrate flux. Daily minimum flow is an important determinant of benthic standing 

crop because of the strong negative effects of desiccation on algae and invertebrates 

(Hardwick and others, 1992; Angradi and Kubly, 1993; Blinn and others, 1995; Benenati 

and others, 1998). Blinn and others (1995) estimated the energy content of various food 

base components in the Lees Ferry reach. They reworked the total Gammarus energy 

content at various minimum flows. Increasing the minimum flow from 142 m3/s to 227 

m3/s leads to a 50% increase in the amount of Gammarus energy present in the Lees 

Ferry reach. Similar patterns were noted for epiphytic diatoms and chironomid larvae. 

Increased daily fluctuations might benefit the food base if it had a strong negative impact 

on New Zealand mudsnails that exceeded potential negative impacts on other 

macroinvertebrates; mudsnail density in Glen Canyon is high (~40,000 individuals/m2—

Benenati and others, 2002) and it seems likely that mudsnails are having an indirect 

negative impact on other aquatic macroinvertebrates via competition for epiphytic algae. 

However, indirect evidence suggests that it is unlikely that mudsnails would be more

adversely affected by increased fluctuations than other macroinvertebrates. In Glen 

Canyon, snails are present across virtually all habitats types (i.e., unstable 

substrates/habitats such as sandy beaches, large mats of Cladophora and other algae, and 

stable substrates such as boulders that will only move during extremely high flows—

Blinn and others, 1995, Kennedy personal observations), whereas Gammarus and other 

macroinvertebrates are strongly dependent on unstable Cladophora for both habitat and 

food (Leibfried and Blinn, 1987). In other words, mudsnails occupy the same unstable 

habitats that will be affected by fluctuations as other macroinvertebrates, along with 

stable habitats that will be unaffected by fluctuations. Further, Blinn and others (1995) 

found that snails readily colonized newly submerged habitats, achieving densities equal 

to those of permanently submerged habitats within a week. In contrast, re-colonization by 

Cladophora, Gammarus, and chironomid larvae was significantly slower, with density on 

newly submerged habitats <30% of permanently submerged habitats even after 4 months 

of submergence. Thus, it seems unlikely that mudsnails will be more adversely affected 

by increasing fluctuations than other invertebrates such as Gammarus. 

Increasing flow fluctuations might have a positive impact on invertebrate flux if they led 

to increases in drift rates without a strong concomitant negative effect on the standing 

stock of benthic invertebrates. Leibfried and Blinn (1987) found Gammarus drift rates 

were positively correlated with the range of discharge variation (i.e., high daily variation) 

and that drift rates were generally higher on the rising limb of the hydrograph for periods 

that followed low discharges. They noted that it was only during periods of widely 

fluctuating flows (minimum discharge of 2,000 cfs and maximum of 18,000 cfs) when 

they observed rates of invertebrate drift that exceeded those during more steady flows. 

For example, in October 1985 when the daily range was 9,000–21,000 cfs rates of 

invertebrate drift did not exceed rates during steady flows. It should be noted that the 

three month period of fluctuating flows that Leibfried and Blinn (1987) studied 

(minimum discharge as low as 1,500 cfs and maximum discharge typically around 20,000 

cfs), were preceded by a three month period of ‘steady’ flows (daily range of less than 

8,000 cfs with minimum flow never below 18,000 cfs). Leibfried and Blinn (1987) 
28

suggested that the three months of high steady flows allowed a large standing crop of 

algae and invertebrates to develop, and when flows began to widely fluctuate large 

numbers of Gammarus that were stranded during the descending limb of the hydrograph 

entered the drift as discharge began to rise. Thus, the high drift rates under fluctuating 

flows documented by Leibfried and Blinn (1987) were probably due in part to the steady 

flows that preceded them. These investigators suggested that the drift rates they observed 

reflected the short term effects of fluctuations, and that under long term fluctuations 

invertebrate drift rates might decline because the standing stock of invertebrates would 

eventually be reduced due to downstream losses. Shannon and others (2001) measured 

high rates of invertebrate drift during high steady flows of June 1996 that approached 

those observed by Leibfried and Blinn (1987) under widely fluctuating flows. Shannon 

and others (2001) concluded that high benthic invertebrate standing stocks associated 

with steady flows can result in invertebrate drift rates that are comparable to those present 

during large daily fluctuations, without the negative impacts of a widely fluctuating varial 

zone. In contrast to the findings of Leibfried and Blinn (1987), McKinney and others 

(1999a) found that drift rates for Gammarus were highest on the descending limb of the 

hydrograph. 

It has been suggested that MLFF has allowed aquatic macrophytes to replace Cladophora

glomerata in Lees Ferry, with negative consequences to the aquatic food base. 

Cladophora glomerata was the single dominant algae in Glen Canyon during the period 

of no action and aquatic macrophytes were virtually absent. With the onset of Interim 

Flows and MLFF, researchers noted that aquatic macrophytes such as Potamogeton 

pectinatus and Chara contraria ‘became co-dominant with Cladophora in the Lees Ferry 

reach by 1996...’ (McKinney and Persons, 1999). However, it appears that aquatic 

macrophytes are generally occupying habitats that Cladophora never did. Cladophora

attach to hard and stable substrates using a holdfast (Blinn and others, 1998), whereas 

aquatic macrophytes are rooted into soft and unstable substrates. During the period of no 

action these soft and unstable substrates in Glen Canyon were no doubt very unstable and 

constantly shifting. With the implementation of Interim Flows and MLFF, the stability of 

sandy substrates increased as compared to the period of no action, so aquatic 

macrophytes were more readily able to colonize (Blinn and others, 1994; McKinney and 

Persons 1999). Thus, the presence of aquatic macrophytes on soft substrates in Glen 

Canyon has actually led to increases in the ‘food base and habitat for benthic invertebrate 

grazers and rainbow trout.’ (McKinney and Persons 1999). Hardwick et al (1992) found 

that large fluctuations caused a shift in the species composition of the epiphytic diatom 

community, with reductions in the abundance of loosely attached upright diatoms and an 

increase in the abundance of adnate or smaller forms; upright diatoms are the preferred 

food of Gammarus lacustris in the Glen Canyon reach (Shannon et al 1994, Pinney 

1991). Ayers and McKinney (1997) in 1993–94 found that small and adnate diatoms 

occurred in proportionally greater densities than large upright species on cobbles and 

artificial substrates, in contrast to previous studies that noted upright diatoms were the 

most common form (Hardwick and others, 1992). However, McKinney and Persons 

(1999) suggested that this shift in species composition might be due to a diatom 

pathogen, rather than the effects of reduced fluctuations. 
29

Increases in daily fluctuations that involve no change to the minimum daily flow may 

lead to increases in invertebrate flux if the antecedent density of Gammarus and other 

invertebrates in Glen Canyon is high. However, observations suggest that the benthic 

standing stock of Gammarus and other invertebrates is presently low relative to historical 

numbers (Scott Rogers, oral communication, 2005). Therefore, under the present 

conditions it seems unlikely that increasing fluctuations will increase invertebrate flux—

there are not many Gammarus available to be captured by fluctuating flows. 

Increases in daily fluctuations that involve an increase in the minimum daily flow may 

lead to measurable increases in benthic invertebrate densities and invertebrate flux. 

Increasing the minimum daily flow will increase the permanently submerged zone, which 

will have a significant positive impact on the total amount of Gammarus in the Lees 

Ferry reach (Blinn and others, 1995), and lead to faster average water velocities, which 

will increase rates of algae growth and production (DeNicola, 1996). If increasing the 

minimum daily flow leads to appreciable increases in benthic standing stock of algae and 

invertebrates, then increasing fluctuations may in fact lead to higher rates of invertebrate 

flux. However, all of this is predicated on increases in the benthic standing stock of 

invertebrates and available evidence indicates that increasing fluctuations alone will not 

accomplish this. 

Although quantitative data are lacking, observations clearly indicate there has been a 

reduction in the standing crop and drift mass of Cladophora and Gammarus in the Lees 

Ferry reach in recent years (William Persons, Scott Rogers, Mark Steffan, oral 

communication, 2005). It is Kennedy’s opinion, based on his reading of the available 

literature, that reductions in daily variation under MLFF are not the cause of this 

reduction in Cladophora and Gammarus abundance. Rather, Kennedy believes that the 

cause of these observed reductions in the Lees Ferry food base may be due to many 

consecutive years of minimum releases and the invasion of New Zealand mudsnails, 

among other things. Widely varying daily fluctuations during the 1980s were often 

associated with high steady releases and it may well have been the high steady releases 

that led to high standing crops of Cladophora and Gammarus. 

Reduced Variation in Monthly Volume - In Glen Canyon, sudden changes in mean flow 

between months were hypothesized to temporarily reduce benthic biomass by either 

exposing large areas of previously wetted substrate following a reduction in flow 

(Hardwick and others, 1992; Angradi and Kubly 1993; Blinn and others, 1995; Shaver 

and others, 1997; McKinney and others, 1999b), or by increasing depth/light attenuation 

and reducing productivity of benthos after a flow increase (Yard, 2003). However, it was 

hypothesized that large increases in monthly volume would have a positive effect on the 

food base in the long-term as this would increase the permanently submerged zone of the 

benthos. The same reasoning holds for Grand Canyon; large decreases in monthly 

volume would have a short and long-term negative impact on the food base, whereas 

large increases in monthly volume would have a short-term negative impact on the food 

base by increasing depth/light attenuation and a long-term positive impact by increasing 

the permanently submerged zone of the benthos. 
30

BHBFs and HMFs - The response of the flux of invertebrates to higher flows associated 

with BHBF’s or HMF’s was uncertain. During and shortly after a high flow event 

invertebrate flux has been show to increase (McKinney and others, 1999, Blinn and 

others, 1999, Shannon and others, 2001). Shannon and others (2001) reported high rates 

of invertebrate drift two months after the 1996 BHBF. Although only limited data on the 

food base were collected following the 2004 BHBF, anglers and AZGFD personnel 

working in Lees Ferry reported that Gammarus were extremely scarce for many months 

after the BHBF (Mark Steffan oral communication 2005, Scott Rogers, oral 

communication, 2005). The 1996 BHBF was conducted in spring whereas the 2004 

BHBF was conducted in late fall. It seems likely that the timing of a BHBF has a major 

impact on food base response; the food base may respond positively and rapidly 

following a BHBF conducted in spring because light levels are high and algae can grow 

rapidly, while it may take many months for the food base to recover from a BHBF 

conducted in winter because of low light levels that limit algae growth. Seasonal 

differences in sunlight to drive primary productivity in Glen, Marble, and Grand canyons 

were reviewed by Yard and others 2005. It has been hypothesized that the community 

that recovers following the disturbance could be more productive than the predisturbance community due to scouring of unproductive and senescent algae and 

macrophytes leading to a long-term increase in flux. This prediction is highly uncertain 

because of the lack of long-term drift data. 

Rogers and others (2003) found that HMFs in 2000 had no impact on macrophyte 

density, periphyton chlorophyll a content, or benthic invertebrate biomass. Total drift 

mass, which was dominated by Cladophora, was reduced in the weeks following the May 

2000 HMF, but this may have been due to the low summer steady flows (Rogers and 

others, 2003). 

Sustained Low Steady Flow - Sustained low steady flows during the summer are 

hypothesized to reduce the flux of invertebrates in both Glen and Grand Canyons, as 

supported by data collected during the LSSF experiment (Benenati and others, 2002; 

Rogers and others, 2003). The direction of response is certain in Glen Canyon, where the 

majority of the food base research was conducted during the LSSF, and is less certain in 

Grand Canyon where the relative importance of autochtonous vs. allochtonous sources of 

carbon is unknown. 

High Sustained Flow During Spring - Food base responses to high-sustained flows during 

spring are likely to be positive. (Footnote: July 2005 Knowledge Assessment 

Workshop participants coded this cell red. After further research on the subject, 

Kennedy decided there was sufficient evidence to code this light green +. See 

discussion below) Rates of algae production and growth are positively related to water 

velocity because faster water delivers more dissolved nutrients for algae to take up than 

slower water (DeNicola 1996). Therefore, high sustained flows will lead to both faster 

rates of algae growth/production and a larger permanently submerged zone. Further, 

Leibfried and Blinn (1987) documented high standing mass of benthic algae and 

invertebrates during high steady releases during the summer months of 1985. Shannon 
31

and others (2001) documented high standing mass of benthic algae and invertebrates and 

high drift rates for invertebrates during high steady flows of spring 1996. 

Non-GCD Actions - Mechanical removal of trout in Marble Canyon is hypothesized to 

increase the availability of drift by reducing losses due to consumption of drift by trout; 

humpback chub and rainbow trout have considerable dietary overlap in Marble Canyon 

(Coggins and Yard unpublished data, Valdez and Ryel 1995). The direction of response 

is uncertain because it is unknown whether the reduction in trout abundance in Marble 

Canyon results in a meaningful increase in food availability for native fish. 

Summary of Food Base Element in Aquatic Matrix - 

Because of the north/south orientation of Glen Canyon, light is generally not limiting to 

primary productivity in the reaches of the Colorado River below Glen Canyon Dam. 

Therefore, flows and temperatures will likely be very important to aquatic food 

production in this reach. The Knowledge Assessment Workshop participants concluded 

that warmer temperatures were likely to be of benefit to food base organisms, but were 

less certain about the effects of flows on the aquatic food base. Studies have been 

conducted during and after many different flow scenarios, compounding the ability to 

draw definitive conclusions. Although greater stability of the river increases the 

permanently wetted area, thereby allowing for more consistent production of algae and 

macrophytes, increased fluctuations seem likely to dislodge and transport this production, 

making it more readily available to fishes. The workshop participants concluded that 

uncertainty remained about what combination of steady and fluctuating flows provided 

the greatest benefit to the food base on which fish depend. Food availability is of great 

importance to both the rainbow trout population in the Lees Ferry reach and to native 

fishes farther downstream. The workshop participants identified that the relative 

contribution of autochthonous and allochthonous material is not well known and needs 

further study. These conclusions led to the development of strategic questions to be 

addressed (Section 4.3.1) and they are summarized and presented graphically in Tables 

3.3 and 3.4. 

3.3.2 Fish 

3.3.2.1 Mainstem Spawning & Incubation 

The mainstem spawning and incubation performance measure represents the conditions 

that promote spawning and the quality of incubation environments before larvae become 

free-swimming fish. Water temperature is the key management action expected to 

improve spawning and incubation for native fish, while the extent of daily fluctuations in 

flows was considered the key determinant for rainbow trout in Glen Canyon. 

Water Temperature - There is certainty that native fish require warmer water 

temperatures for spawning than is currently available (Valdez and Carothers, 1998; 

Valdez and Ryel, 1995). Flannelmouth suckers can spawn at temperatures as low as 8o C 

but require a temperature of 14o C for hatching. Humpback chub require temperatures 16o 

C or higher for successful spawning and hatching (Clarkson and Childs, 2000). 
32

Significant increases in water temperature from a TCD or low reservoir elevation are 

unlikely to occur during the months of spawning (Feb.–Apr.) and incubation (Mar.–Jun.) 

for rainbow trout (Korman and others, 2005a) and therefore will not influence 

reproductive success of that species. However, predictions for rainbow trout are uncertain 

because higher temperatures would be expected to increase metabolic energy demand of 

adults and possibly affect maturation schedules or fecundity. 

 

Increased Daily Fluctuations - The effects of increased daily fluctuations in flow on 

spawning and incubation success for humpback chub are unknown. Fluctuations could 

expose eggs and larvae of broadcast spawners like humpback chub and flannelmouth 

sucker but the extent of this impact is uncertain. Spawning of flannelmouth sucker occurs 

over shallow riffles where exposure of eggs or early larval stages due to fluctuating flows 

is more likely (Scott Rogers, oral communication, 2005). Increased fluctuations can 

reduce incubation survival for rainbow trout in Glen Canyon if the extent of fluctuations 

is large (Korman and others, 2005a). A model predicting the magnitude of the effect is 

available but has not been validated. In Marble Canyon, the vast majority of mainstem 

spawning habitat is below the 5,000 cfs stage (Korman and others, 2005a); thus, 

spawning and incubation success is unlikely to be effected by daily operations of GCD 

conducted at higher volumes. 

Reduced Variation in Monthly Volume - The effects of changes in monthly volumes on 

spawning and incubation success of native fish is unknown. It is uncertain whether 

increased flow during springtime, as recommended as part of the Seasonally Adjusted 

Steady Flow alternative, is required to stimulate humpback chub spawning. Reduced 

volumes during summer months reduce mean flow and increase travel time and therefore 

exposure to solar insolation, resulting in warmer water at downstream locations that may 

promote successful spawning for some native fish species (Vernieu and others, 2005). 

Decreases in monthly volumes during the incubation period of rainbow trout has the 

potential to increase incubation mortality in Glen Canyon, but not in Marble Canyon 

where almost all spawning habitat is below 5,000 cfs (Korman and others, 2005a). 

BHBFs and HMFs - The effects of these high flows on spawning and incubation success 

for native fish and rainbow trout is very uncertain. High flows have the potential to 

stimulate spawning of native fish or displace eggs or larvae with limited mobility. Shortterm changes in food availability caused by high flows could influence maturation 

schedules and are hypothesized to have reduced the condition and spawning intensity of 

rainbow trout in Glen Canyon. 

Sustained Low Steady Flow - Lower and steady mainstem flows occurring during native 

fish spawning periods would lead to an increase in water temperatures that may promote 

spawning and minimize exposure of incubating and early larval stages (Trammell and 

others, 2002). There is less certainty for flannelmouth sucker because it is uncertain 

whether temperatures during the spring will be high enough to promote spawning. 

However, these fishes have been documented spawning later in the year when mainstem 

water temperatures would be more conducive under sustained low steady flows (Douglas 

and Douglas 2000). Steady flows during the rainbow trout spawning period would 
33

increase incubation survival (Korman and others, 2005a). It is unlikely that steady flows 

would affect spawning and incubation of rainbow trout in Marble Canyon, as the majority 

of spawning habitat is below 5,000 cfs. 

High Sustained Flow During Spring - The effects of higher flows during spring on 

spawning and incubation for native fish is highly uncertain. Higher flows during the 

spring could potentially reduce incubation mortality for rainbow trout in Glen Canyon, 

but high flows could also have no impact depending on timing relative to spawning 

(McKinney and others, 1999; Korman and others, 2005a). The effect of higher flows on 

trout spawning in Marble Canyon is highly uncertain. Higher flows could increase access 

to tributaries (positive effect) or could scour redds deposited in the mainstem (negative 

effect). 

Non-GCD Actions - Removal of nonnative fishes is hypothesized to potentially reduce 

predation rates on eggs and early life stages of native fish, but the importance of this 

effect is uncertain (McKinney and others, 1999). 

3.3.2.2 Young-of-Year/Juvenile Nearshore Rearing 

The YoY/Juvenile nearshore rearing performance measure represents the physical 

conditions that promote growth and survival of young fish in nearshore environments. 

Water temperature and greater diurnal flow stability were hypothesized to be the key 

determinants. Responses of native fish and rainbow trout in both Glen and Grand 

Canyons were very similar across most management actions that were evaluated. 

Water Temperature - Increased water temperature is known to increase growth rates of 

juvenile native fish and reduce thermal shock for YoY immigrating from the LCR into 

the mainstem (Valdez and Carothers, 1998). Increased temperatures will increase 

metabolic demand. Thermal optimum for trout is less than those for native fish but higher 

than normal GCD release temperatures. Increased temperatures combined with sufficient 

food availability would improve growth rates. This was considered likely to occur in 

Glen Canyon but there was increased uncertainty of this effect for rainbow trout in 

Marble Canyon where the food supply is lower and where temperatures can exceed 15o 

C, the thermal optimum for rainbow trout. 

Increased Daily Fluctuations - Increases in daily fluctuations destabilize nearshore habitat 

(Hoffnagle 2000) and are hypothesized to reduce the growth and survival of young fish 

(McKinney and others, 1999; Stone and Gorman, 2005). This hypothesis is weakly 

supported by habitat use and modeling studies for native fish (Converse and others, 1998; 

Korman and others, 2003). Nearshore use in the LCR observed by Stone and Gorman 

(2005) shows that humpback chub young-of-year have a high affinity of shoreline 

habitats and use these areas as a refuge from predation. Stone and Gorman (2005) 

hypothesize that destabilization of shoreline habitats in the mainstem Colorado River by 

load following will impact young-of-year humpback chub survival rates. Strong year 

classes of rainbow trout in 2000 were possibly in response to steady flows during the 
34

LSSF experiment (S. Rogers, oral communication). Korman and others (2005a) have 

shown improved growth of rainbow trout during Sunday steady flows based on analysis 

of otolith microstructure. The daily variation in flow and the daily minimum flow were 

hypothesized to be the most important components determining nearshore rearing habitat 

quality. 

Warming in nearshore habitats has been shown to be substantially increased by decreased 

diel flow variation (Hoffnagle 2000; Korman and others, 2005b; Vernieu and others, 

2005). Nearshore water temperatures increase under steadier flows. Flow stabilization 

therefore not only allows native young-of-year to remain in their preferred shoreline 

habitat, but provides warmer water temperatures which promote growth. 

 

Reduced Variation in Monthly Volume - A reduction of changes in mean monthly 

volumes was hypothesized to improve nearshore rearing habitat by providing increasing 

shoreline stability (Stone and Gorman 2005; Korman and others, 2005a). Stranding or 

evacuations of juvenile fish from backwaters that are isolated from the mainstem 

following a flow reduction have been observed (Hoffnagle, 1996). Korman and others 

(2005a) found a statistically significant reduction in survival rates of YoY rainbow trout 

in Glen Canyon associated with the large change in monthly volumes from August to 

September in 2004. 

BHBFs and HMFs - The effects of BHBFs and HMFs on nearshore rearing habitat for 

native fish are dependent on sand supply and the timing of the flood. High flows were 

hypothesized to potentially result in a short-term negative impact on juvenile fish (due to 

displacement and reduced food supply), especially if the flow occurred in the fall when 

smaller YoY fish are present in the mainstem. High flows in the absence of an adequate 

sand supply have been shown to result in only limited backwater development. Thus, 

high flows that do not provide any long-term improvement in backwater development 

were hypothesized to have an overall negative effect for both native fish and trout. The 

importance of backwaters to juvenile native fish is unknown, thus the direction of 

response for high flow events that were coupled with an adequate sand supply was 

uncertain. Because young trout are not as dependent on sandy or backwater habitats as 

are native fish, the negative impacts of high flows associated with displacement were not 

potentially countered by an increased availability of these habitats. 

Sustained Low Steady Flow - Sustained low and steady flows would result in shoreline 

stabilization and higher temperatures that are hypothesized to improve nearshore rearing 

for native fish (Stone and Gorman 2005) and rainbow trout (Korman and others, 2005a). 

This is supported by strong year classes of flannelmouth suckers (S. Rogers, unpublished 

data) and rainbow trout (Speas and others, 2004) produced in the same year as the 2000 

LSSF experiment was conducted. 

High Sustained Flow During Spring - High and steady flows during the spring were 

hypothesized to provide greater shoreline stability for rainbow trout and allow juvenile 

fish to access flooded vegetation for rearing. It is uncertain whether ponding of tributary 

mouths improves juvenile survival rates of native fish in tributaries. Juvenile and young-
35

of-year that remain in the LCR because of ponding would very likely have better growth 

rates than if they migrated to the mainstem, but predation rates in the LCR could be much 

higher (Stone and Gorman, 2005), offsetting any survival advantage associated with 

improved growth. 

Non-GCD Actions - Mechanical removal of warmwater and coldwater nonnative fishes 

was hypothesized to improve the quality of nearshore rearing environments for native 

fishes through reduced competition and predation. 

3.3.2.3 Invasive Fish Species 

The invasive fish species performance measure reflects the population response for 

coldwater and warmwater nonnative species. 

Water Temperature - Warmer water temperatures in the range of 14–18oC are 

hypothesized to increase the abundance of both coldwater and warmwater nonnative 

fishes. Higher temperatures were hypothesized to be the most important factor controlling 

the abundance of warmwater nonnatives. 

Increased Daily Fluctuations - Increased daily fluctuations in flow are hypothesized to 

reduce the abundance of both warmwater and coldwater nonnative fishes. The 

hypothesized mechanism is degradation of nearshore rearing environments. The daily 

variation in flow and the minimum flow were hypothesized to be the most important 

constraints. 

Reduced Variation in Monthly Volume - The effects of changes in mean monthly flow on 

nonnatives is unknown. 

BHBFs and HMFs - As coldwater nonnative fish species are not as dependent on 

backwater habitat and vegetated sandy shorelines as native fish, changes in these habitats 

resulting from BHBFs or HMFs were hypothesized to have little effect on abundance. 

The effect on warmwater nonnative fish that are more dependent on these habitats is 

unknown. 

Sustained Low Steady Flow - Low and steady flows tested during the summer of 2000 

have been shown to increase the abundance of rainbow and brown trout in Grand Canyon 

(S. Rogers, unpublished data; Speas and others, 2004) and the abundance of warmwater 

nonnative fishes (Trammell and others, 2002). 

Non-GCD Actions - Mechanical removal of rainbow and brown trout in Marble Canyon 

has been shown to result in a substantial reduction in population sizes for these species 

(GCMRC, unpublished data). The effect of reductions in rainbow and brown trout 

abundance on warmwater nonnative fish species was considered highly uncertain and 

vice-versa. The efficacy of mechanical removal of warmwater nonnatives is uncertain. 
36

3.3.2.4 Disease 

The disease performance measure predicts the potential incidence of Asian Fish 

Tapeworm in humpback chub and whirling disease in rainbow trout (McKinney and 

others, 2001a). Water temperature was considered the driving factor for both cases. 

Water Temperature - Increases in water temperature were hypothesized to potentially 

increase the incidence of disease in both humpback chub and rainbow trout although the 

extent of the response is uncertain. Currently, water temperatures in the mainstem 

Colorado River are too low for the Asian tapeworm to complete its life cycle and Asian 

tapeworm sources are thought to exist only in Colorado River tributaries. Increased water 

temperature in the Colorado River may allow the Asian tapeworm distribution and 

sources to expand. 

Increased Fluctuations and other Flow Actions - The response of disease to changes in 

flows from GCD was highly uncertain. 

3.3.2.5 Adult Populations 

Adult population performance measures predict the response of humpback chub and 

flannelmouth suckers in Grand Canyon and rainbow trout abundance in Glen and Marble 

Canyons. The size of trout in Glen Canyon, an important determinant of angling quality, 

was also predicted. 

The direction of response of native fish to all the management actions that were 

considered in almost all cases was highly uncertain. Potential benefits of improvements 

in the food base and improved spawning/incubation and/or YoY/juvenile rearing 

environments could potentially be outweighed by increased incidence in disease or 

increased abundance of nonnative fish populations. 

Water Temperature - A significant increase in the abundance of juvenile age classes in 

the flannelmouth sucker population in Grand Canyon have been observed over the last 

few years (S. Rogers, unpublished data, Trammell and others, 2002). It is possible that 

these increases could be the result of lower flows and higher release temperatures from 

Glen Canyon Dam. The observed stabilization of the humpback chub population (Melis 

and others, 2006) may have resulted, in part, from warmer water temperatures during low 

steady summer flows in 2000 and warmer Glen Canyon Dam releases in 2003–5. 

Rainbow trout abundance and size were hypothesized to increase in Glen Canyon in 

response to warmer temperatures. This hypothesis is supported by the strong recruitment 

in 2000 (Speas and others, 2004). The response in Marble Canyon was highly uncertain 

because increased metabolic costs associated with higher temperatures could result in a 

negative overall effect in the absence of a concomitant food supply increase to meet 

increased metabolic energy demand. 

Increased Daily Fluctuations - Reductions in daily fluctuation in flow have been shown to 

increase the abundance of trout in Glen and Marble Canyons with a subsequent reduction 
37

in fish size (McKinney and others, 1999; McKinney and others, 2001b). It has been 

hypothesized that some level of higher daily fluctuations will increase food availability 

and adult size-at-age. Responses of native fish to increased daily fluctuations are highly 

uncertain. 

Reduced Variation in Monthly Volume - The reduction in mean volumes between August 

and September 2004 were shown to result in a significant decrease in YoY survival rates 

for rainbow trout in Glen Canyon, however, the effects of such a decrease on the adult 

population size is uncertain (Korman and others, 2005a). No data on population response 

for native fish are available. 

BHBFs and HMFs - The effects of occasional high flows to build sandbars was not 

considered to have any long-term and significant effects on rainbow trout populations in 

Glen or Marble Canyons. There was no evidence of any significant effect associated with 

the 1996 flood. Responses of native fish populations are highly uncertain because the 

importance of backwaters and vegetated shorelines to overall recruitment has not been 

determined. 

Sustained Low Steady Flow - Strong year classes of flannelmouth suckers and rainbow 

trout were produced from the 2000 LSSF experiment (S. Rogers, unpublished data, 

Trammell and others, 2002). The 2000 flow experiment may also have contributed to the 

stabilization of the Grand Canyon humpback chub population (Melis and others, 2006). 

Low and steady flows during the summer and fall months is part of the reasonable and 

prudent alternative in the biological opinion for humpback chub. Recent habitat use 

studies document the importance of nearshore habitats for humpback chub young-ofyear, which was used to support a recommendation to implement a low and steady 

summer flow test to increase adult population size (Stone and Gorman 2005). The overall 

effect of low and steady flows on the adult humpback chub is uncertain because 

improvements in young-of-year survival rate or adult growth could be offset by an 

increased incidence of disease or an increase in the abundance of warmwater nonnative 

fish which compete with and prey on humpback chub and other native fishes. 

Sustained High Flows During Spring - The effects of sustained high steady flows on fish 

populations in Glen and Grand Canyons is unknown. 

Non-GCD Actions - Mechanical removal of rainbow trout in Marble Canyon has been 

shown to significantly reduce the population size of trout in this area and the extent of the 

reduction can be quantified (GCMRC, unpublished data). However, the response of 

humpback chub and other native fish to this reduction is unknown. The effect of reducing 

the abundance of warmwater nonnative fishes on the population of rainbow trout in 

Marble Canyon is also unknown. 

3.3.3 Angling Opportunity and Quality 

Criteria used for this performance measure were the abundance and size of rainbow trout 

in Glen Canyon, the availability of drift, and the access of anglers to fish. 
38

Water Temperature - Increased water temperature is predicted to increase the size and 

abundance of rainbow trout in Glen Canyon that would result in benefits to the fishery. 

Increased temperature will also increase metabolic demand and feeding rate that may 

improve “catchability.” (See the food base section for specific references on the expected 

relationships between water temperature and aquatic productivity). 

Increased Daily Fluctuations - Increased fluctuations are hypothesized to reduce angling 

opportunities. Increases in the hourly up ramp rate decrease the period of high drift rates 

when fish are feeding most actively and the duration of angling during this period. 

Increasing the daily flow to above ca. 15,000 cfs significantly reduces the number of 

angling locations. Increases in the daily maximum and range in flows, or a decrease in 

the daily minimum flow, increase the distance between the areas where adult fish are 

holding and where anglers can fish from. This increase in distance reduces angling 

opportunity and catchability. This assessment is backed up by two different studies 

(Bishop and others, 1987; Stewart and others, 2000) that relied on statistically reliable 

social science survey methods to determine angler preference for certain types of flows. 

In both studies, most anglers expressed preference for constant flows over fluctuating 

flows. Changes in flow (rising or falling water) were considered an important attribute of 

an excellent fishing experience for slightly more than half of all anglers, but rising or 

falling water ranked behind several other attributes that were considered more important 

for an excellent trip, such as “good weather” and “low water”. In the Bishop et al. (1987) 

study, anglers were specifically asked to consider the likelihood that fluctuating flows 

promoted more drift and therefore improved feeding behavior, when making their 

assessment of flow preferences, yet even when presented with this information, the 

majority of respondents preferred moderate to moderately low (25,000 to 10,000 cfs) 

stable flows over fluctuating flows. Fluctuating flows were also perceived to be more 

problematic at higher flows (averaging around 25,000 cfs) than at lower volume flows. 

Reduced Variation in Monthly Volume - Reductions in the change in monthly volumes is 

likely to result in a more stable food supply for rainbow trout and a decrease in the 

distance between the bank and the area where fish are feeding. 

BHBFs and HMFs - High flows eliminate angling opportunities during the high flow 

event and result in reductions in food availability for weeks to months (depending on 

timing) following the high flow. This negative assessment is supported by data from two 

different social science studies (Bishop and others, 1987; Stewart and others, 2000) that 

relied on statistically reliable samples and survey methods to determine angler preference 

for certain types of flows. In both studies, high flows (flows of 25,000 cfs and above) 

were considered by the majority of anglers to be somewhat or very unsatisfactory (with 

the percentage of very unsatisfactory responses increasing steadily as flow levels 

increased.) 

As flow levels increased above 15,000 cfs, angler satisfaction ratings decreased, 

with the highest levels of dissatisfaction correlated with the highest flows. At flows of 

40,000 cfs, 50,000 cfs, and 60,000 cfs, the percentage of anglers characterizing these 

flows as “very unsatisfactory” was 65%, 69%, and 71% respectively (or 72%, 74%, and 
39

74% when “very unsatisfactory” and “somewhat unsatisfactory” responses were 

combined.) 

Sustained Low Steady Flow - Steady flows reduce the availability of drift and 

catchability as experienced during the 2000 LSSF experiment. Additionally, low steady 

flows during summer promote the establishment of seedling tamarisk trees at low 

elevation. These seedlings then temporarily hinder fishing activity following a return to 

higher flows. This negative assessment must be qualified in relation to previous findings 

by Bishop and others (1987) and Stewart and others (2000), in which anglers expressed 

preference for moderate (10,0,000–15,000 cfs) and moderately low (7,500–10,000) flows 

over lower (<7,500), higher (>15,000 cfs), or fluctuating flows. In the Stewart study, 39% 

of anglers rated flows of 7,500 cfs as very or somewhat satisfactory, 38% rated 7,500 cfs 

as “neutral” (neither satisfactory nor unsatisfactory), and less than a quarter (24%) rated 

this flow level as somewhat or very unsatisfactory. As flow levels decreased below 7,500 

cfs, however, anglers expressed increasing dissatisfaction, e.g., 40% rated flows of 5,000 

cfs as somewhat or very unsatisfactory, 50% rated flows of 4,000 cfs as somewhat/very 

unsatisfactory, 54% were very or somewhat unsatisfied with 3,000 cfs, and 56% 

somewhat to very unsatisfied with 2,000 cfs. According to the Bishop and others (1987) 

study, the increasing dissatisfaction with lower flows has more to do with increasing 

difficulty of boat access upriver and increasing potential for equipment (motor/prop) 

damage as flows decreased than with decreasing drift or fish catchability. 

High Sustained Flow During Spring - Flows in excess of ca. 15,000 cfs increase the 

distance between areas from which anglers can fish and the area where fish are holding, 

thereby reducing catchability. This negative assessment is supported by data from the 

Bishop and others, 1987, and Stewart and others, 2000, studies, both of which employed 

statistically reliable social science sampling and survey methods to determine angler 

preference for certain types of flows. In both studies, anglers expressed a strong 

preference for flows in the 10,000–15,000 cfs range. As flow levels increased above 

15,000 cfs, angler satisfaction ratings decreased, with the highest levels of dissatisfaction 

correlated with the highest flows. At flows of 20,000, 25,000 and 30,000 cfs, the 

presumed range of these sustained high flows, the percentage of anglers characterizing 

these flows as “very unsatisfactory” was 19%, 39% and 54% respectively (or 36%, 56% 

and 67% when “very unsatisfactory” and “somewhat” unsatisfactory responses were 

combined), compared with 6% “very unsatisfactory” responses for flows in the 15,000 

range. Conversely, one third of all respondents (33%) applied the “very satisfactory” 

rating to flows in the 15,000 cfs range, whereas 17% rated flows in the 20,000 cfs range 

as “very satisfactory” and only 5% and 3% of respondents rated flows of 25,000 and 

30,000 cfs as “very satisfactory.” 

Summary of Fishery Portion in Aquatic Matrix - 

The Glen Canyon Dam Adaptive Management Program has identified maintenance and 

improvement of two distinct fish populations that both inhabit the Colorado River below 

Glen Canyon Dam in the Program Goals: the introduced rainbow trout population in the 

Lees Ferry reach (subject to recreational angling) and the native fish population in Grand 

Canyon, especially the federally listed endangered humpback chub. The Knowledge 
40

Assessment Workshop participants recognized the fundamental linkage of these 

populations with their food sources (primarily the production of algae, macrophytes, and 

invertebrates, but also allochthonous material produced in the riparian zone and organic 

material delivered from tributary flows) although the food base is presented as a distinct 

resource in this report. This linkage is reflected in some of the strategic questions 

presented in Section 4.3. Because metabolic rates in fishes is dependent on water 

temperature, increased release temperatures from Glen Canyon Dam can be expected to 

increase growth rates for both rainbow trout and native fishes. Increased water 

temperatures can also be anticipated to provide some benefit to nonnatives adapted to 

warmwater, and so require ongoing research to determine how much they limit survival 

of native fish species. The effects of various flow regimens on fish is less certain, and 

should be the subject of additional study. Rainbow trout in the Lees Ferry reach appear to 

benefit from additional food that can be made available by fluctuating flows, though 

apparently not the artificial floods, but this deserves additional research. Workshop 

participants recognized that humpback chub reproduction is being naturally maintained in 

the Little Colorado River but that survival and recruitment of these fish to maturity is 

limited. Humpback chub reproduction in the mainstem Colorado River in Grand Canyon 

appears to be dramatically reduced from historic levels. What is less certain is which 

factor or suite of factors is most limiting to these young fish. It is not clear whether 

mainstem water temperatures, predation by nonnatives, displacement by flows, habitat 

availability/quality, parasites, or some combination of factors is most limiting to 

maturation and reproductive success of native fish, and so elucidation of limiting factors 

deserves additional study to help guide management. Researchers in this area will need to 

be cognizant of the potential impacts of their sampling on fish populations. Strategic 

questions associated with these resources are presented in Sections 4.3.1 – 5, and the state 

of knowledge is summarized graphically in Tables 3.3 and 3.4. 

3.4 Riparian Habitat 

Riparian vegetation was initially separated into large geomorphic reach designations 

(e.g., Glen and Grand Canyon) to represent the response of riparian vegetation to local 

and large scale variables. However for the purposes of this exercise, it was determined 

that the focus should be on garnering a general understanding of the response of riparian 

vegetation, throughout the CRE, to operations. To identify where uncertainty exists 

within riparian vegetation, the shoreline was divided by stage elevations. The categories 

are: fluctuation zone with stage elevation to 708 cms (<25,000 cfs,); lower riparian zone 

with stage elevation to 708–1,274 cms (25,000–45,000 cfs); upper riparian zone with 

stage elevation to 1,274–1,680 cms (45,000–60,000 cfs); predam high water zone with 

stage elevation to >1,680 cms (>60,000 cfs); and uplands. In the case of the last three 

stage elevation categories, the flow options considered during this workshop were 

generally not applicable (Table 3.5). However, the lack of flows above 1,274 cms on 

these zones is discussed briefly. This section concludes with a discussion of wildlife 

resources and dam operations, but is restricted to southwestern willow flycatcher 

(Empidonax traillii extimus) and Kanab ambersnail (Succineidae: Oxyloma haydeni 
41

kanabensis). It should be noted that these resources were not a focus of discussion during 

the workshop and are not included in the matrix. 

Water Temperature - The effect of warmer temperature on riparian habitat is largely 

unknown for vegetation below the 1,274 cms stage elevation, in the CRE. Studies in the 

Walker River Delta in Nevada (Young and others, 2004) indicate that tamarisk 

germinates throughout a range of seed bed temperatures (0–40°C). These temperatures 

may be constant or vary diurnally. Optimal germination was diurnal with 16 hours of 

10°C and 8 hrs of 20°C. Germination was variable by seed lots. A similar response was 

noted for coyote willow Salix exigua) a species native to the CRE (Young and Clement 

2003). Other factors contributing to the uncertainty include physiological responses 

associated with germination and clonal growth of wetland and riparian species 

(Farnsworth and Meyerson, 2003; Güsewell and others, 2003). Vegetative growth rates 

would likely increase under a warmer regime, resulting in denser marsh and riparian 

vegetation within the fluctuation zone. Besides temperature, nutrient concentrations in 

water also come into play and would likely differentially affect native and exotic species 

found in the fluctuation and lower riparian zones (Biondini, 2001; Farnsworth and 

Meyerson, 2003; Güsewell and others, 2003). This effect would likely be more 

pronounced in the fluctuation zone that is also subject to daily disturbance. These 

changes could cascade up the trophic food web by changing invertebrate composition and 

types of plant species that provide nearshore cover for aquatic species. 

Increased Daily Fluctuations - Overall riparian vegetation in the fluctuation and lower 

riparian zone would respond positively to increased fluctuations. The group agreed that 

the “certain” category could be applied to the fluctuation zone, citing Stevens and others 

(1995). The lower riparian zone response was likely trending to “uncertain” with a 

potential increase in vegetation volume among woody species (Kearsley, 2004). GCMRC 

has in hand several annual reports (Waring, 1995; Kearsley and Ayers, 1996; 1998; 

Kearsley and others, 2003; 2004), but few peer reviewed articles (Turner and Karpiskac, 

1980; Johnson, 1991; Ralston, 1995) for this zone. 

Reduced Variation in Monthly Volume - For all zones the group identified that 

uncertainty existed and, like temperature, the amount of available information for the 

CRE was too small to know or infer positive or negative effects. 

BHBFs and HMFs – Beach/habitat-building flows have only occurred twice, under 

different sediment enrichment conditions, and at different times of the year. Habitat 

maintenance flows took place under depleted sediment conditions, and at different times 

of the year. Seasonal differences confound interpretation of the effects on vegetation. The 

participants felt that short duration high flows were ranked as uncertain, but with positive 

effects. There is some peer-reviewed literature available regarding the 1996 event 

(Kearsley and Ayers, 1999; Stevens and others, 2001). The authors found significant 

changes in vegetative cover immediately following the high flow, but species 

composition remained little affected. The number and extent of sampled area (i.e., 

polygons) dominated by obligate wetland species (e.g., cattails, common reed) did 

increase following the high flow, but these increases were not statistically significant. 
42

Understory cover was eliminated, largely through burial. Substrate grain size was more 

homogeneous following the high flow event. Recovery from burial was by species that 

could withstand burial including both native species such as coyote willow (Salix exigua) 

and introduced species, such as tamarisk (Tamarix ramosissima). Operations appear to 

affect water availability up to approximately 400 cms above river surface elevation 

(Kearsley, 2004, Ralston, 2005). A flow that exceeds 1260 cms (45,000 cfs) may be 

beneficial to the predam high water zone, but this certainty falls in the yellow to light 

green color designation. Higher flows would result in a loss of vegetation in the lower 

and upper riparian zone (Waring, 1995), but the magnitude of loss is difficult to predict. 

A lack of high flows has, with certainty, limited recruitment in the predam high water 

zone (Anderson and Ruffner, 1988). 

In the absence of sediment enriched conditions, questions remain regarding how 

coarsening of the substrate changes vegetation composition and dominance. In the same 

vein, silt and clay contribution to nutrient cycling and riparian dynamics would likely be 

different under enriched versus starved sediment conditions. The seasonality question is 

also an area of uncertainty. Although high flows historically occurred in late spring, the 

daily range in July through October (during the monsoon period) was also extreme 

(Topping and others, 2003) but of shorter duration. The high flow tests that are of short 

duration may need to be considered in this context rather than in the context of a spring 

flood scenario. In general, high flows were thought to likely result in some reworking of 

return channel/marsh sites and a redistribution of carbon sources. There would be less 

material involved, but these high flows would still deliver nutrients and transport seeds, 

implying a benefit to riparian vegetation. 

Sustained Low Steady Flows - The only point of reference for sustained low steady flows 

is for the summer of 2000. In this context, the group felt that there was certainty that the 

flows had a negative affect on the fluctuation zone and a neutral to negative affect on the 

lower riparian zone. The negative affect was attributed to invasive species establishment, 

particularly tamarisk (Porter and Kearsley, 2001) that germinates under constant moisture 

conditions. Observations of tamarisk seedling establishment in the Glen Canyon Reach in 

the cobble bars and along the shoreline were also made by fishing guides. The response 

of riparian vegetation to sustained flow during other parts of the year, fall for instance is 

unknown. 

High Sustained Flow During Spring - High sustained flow effects were considered certain 

and positive for the fluctuation zone, subject to more specific delineation of the duration 

and timing. The marsh/fluctuation zone would likely increase in area shoreward and 

upslope (Stevens and others, 1995; Kearsley and Ayers, 1999). A similar level of 

certainty was given for the lower riparian zone with respect to high sustained flows, but 

again the duration and timing would require definition. High sustained flows during 

spring and summer may support increased diversity in the lower riparian zone (Kearsley 

and Ayers 1998) and may have, with some certainty, negative affects on exotic species 

like camelthorn (Alhagi maurorum) and pepperweed (Lepidium latifolium), through loss 

of these seeds in the seedbank, though the weeds studies had low representation at the 

sample sites (Kearsley and Ayers, 1998). 
43

Non-GCD Actions - These were not applicable for all zones. 

Wildlife Resource Linkages – Two wildlife resources of concern that were not the focus 

of discussions during the workshop, owing to time constraints, are the southwestern 

willow flycatcher (Empidonax traillii extimus) and Kanab ambersnail (Succineidae: 

Oxyloma haydeni kanabensis). Both are impacted by high flows (BHBF) with some 

certainty. 

The impacts to southwestern willow flycatchers are associated with inundation of rearing 

habitats located in the upper riparian zone and possibly foraging habitats within the 

fluctuation zone. The latter is more uncertain. The willow flycatcher is a riparian 

breeding bird that has had historic nesting sites along the river corridor since 1963 and 

possibly before the dam (Sogge and others, 1997; Holmes and others, 2005). The bird’s 

nests are usually found in tamarisk trees, at heights well above inundation levels (Sogge 

and others, 1997; Stevens and others, 2001). Timing and magnitude are variables 

associated with high flows that might impact willow flycatchers. If flows exceed 1274 

cms (45,000 cfs), and occurred during the breeding or into the period when young birds 

fledge (May – August or September), then this species could be affected with some 

certainty. Stevens and others (2001) reported nominal impacts associated with the March 

1996 high flow including reduction in groundcover and branches located lower than 0.6 

m. The high flows did not reach nest trees. Stevens and others (2001) speculated that the 

high flows might have had an impact on foraging habitat (i.e., marsh areas), but studies 

that make these linkages are not available (Holmes and others, 2005). 

The linkages between riparian vegetation and breeding bird habitat quality are not well 

defined for the river corridor. However, vegetation density (volume) associated with trees 

rather than shrubs, and vegetated area are two variables that correlate well with breeding 

bird abundances and diversity (Mills and others, 1991; Sogge and others, 1998). This 

correlation was strongest with tamarisk and mesquite (Prosopis glandulosa) and less so 

with willow, arrowweed (Pluchea sericea), or seep willow (Baccharis emoryi) (Sogge 

and others, 1998). Operations that would promote expansion of trees over shrubby 

species might include high flows that promote seed scour and redistribution of mesquite 

or acacia (Acacia greggii) and habitat reworking, though this is uncertain. A retrospective 

analysis following the 1984 high flow event of mesquite distribution would help elucidate 

this question. Fluctuating flows like those seen in the winter of 2002–4 would also 

promote growth of existing vegetation (Kearsley, 2004), but this would include both 

shrubs and trees. This can be stated with some certainty. 

The habitat of Kanab ambersnail at Vaseys Paradise is affected by both spring discharge 

and river stage elevations. Spring discharge influences available wetted area above river 

stage elevation, while river stage determines how far down the slope vegetation can be 

established along the river’s edge. The available wetted area and amount of moisture 

influences plant composition which in turn influences snail distribution (Stevens and 

others, 1997b; 1998; Meretsky and Wegner, 2000). High flows that occur in low 

frequency may temporarily reduce snail habitat for several years (Stevens and others, 
44

2001). However these impacts may be alleviated through temporary removal and 

replacement of vegetation. A conservation effort associated with the 2004, high flow test 

that involved temporary removal and replacement of habitat appears to have promise for 

the conservation of this species (Cox and others, 2005). 

Summary of Vegetation Element of Terrestrial Matrix - 

Knowledge Assessment Workshop participants found it practical to assess resource 

impacts in this area within the context of various flow elevations, where the lowest 

elevations are closest to the river and therefore are most affected by dam operations. The 

participants considering riparian vegetation were uncertain how various temperature 

releases would affect this resource, and so developed a strategic question focused on this 

habitat aspect. Some increased flows and flooding appear to benefit riparian vegetation 

through seed distribution and mechanical disturbance, which might be anticipated of 

species adapted to a historically fluctuating environment. Steady flows, conversely, are 

thought to have negative impacts on riparian vegetation, especially through the 

mechanism of allowing for establishment of nonnative invasive species, particularly 

tamarisk. These responses to flows are not certain, and so managers can benefit from 

additional research. Strategic questions associated with riparian resources are presented 

in Section 4.4, and the state of knowledge of effects of various treatments is presented 

graphically in Table 3.5. 

3.5 Recreation 

3.5.1 Campable Area 

The campable area performance measure represents the conditions that promote the 

maintenance (or increase) of sandy areas suitable for use as campsites. In general, the 

results of this analysis track closely with the sediment matrix findings, since conditions 

that promote sediment retention on a systemwide basis are generally thought to be 

beneficial for campsite retention, as well. This perception is supported by data from 15+ 

years of campsite area monitoring at a sample of popular riverside campsites (Kaplinski 

and others, 2005a; 2005b) BHBFs are the key management action expected to maintain 

or improve campsite areas over the long run. 

Water Temperature - There is certainty that warmer temperatures decrease water 

viscosity, allowing fine sediment to settle out faster, and thereby reducing the suspension 

of sediment in the water column and the amount of sediment available for downstream 

transport (Lane and others, 1949; Hubbell and Ali, 1961; ASCE, 1975). In theory, 

warmer water reduces the amount of sediment transported from sandbar deposits (S. 

Wright, oral communication, 2006), thereby increasing the longevity of sandy deposits 

used as camping beaches. 

Increased Daily Fluctuations - The effects of increased daily fluctuations on campable 

area is reasonably well known from campsite area data collected sporadically during the 

1980s and early 1990s (Beus and Avery, 1992), as well as from recently and historically 
45

compiled sediment transport data (Topping and others 2000a; 2000b; 2003, 2006; Hazel 

and others 2006). It is well established that increased fluctuations will result in higher 

sediment transport rates, with sediment derived from eddy storage complexes, which are 

the primary setting of most campsites in the CRE. However, as discussed by Beus and 

Avery (1992) responses of sandbars to higher fluctuating flow regimes are complex and 

highly variable. For example, during experimental fluctuating flows in 1990–91, higher 

fluctuations under sediment enriched conditions allowed some bars to aggrade, while 

others eroded or showed no net change. On the other hand, higher fluctuations under 

sediment depleted conditions generally resulted in net erosion, while under lower steady 

flows, bars can not aggrade above the level of the highest low flow, and therefore, over 

time (without occasional higher flows), the subaerial volume and height of sandbars will 

likely decrease even though sediment transport out of the system has been reduced.. 

Based on the work of Budhu (1992), Beus and Avery also concluded that “seepage 

induced erosion processes intensified as the range of daily stage fluctuations and downramping rates increased” while tractive erosion processes increased with higher 

upramping rates. Ultimately, these data led Beus and Avery (1992, Chapter 10, p.15) to 

conclude that: “Without bar building flows, the dynamic equilibrium condition achieved 

through seepage-driven erosion will keep the system at or near the minimum sandbar area 

and volume for this system. Large fluctuations or bar building flows may be used to 

maintain or rebuild the remaining sandbars . . . provided that adequate sediment supplies 

are available for bar building. In situations where sediment supplies are low or unknown, 

a general strategy of sediment storage is recommended [including] low ramping rates, 

low range of daily flows, and low maximum flows.” 

 

Reduced Variation in Monthly Volume - The effect of changes in monthly volumes on 

availability of camping beaches is unknown, because the changes in monthly volumes 

were not clearly defined. Decreases in monthly volumes during the summer high-use 

season would increase the available campable area during the peak recreational use 

season, but since the distribution of monthly volumes was not specified, the relative 

effects of monthly volume changes could not be gauged. 

BHBFs - The effects of high flows above powerplant capacity is relatively well 

documented and appears to be the only mechanism available for restoring sand to 

elevations above the highest level of normal dam operations (Beus and Avery 1992; 

Kaplinski and others, 2005b). BHBFs have been demonstrated to increase campable areas 

on a systemwide basis, at least temporarily (Kearsley and Quartoroli, 1997; Hazel and 

others 1999; Kaplinski and others, 2005a; 2005b). 

HMFs - The effect of powerplant capacity flows on campsite size is reasonably well 

documented (Kaplinski and others, 2005b), at least for HMFs under sediment depleted 

conditions. HMFs under sediment depleted conditions have been shown to not 

appreciably increase campable area (Kaplinski and others, 2005b). The work of Beus and 

Avery (1992, Chapters 6 and 10) suggests that under sediment enriched conditions, shortduration higher flows or higher fluctuating flows could potentially benefit some sandbars, 

while others would either continue to erode or show minimal measurable change. 
46

Sustained Low Steady Flow - Results reported by Wright and others (2005); Schmidt and 

others (in review); Hazel and others (2006); Topping and others (2066) indicate that new 

tributary sand inputs are clearly conserved and will accumulate in the main channel at 

stable flows of 8,000 cfs (summer 2000, low summer steady flowtest) and under 

fluctuating flows ranging between 5,000 and 10,000 cfs (fall 2004 period before highflow sediment test). Given that sustained low steady flows below 9,000–10,000 cfs are 

known with certainty to constrain the amount of sediment transported out of the system to 

negligible amounts (Topping and others, 2003), it follows that low sustained flows would 

significantly reduce the rate of sediment loss from campable sandbar areas. In addition, 

sustained low flows would expose more campable area on a sustained basis, increasing 

the amount of campable area exposed at lower elevations (Hazel and others 2001). 

However, over time, sustained low steady flows may result in decreases to the size and 

volume of sandbars above the low flow level due to the cumulative effects of repeated cut 

bank failures and the consequent redistribution of higher elevation sediment to lower 

elevations (at or slightly below the level of the highest flows). In other words, unless low 

steady flows are accompanied by occasional periods of higher fluctuating flows, the 

conserved sand may ultimately end up stored in the river system at or below a level that 

is beneficial for use by campers. Low steady flows will also permit tamarisk and other 

vegetation to encroach on sandbars (Porter and Kearsley, 2001), which could eventually 

offset any gains resulting from greater bar exposure at lower elevations. Unfortunately, 

measured information that would allow us to quantify rates of vegetation encroachment 

on sandbars under low flows (or any other flows, for that matter) are lacking, so the 

overall impact of vegetation encroachment on campable area is unknown. 

Sustained High Flow - Sustained higher flows are certain to increase the rate and volume 

of sediment transported out of the system (Wiele and others 2002; Topping and others 

2003; Hazel and others, in press), therefore sustained high flows would increase the rate 

of sediment loss from sandbars used for camping. Furthermore, sustained high flows 

would consistently inundate lower elevation campable areas, resulting in a sustained loss 

in campable area up to the level of the highest flows (Kaplinski and others, 2005b). 

Sustained high flows would obviously impact and remove vegetation growing below the 

zone of inundation, but since lower areas of sandbars would be constantly inundated, the 

benefits of removing vegetation would not be realized for campable area. 

Non-GCD Actions - Mechanical removal of trout was the only type of mechanical 

removal considered in the matrix development, and obviously, trout removal would not 

affect campable area. However, we did not consider the option of mechanical removal of 

terrestrial exotics (such as tamarisk). Mechanical removal of tamarisk thickets or other 

exotics such as tumbleweed and camel thorn could hypothetically increase campable 

area, but the practicality and sustainability of applying such an approach with the goal of 

increasing campable area is untested and therefore unknown at this time. 

3.5.2 Access to Attraction Sites 

The access to attraction sites performance measure represents the conditions that enhance 

accessibility to popular recreation sites in the CRE. 
47

Water Temperature - The assumption is that warmer water would not measurably 

enhance or detract from access to attraction sites, but this assumption has not been 

subject to serious scrutiny. 

Increased Daily Fluctuations - The effects of increased daily fluctuations on access to 

attraction sites is not well documented, but anecdotal information suggests that increased 

fluctuations would reduce access to attraction sites by creating unstable and less 

predictable boat mooring conditions, resulting in the need to shortened side trips and/or 

leave some members of a boating party with their boats at all times, so that they could 

deal with the changing parking conditions under fluctuating flows. 

Reduced Variation in Monthly Volume - The effect of changes in monthly volumes on 

access to attraction sites is unstudied and unknown. 

BHBFs and HMFs - The effects of high flows at or above powerplant capacity on access 

to attraction sites is not well documented, but anecdotal information suggests that it 

would have neither a generally positive nor negative effect. At specific attraction sites, 

pulling over and parking boats may be easier or more difficult, but available anecdotal 

information does not indicate a dominant trend in one direction or another. 

Sustained Low Steady Flow - Sustained low steady flows reduce the amount of time row 

boaters have available to spend at attraction sites, because the slower current requires 

boaters to spend more time rowing to get downstream. Low flows do not appreciably 

affect motor boaters’ use of attraction sites, since they can make up for the slower flow 

velocity by motoring faster and floating less often (Jonas and Stewart, 2002, p.23). Lower 

flows may improved the ease with which boaters can pull over and park their boats, and 

the parking at attraction sites would be stable, therefore low sustained flows may 

somewhat improve access to attraction sites overall. The relationship between low flows 

and access to attraction sites remains to be evaluated through formal study.

Sustained High Flow - The effects of sustained high flows at or above powerplant 

capacity on access to attraction sites is not well documented, but anecdotal information 

suggests that it would have neither a generally positive nor significant negative effect. In 

theory, higher flows would allow rafters to spend less time on the river and more time 

exploring attraction sites, but higher flows may also make some sites less accessible due 

to loss of mooring sites. Once again, the hypothesized relationship between specific flows 

and access to attraction sites remains to be evaluated through formal study.

 

Non-GCD Actions - Mechanical removal of trout was the only type of mechanical 

removal considered in the matrix development. It seems self-evident that trout removal 

would not affect access to attraction sites, except perhaps on a short term, infrequent 

basis. However, we did not consider the option of mechanical removal of terrestrial 

exotics (such as tamarisk). Mechanical removal of tamarisk thickets or other exotics such 

as tumbleweed and camel thorn could hypothetically increase accessibility to certain 

attraction sites that are currently difficult to access due to the density of nearshore 
48

vegetation; however, it is doubtful that the NPS would support the use of mechanical 

vegetation removal solely to improve accessibility to attraction sites. 

3.5.3 Rafting Navigability 

The rafting navigability performance measure represents the conditions that enhance 

navigability of the river in general and the white water rapids specifically. Initially, this 

variable was combined with human health and safety, but the knowledge assessment 

participants found it difficult to address this multi-component variable due to the fact that 

for any given flow scenario, the outcomes from a health perspective could be very 

different from those for navigability or safety. There was also considerable debate among 

the knowledge assessment workshop participants about how to define this multicomponent variable and what indicators could best represent this variable. There were 

also concerns with how health and safety elements overlap with the visitor experience 

performance measure. In the end, we agreed that the original health and safety variable 

should be split into three separate components – navigability of rapids, human health, and 

safety (the latter interpreted as numbers of injuries and/or documented incidents putting 

human safety at risk) – and that the navigability variable would be limited to evaluating 

the ease with which boats are able to navigate downstream without getting stranded, 

damaged or overturned. 

Water Temperature - Common sense suggests that water temperature does not affect the 

navigability of rapids. Therefore, temperature is considered to be “not applicable” or a 

known “non-issue” relative to navigability. 

Increased Daily Fluctuations - Effects of increased daily fluctuations on rafting 

navigability are not well documented. We do know (from anecdotal information and 

common sense) that higher fluctuations improve the possibility of a stranded boat 

becoming unstuck within a reasonable timeframe (less than 24 hours); on the other hand, 

increasing the down ramping rates increases the likelihood of rafts getting stuck in the 

first place. One workshop participant expressed the opinion that higher fluctuations 

would improve navigability because it offered boaters more options on when and where 

to run a given rapid. However, the general consensus of workshop participants was that 

the effects of increased fluctuations on navigability were too poorly understood to 

confidently evaluate at this time. 

 

Reduced Variation in Monthly Volume - The effect of changes in monthly volumes on 

rafting navigability is unknown. While there is anecdotal information suggesting that 

navigability may improve or decrease under higher or lower flow levels, the change in 

distribution of monthly volumes was not specified at the time this knowledge assessment 

was undertaken, therefore the effects of monthly volume changes on navigability could 

not be gauged. 

BHBFs and HMFs - The effects of high flows above powerplant capacity on navigability 

is not well documented in the peer-reviewed literature, but anecdotal information and 

several in-house NPS studies (that have not yet been subject to peer review) (Jalbert, 
49

1996; Jalbert, 2001) suggest that higher flows improve the navigability of most rapids by 

covering rocks that would otherwise be exposed and by creating more channels for 

boaters to chose from as they navigate downstream. Also, Webb’s work (1999) shows 

that BHBFs can clear channels of rock debris accumulations, which generally (but not 

always) creates easier passage for boats after flows diminish. The NPS studies found a 

slight increase in flipped row boats and inadvertent swimmers under experimental high 

flows in the 45,000 cfs range, but the difference in numbers of these incidents under high 

and lower flows was not statistically significant (Jalbert, 1996.) 

Sustained Low Steady Flows - Studies by Jonas and Stewart (2002) and Stewart and 

others (2000) provide limited data on rafters’ perceptions of navigability under low 

sustained steady flows, and Jalbert (2001) has provided some preliminary data on 

accident rates during the 2000 LSSF experiment (showing a 100% increase in “incidents” 

under low flows), but most of the currently available information is anecdotal and in 

some instances, also contradictory. Lower flows certainly expose more rocks in the 

channel, and they constrain the options available to boaters for navigating the rapids, but 

at the same time, these flows slow the velocity of the current, thereby increasing the ease 

with which boaters can avoid obstacles in their path. There was general agreement among 

workshop participants that lower flows decreased the navigability of rapids for motor 

boats, but effects to oar powered crafts were strongly debated (e.g., Larry Stevens was 

adamant that lower flows improved navigability, while Andre Potochnik argued for the 

opposing viewpoint.) Workshop participants were in general agreement that lower flows 

would likely allow rocks to accumulate in the rapids more quickly than at higher flows, 

due to the reduced carrying capacity of the river, thereby decreasing navigability of 

rapids over time. This conclusion is supported by the work of Webb and others (2005), 

although Webb and others (2005, p.145) also found that several factors besides flow 

velocity and volume influence the rate and amount of debris fan reworking. The 

knowledge assessment participants generally concluded that low sustained steady flows 

would have either no appreciable effect or would have a negative effect on navigability 

overall, but solid consensus was lacking on this issue. 

Sustained High Flows - The effects of sustained high flows at or slightly above 

powerplant capacity on rafting navigability is generally thought to be positive (Bishop 

and others, 1987; Stewart and others, 2000), with Crystal Rapid being a notable 

exception. The available anecdotal information suggests that high sustained flows would 

have either a generally positive effect or a neutral effect, for the same reasons as noted 

under BHBF. 

 

Non-GCD Actions - Mechanical removal of trout was the only type of mechanical 

removal considered in the workshop, and it seems self-evident that trout removal would 

not affect navigability. We did not consider the option of mechanical removal of rocks or 

channel clearing with dynamite as viable nonflow options, as it seemed doubtful that the 

NPS would support the use of such approaches to improve navigability. 
50

3.5.4 Quality of Water and Human Health Issues 

This performance measure addresses conditions that influence the quality of water from a 

human health perspective, as well as the conditions that may influence the retention or 

dissolution of human pathogens in shoreline deposits, beach sand, etc. To our knowledge, 

no studies specifically evaluating the effects of flows on human health are available from 

the CRE, so this assessment is based on information from sources outside the system and 

from anecdotal information. 

Water Temperature - An increase in waterborne pathogens is certain to occur with 

warmer water temperatures, based on information from EPA and elsewhere, therefore 

this cell was coded as negative light green. 

Increased Daily Fluctuations - It was hypothesized that higher ramping rates and 

increases in the range of fluctuations could possibly improve water quality (through 

increased aeration of the water) and could reduce concentrations of bacteria near the 

shore lines of campsites where organic refuse and human pathogens tend to concentrate 

by constantly reworking and repeatedly “flushing” the sediments under fluctuating flows. 

Reduced Variation in Monthly Volume - The effect of changes in monthly volumes on 

quality of water and human health is unstudied and unknown. 

BHBFs and HMFs - The effects of high flows at or above powerplant capacity on quality 

of water and human health has not been scientifically documented, but scientific studies 

demonstrate that higher flows significantly rework shoreline deposits (Beus and Avery 

1992). Anecdotal information suggests that this reworking of shoreline sediments flushes 

out viral and bacterial loads that tend to concentrate along the shorelines of camping 

areas under lower flows. This hypothesis warrants testing through formal study. 

 

Sustained Low Steady Flow - Sustained low steady flows are assumed to negatively 

affect human health and quality of water through increasing temperatures, thereby 

increasing the volume of pathogens in the water. Also, sustained low flows reduce the 

volume and velocity of water flowing along the shorelines, as well as limiting the change 

in shoreline levels, thereby allowing pathogens to accumulate in shoreline sediment, 

especially in the vicinity of campsites. River guides have observed algae blooms 

associated with human urine along shorelines that are not regularly inundated by periodic 

higher flows, and they experienced increased incidents of waterborne viral infections 

during the 2000, LSSF, adding further anecdotal support to the notion that low steady 

flows promote conditions that are not conducive to maintaining optimal human health 

conditions. 

Sustained High Flows - For the opposite reasons of those noted above (e.g., colder water, 

higher volumes, and increased velocity near shorelines), sustained high flows at or 

slightly above powerplant capacity are assumed to have a positive effect on quality of 

water and human health, but once again, actual data to support or refute this hypothesis, 

which is currently based on “commonly accepted river guide knowledge”, is lacking. 
51

Non-GCD Actions - Mechanical removal of trout was the only type of mechanical 

removal considered. It is assumed that this activity does not affect human health either 

positively or negatively; however, once again, data is lacking to support or refute this 

assumption. 

3.5.5 Human Safety 

This category evaluates the likelihood of recreationists sustaining injuries or death from 

accidents associated with various flow regimes. 

Water Temperature - Hypothermia and heart attacks due to thermal shock are some of the 

greatest risks to boaters and anglers who fall out of their boats in the CRE (Meyers and 

others, 1999, p. 37). Under normal dam release temperatures of 8–12°C, people can lose 

motor control in a matter of minutes and may lose consciousness in less than 10 minutes. 

This reality forces potential rescuers to take additional risks in their attempts to get 

swimmers out of the water as quickly as possible. Although no specific studies have been 

done in the CRE to document the relationships between water temperature and safety 

incidents, the well-known and well-documented hypothermia risks associated with 

typical dam release temperatures (Meyers and others, 1999) leaves little room for doubt 

that warmer water would significantly reduce safety risks associated with Colorado River 

recreation. Therefore, this cell was coded as light green and positive. 

Increased Daily Fluctuations - The effects of increased fluctuations on visitor safety are 

unknown. There is some anecdotal information suggesting that higher ramping rates 

increase safety risks for anglers (as anglers are more likely to become stranded on gravel 

bars due to rapidly rising water, and anglers’ boats are more likely to swamp if anchored, 

or to drift away if not anchored), but this relationship has not been formally documented. 

There is also anecdotal information suggesting that the increased unpredictability of 

flows increases risks for downstream boaters, but it is unclear whether either of these 

perceived increased risks would be substantiated upon further study. 

 

Reduced Variation in Monthly Volume - The effect of changes in monthly volumes on 

human safety is unstudied and unknown. 

BHBFs and HMFs - Several “in-house” studies have been undertaken by the NPS that 

evaluated the effects of high flows at or above powerplant capacity on safety (e.g., Brown 

and Hahn-O’Neill, 1987; Jalbert, 1996.) The results of these studies are somewhat 

difficult to evaluate because the studies were relatively short term, the sampling strategy 

was not random, the studies did not take into account nonflow factors such as boater 

experience, and the results were not subject to rigorous independent peer review. In 

addition, various studies have evaluated boaters’ perceptions of risk at high flows (e.g., 

Bishop and others, 1987; Shelby and others, 1992; Stewart and others, 2000), but the 

findings from these studies have not been independently evaluated through actual 

monitoring of safety incidents during non-experimental flow events. Based on a 

comparison of data from 1987, when flows in the low 30,000 cfs range were common, 

with incident data collected during the 1996 BHBF, Jalbert (1996, p. 16) concluded that 
52

more accidents were likely to occur under flows of 31,500–33,000 cfs than at 45,000 cfs. 

The 1996 NPS study concluded that despite observing a slight increase in boat flips and 

unintentional swims at a couple of rapids during the 1996 BFBH, the overall numbers of 

incidents at 45,000 cfs were not significantly different from those reported during nonexperimental flow conditions (Jalbert, 1996.) Studies specifically designed to evaluate 

safety issues at a variety of different flows are needed to substantiate these conclusions. 

 

Sustained Low Steady Flow – Effects of sustained low steady flows on safety were rated 

as positive by workshop participants because the river is warmer and slower, and the runs 

in the rapids are more predictable than under fluctuating flows. However, this conclusion 

contradicts data collected by NPS staff (Brown and Hahn, 1987; Jalbert 1996) and 

cooperating scientists (Shelby and others, 1992; Jonas and Stewart 2002) since the mid 

1980s which indicates that boaters have a greater likelihood of incurring equipment 

damage and hitting rocks at flows below 9,000 cfs than at flows above 9,0000 (Jalbert, 

1996, p.16). Also, preliminary data from the NPS LSSF safety study (Jalbert, 2001) 

documented a 100% increase in incidents such as hitting rocks in rapids and damage to 

equipment during the LSSF experiment (flows of 5,000–8,000 cfs), compared with 

normal ROD (MLFF) flows, adding further support to the “common consensus” of river 

guides that more accidents and injuries occur under low flow conditions. Once again, a 

formal, up-to-date, and comprehensive study evaluating the effects of flows on safety is 

currently lacking; a rigorous, peer-reviewed study to test the hypothesis that low steady 

flows positively affect safety is warranted in the future. 

Sustained High Flows - For the opposite reasons of those noted above (e.g., colder water, 

greater velocity), sustained high flows at or slightly above powerplant capacity are 

assumed to have a somewhat negative effect on human safety. Actual data supporting this 

conclusion are limited to a single, non-peer-reviewed study (Brown and Hahn, 1987), 

which showed a correlation between flows in the low 30,000 cfs range and increased flips 

and swims at selected rapids. At flows in the low to mid 20,000 there was also a slightly 

elevated risk of having an incident, compared with flows in the 10,000 to 17,000 cfs 

range (which were rated as the safest flows by Brown and Hahn, (1987)). With higher 

flows, there are less rocks exposed, hence the increased safety risks that come with 

colder, swifter water may be offset to some degree by the simultaneous reduction in risk 

from hitting rocks during lower flow conditions. Once again, a formal study to evaluate 

safety risks associated with different flows is currently lacking and is clearly warranted in 

the future. 

 Non-GCD Actions - Mechanical removal of trout was the only type of nonflow action 

considered in the assessment. Several workshop participants felt that risks to visitor 

safety increased due to the use of electricity in the water near shorelines and camps, 

however, other workshop participants argued that this risk was being mitigated through 

public outreach and education, and therefore, recreational safety was not relevant in the 

evaluation of this activity. To date, no injury incidents associated with electro-fishing in 

the CRE have been reported. 
53

3.5.6 Recreational Experience 

The recreational experience category proved difficult to evaluate for several reasons. First 

and most importantly, there is no universally agreed upon definition of what this visitor 

experience variable encompasses. Secondarily, the concept of recreational experience is 

multidimensional, i.e., recreational experiences in the CRE are framed by distinct but 

interrelated physical, place-based attributes (beaches, scenery, local environmental 

conditions) as well as an array of different recreational activities (camping, rafting, 

angling, wilderness exploration, social interactions with others, etc.) making a single 

evaluation in relation to flows problematic (i.e., a given flow regime may benefit one 

aspect of the overall experience while simultaneously causing negative effects to others.) 

Furthermore, although we have ample evidence to demonstrate that flows are important 

influences—both directly and indirectly—on the attributes considered important for a 

high quality visitor experience (Bishop and others, 1987; Shelby and others, 1992; 

Stewart and others, 2000), we lack a solid understanding of the relative importance of 

flows compared with other potential experiential “drivers” such as visitor use levels, 

scenery, weather, quality of social encounters, and so forth. The workshop participants 

felt that the GCDAMP should undertake some basic, foundational studies to do this 

assessment exercise properly. Nevertheless, we attempted to evaluate how various 

regimes might affect recreational experience, as follows: 

Water Temperature - Studies documenting the relationship between water temperature 

and recreational experience are lacking, but anecdotal information, plus some social 

science survey data, indicate that warmer water would likely improve the boating and 

angling experience because warmer water would allow visitors to enjoy being in the 

water and getting soaked in the rapids. Furthermore, warmer water would expand the 

range of river-based recreational opportunities, as swimming in the river is currently 

discouraged by most commercial outfitters due in part to the coldness of the water and 

associated risks of hypothermia. Anecdotal information suggests that most visitors 

currently manage to enjoy their recreation experience in spite of the very cold water 

temperatures. The Stewart and others (2000) study found that a small majority of 

commercial passengers (53%) did not believe that their trip would have been any more 

enjoyable with warmer water, but a slightly larger majority of private boaters and river 

commercial guides shared the opposite opinion (i.e., 57% of private boaters and 58% of 

commercial guides felt that warmer water would have improved their overall experience.) 

The trade offs associated with different aspects of visitor experience affected by warmer 

water (i.e., improved “swimmability” vs. possible increase in viral or bacterial infections) 

have never been studied and remain unknown. 

Increased Daily Fluctuations - Visitors’ perceptions about fluctuating flows have been 

documented to a limited degree in studies by Bishop and others (1987) and Stewart and 

others (2000). Both studies concluded that most rafters preferred stable, predictable flows 

over fluctuating flows. Both the Bishop and Stewart studies also demonstrated that, 

contrary to the opinions of many members of the Lees Ferry fishing guide community, 

anglers generally preferred stable flows over highly fluctuating flows (Bishop and others, 
54

1987, p.132–133; Stewart and others 2000, p.49). This opinion held even when anglers 

were reminded that higher fluctuations could potentially improve the amount of drift in 

the water and thereby might increase the quality of the fishing experience (Bishop and 

others, 1987, p.126). 

 

Reduced Variation in Monthly Volume - The effects of changes in monthly volumes on 

recreational experience are unknown. 

BHBFs and HMFs - Several studies by the NPS have evaluated the effects of high flows 

at or above powerplant capacity on visitor safety, an important component of visitor 

experience. Studies by Bishop and others (1987) and Stewart and others (2000) 

specifically evaluated the effects of different flows on recreational experience, and these 

studies concluded that for both boaters and anglers, moderate flows of 10,000 to 20,000 

cfs were preferred over higher flowers; however, Jalbert (1996) found that most boaters 

who were interviewed during the 1996 BHBF were excited to be running on the high 

experimental flows, and some deliberately scheduled their trip to maximize time spent on 

the high flow. The results of the Jalbert study (which was not subject to independent peer 

review), plus additional anecdotal information from river guides, indicates that flows 

over 40,000 cfs are perceived as both positive and negative by different user groups, with 

boaters who have more experience generally having a more positive perception of the 

high flow experience than less experienced boaters. This issue warrants a future focused 

study. 

Sustained Low Steady Flow – Jonas and Stewart’s (2002) study on the 2000 low summer 

steady flow experiment examined effects to various attributes of the recreational 

experience (perceptions of safety, campsite quality, crowding and intergroup encounter 

rates, etc.). Jonas and Stewart (2002, p.28) concluded that there was relatively little 

impact to visitor experience at sustained flows of 8,000 cfs, although this conclusion 

might change if flows were significantly lower than 8,000 cfs. Several participants in the 

knowledge assessment workshop (river guides) felt that low flows detracted from the 

recreational experience because boaters had to spend considerably more time on the 

water, and therefore they had less time to spend exploring side canyons or enjoying camp 

life. Also, there was an increased likelihood of having equipment damage or injury 

(discussed in previous safety section) that further detracted from the visitor experience. 

However, it was also noted that the improvement in size and availability of campsites at 

low flows countered the negative attributes of low flows to some unknown extent. 

Workshop participants felt that a study involving a trade-off analysis was needed to 

properly evaluate the effects of sustained low flows on recreational experience. 

Sustained High Flows - Both Bishop and others (1987) and Stewart and others (2000) 

concluded that rafters preferred moderately high sustained flows (16,000–33,000 cfs in 

the Bishop study, 20,000–25,000 cfs in the Stewart study) over fluctuating flows, low 

flows (<10,000 cfs), or very high flows (>33,000 cfs). However, they also expressed a 

general preference for moderate to moderately low flows (10,000–20,000 cfs) over higher 

flows. These studies did not examine the potential negative trade offs to visitor 

experience that could accompany sustained high flows (i.e., smaller and less numerous 
55

camping opportunities, increased competition for camp sites, etc.) Once again, the 

workshop participants felt that a study involving a trade-off analysis was needed to 

properly evaluate the effects of sustained high flows on recreational experience. 

Non-GCD Actions - The effects of mechanical trout removal (or mechanical removal of 

exotic species in general) on visitor experience is a subject of continuing speculation, but 

one for which no actual data is currently available. 

Summary of Recreation Matrix Elements 

Effects of flows on recreation are multidimensional, but this topic has received relatively 

little attention from social science researchers in the past 25 years. Studies evaluating the 

trade offs between attributes that are important to maintaining a high quality recreational 

experience are very limited, therefore these multidimensional effects are difficult to 

quantify. Nonetheless, available data on flow impacts to single attribute components are 

fairly robust in some cases (such as campable area) and there is abundant anecdotal 

information related to others, allowing for a general evaluation of flow effects to 

recreation. The available data suggest that flows that conserve sediment and allow for its 

redeposition and maintenance at higher elevations benefit the recreational experience, as 

do flows that improve navigability and safety. Warmer, moderately low (8,000–15,000 

cfs) and steadier flows are likely to benefit recreation in many (but not all) respects. 

Conversely, higher fluctuating flows with higher daily ranges, lower lows, or higher 

peaks are not considered beneficial to the overall recreational experience. Warmer 

temperatures are generally believe to improve the recreational experience, both in terms 

of enjoyment and safety, but there may be negative consequences from warmer, steadier 

flow in relation to water quality and human health issues. All of these assessments 

warrant further evaluation and rigorous testing through future focused research efforts. 

The strategic questions that resulted from the workshop focus on the key areas of 

uncertainty that remain to be resolved through future experimentation and research. 

3.6 Cultural Resources (National Register Historic Properties) 

The historic properties performance measure represents the conditions that support the 

preservation of National Register eligible properties within the CRE. In general, the 

results of this analysis track closely with the sediment matrix. This is because the 

conditions that promote systemwide sediment retention are also thought to benefit 

historic properties, specifically archaeological sites, by helping to preserve the 

sedimentary matrices that bury, and to some degree, stabilize, these actively 

deteriorating, non-renewable resources. Specific quantified studies tracking rates of 

change to historic properties under varying flow regimes are currently lacking; therefore, 

the matrix evaluations were mostly coded yellow to reflect reliance on anecdotal 

knowledge as well as knowledge based on interpretation of general conceptual models 

about how the terrestrial system operates in physical and biological realms under current 

and experimental flow scenarios. 

The knowledge assessment evaluated likely outcomes for various flow parameters in two 

separate settings: above the 45,000 cfs flow level and below the 45,000 cfs flow level 
56

(Table 3.7). In the end, the assessments were identical for both settings in all but one 

instance (sustained low steady flows.) 

Periodic re-deposition of sand at elevations above powerplant capacity are considered to 

be an essential component of future management actions designed to maintain or improve 

the condition of archaeological sites in the CRE, but the long term effects of conducting 

periodic, sediment-enriched high flows on cultural resource sites remains largely 

untested. 

Actions required for maintaining the integrity of traditional cultural properties (TCPs) in 

the CRE, other than archaeological sites, are largely unknown at this time, due to the lack 

of clear TCP definition in the CRE. Mechanical removal of trout is the only proposed 

management activity for which there is any degree of certainty about its potential effect 

on a known tribally valued TCP (the mouth of the Little Colorado River). 

Water Temperature - Warmer temperatures decrease water viscosity, allowing fine 

sediment to settle out faster (Lane and others, 1949; Hubbell and Ali, 1961; ASCE, 

1975), thereby reducing the suspension of sediment in the water column and the amount 

of sediment available for downstream transport. In theory, warmer water should therefore 

reduce the amount of sediment transported from the system (potentially by as much as 

5%, according to S. Wright, personal comm. 2006) and increase the longevity of sandbars 

and sandy deposits along the shoreline. With larger sandbars, there is more sand 

potentially available for downwind transport to terrestrial settings, where archaeological 

sites and other terrestrial resources may ultimately benefit from the addition of wind 

transported sediment. 

Increased Daily Fluctuations - The effects of increased daily fluctuations on sandbars is 

reasonably well known from survey data collected during the 1980s (Schmidt and Graf 

1990) and early 1990s (Beus and Avery 1992), as well as from historical and recently 

compiled sediment transport data (Topping and others, 2000a, 2000b, 2003; Hazel and 

others, 2006; Topping and others, 2006). Increased fluctuations would result in higher 

sediment transport rates, reducing the overall sediment supply in the CRE. Also, 

increased fluctuations prevent sandbars from drying out, thereby decreasing opportunities 

for the sandbars to serve as sources for fine sediment to be picked up, transported and redeposited by wind into areas where archaeological sites are located. 

Reduced Variation in Monthly Volume - The effect of changes in monthly volumes on 

historic properties is unknown. Decreases in monthly volumes during the summer highuse season would certainly increase the exposed sandbar area, making more sediment 

available for transport by wind to higher elevations within the CRE; however, since the 

distribution of monthly volumes was not specified for the cultural matrix, the effects of 

monthly volume changes on historic properties could not be gauged. 

BHBFs - The effects of high flows above powerplant capacity on sandbars and high 

elevation sediment supply is relatively well documented (Webb and others, 1999; 

Topping and others, 2006). Various short term studies to evaluate the effects of the 1996 
57

experimental flood on historic properties within the CRE documented either no effects, 

no adverse effects, or potentially beneficial effects from the BHBF (Balsom and Larralde, 

1996). 

BHBFs appear to be the only mechanism available for restoring large volumes of sand to 

elevations above the highest level of normal dam operations. BHBFs have been shown to 

increase higher elevation sandbar areas systemwide, at least on a temporary basis (Hazel 

and others, 1999; Kaplinski and others, 2005b). BHBFs have also been demonstrated to 

deposit sand in arroyo mouths up to the highest flow level (Yeatts, 1997; Hazel and 

others, 2000), thereby temporarily reducing the gradient of arroyos and potentially 

slowing rates of down cutting in the lower reaches of the drainage. More focused studies 

are needed to evaluate the long-term effects of BHBFs on cultural resources in the CRE. 

HMFs - HMFs under sediment depleted conditions have been shown to not appreciably 

increase sandbar area or channel margin deposits (Kaplinski and others, 2005b; Hazel 

and others, in press) while increasing sediment transport rates relative to MLFF (Wiele 

and others, 2002; Hazel and others, in press); hence HMFs were considered to be 

potentially detrimental to historic properties. 

Sustained Low Steady Flow - Sustained low steady flows at or below 9,000–10,000 cfs 

constrain the amount of sediment transported out of the system to negligible amounts 

(Topping and others, 2000a; 2000b; Rubin and others, 2002; Topping and others, 2003), 

therefore low sustained flows would allow sediment to accumulate in the channel and, 

after a period of adjustment to the lower flow regime, would reduce the rate of sediment 

loss from sandbar erosion. In addition, sustained low flows would expose more dry sand 

at the higher elevations of bars, increasing the amount of sand available for potential 

transport by wind. On the other hand, sustained low flows without occasional higher 

flows (under sediment enriched conditions) would be likely to eventually result in 

systemwide reduction of sandbar area at higher elevations (Beus and Avery, 1992, 

Chapter 6, p.49) and a concomitant increase in bar area and volume at lower elevations; 

this would potentially reduce the amount of dry sand available for aeolian transport over 

time. Also, low steady flows would permit tamarisk and other vegetation to encroach on 

sandbars, eventually reducing the sand available for wind transport over time. We 

currently lack measured information that would allow us to quantify rates of vegetation 

encroachment under low flows (or any other flows, for that matter), so the relative 

importance of vegetation encroachment in reducing aeolian transport is uncertain. 

Sustained High Flows - Sustained higher flows are certain to increase the rate and volume 

of sediment transported out of the system (Rubin and others, 2002; Topping and others, 

2003); therefore sustained high flows would increase the rate of sediment loss from 

sandbars and ultimately diminish the overall sand supply in the CRE. Furthermore, 

sustained high flows would consistently inundate lower elevation sandbars and channel 

margin deposits, resulting in a sustained loss in sand available for wind transport up to 

and slightly above the level of the highest flows. 
58

Non-GCD Actions - Mechanical removal of trout was the only type of mechanical 

removal considered in the matrix development. Obviously, trout removal would not 

affect the physical integrity of archaeological sites directly, although there is some 

(slight) potential of impacts from on-shore activities associated with the trout removal 

project. 

The killing of trout and other life forms in the vicinity of the mouth of the Little Colorado 

River has been identified by several Native American tribes as an issue of concern and a 

potentially detrimental impact to traditional cultural property values associated with this 

area. Therefore, the matrix shows a negative impact to TCPs from this activity. This is 

the only matrix variable for which an assessment of knowledge about potential benefits 

or negative impacts to TCPs could be made with a fair degree of confidence. 

We did not consider the option of mechanical removal of terrestrial exotics (such as 

tamarisk). Mechanical removal of tamarisk thickets or other exotics such as tumbleweed 

and camel thorn could hypothetically increase sediment transport from lower to higher 

elevations within the CRE, but the practicality and sustainability of applying such an 

approach with the goal of increasing aeolian sand transport is untested and therefore 

unknown at this time. 

Summary of Cultural Resource Portion of the Terrestrial Matrix 

Effects of flows on cultural resources are poorly understood, despite many years of 

monitoring cultural resource condition in the CRE. This is because, with few exceptions, 

cultural resource condition has not been monitored in direct relation to flow variables. 

Nonetheless, because archaeological site integrity is closely linked to the stability of the 

sediment matrix in which most archaeological sites and other types of cultural resources 

are found, and because many cultural resources are embedded in sediments deposited by 

or derived from the Colorado River, flows that conserve sediment throughout the 

ecosystem and allow for its redeposition and maintenance at higher elevations are 

considered to be most beneficial for maintaining or improving cultural resource 

condition. Thus, the assessment of flow effects tracks closely with those in the sediment 

matrix, even though these assessments were developed independently by different groups 

of participants during the Knowledge Assessment Workshop. The hypothesis that flows 

benefiting high elevation sediment retention will also benefit cultural resources warrants 

further testing and refinement through future focused research, experimentation, and 

monitoring. The science questions that resulted from the K knowledge Assessment 

workshop reflect the key areas of uncertainty that remain to resolved in order to improve 

future understanding of how dam controlled flows effect cultural resources. 
59

4.0 Science Questions (revised after comment by the Science 

Advisors) 

The key uncertainties highlighted in the knowledge assessment matrices were helpful in 

framing key scientific questions that need to be resolved. There was not sufficient time at 

the workshop to develop a complete list of questions for each sub-model. What follows is 

a revised list of strategic science questions developed at the workshop as well as others 

developed by the authors following the meetings. These questions were reviewed by the 

Science Advisors at the request of the GCMRC and the questions were subsequently 

revised in response to those comments and suggested revisions. Following the 2005 

Knowledge Assessment workshop, those science questions determined by the GCMRC 

and Science Planning Group were incorporated into the GCMRC’s 2007–11 Strategic 

Science and Monitoring and Research Plans. 

The critical experimental section of the Monitoring and Research Plan is still in 

development and the information in this report provides the basis for current knowledge 

with respect to evaluations of resource responses under three experimental design 

options. A recommendation in support of one of the options is anticipated in fall 2006, on 

the basis of resource evaluations for each of the options, including economic implications 

to hydropower resources. 

Both the Strategic and Monitoring and Research Plans provide the basis for development 

of the GCMRC’s FY2007 annual work plan, as well as future work plans, such as the 

proposed Biennial FY 2008–9 work plan that is currently in development. 

4.1 Physical Resources (fine sediment and downstream quality of water) 

Fine Sediment (sandbars and related habitats):

1) Is there a “Flow-Only” operation (i.e. a strategy for dam releases, including managing 

tributary inputs with BHBFs, without sediment augmentation) that will restore and 

maintain sandbar habitats over decadal time scales? 

Several related, but subordinate questions related to fine sediment were also identified 

by participating sediment researchers during the workshop: 

• What are the short-term responses of sandbars to BHBFs? 

• What is the rate of change in eddy storage (erosion) during time intervals between 

BHBFs? 

• How does the grain-size distribution of the deposits affect sandbar stability? Main 

channel turbidity? 

• What are the effects of ramping rates on sediment transport and sandbar stability? 

• Can we develop a relationship between suspended sediment concentration and 

turbidity to support fisheries research? 

At the summer 2005 Knowledge Assessment workshop, sediment researchers 

recommended that the first priority strategic science question for sediment, be addressed 

by replication of the 2004 High Flow test, with respect to the level of sand enrichment for 
60

the replicate being equal or perhaps greater than antecedent sand supply conditions before 

that test. The schematic diagram below, shows the proposed strategy for an experimental 

approach to answering the sediment question. 

The experimental approach to answering the fine sediment question might also be 

supported with sediment and flow models for identifying an optimal prescription for 

beach/habitat-building flows if replication of the sand enriched High Flow test results in 

net positive response in sandbar resources. 

Downstream Quality of Water:

2) How do dam release temperatures, flows (average and fluctuating component), 

meteorology, canyon orientation and geometry, and reach morphology interact to 

determine mainstem and nearshore water temperatures throughout the Colorado River 

ecosystem (CRE)? 

At the 2005 workshop, physical scientists also suggested that addressing the 

priority quality of water question was best undertaken through a modeling development 

research phase. Existing and future temperature monitoring data for the main channel and 

selected nearshore areas of the river was also identified as a critical element of the model 

development and support activity. Downstream temperature modeling was initially 
61

undertaken by the Bureau of Reclamation before the 2005 workshop and following the 

workshop in January 2006, a second phase of modeling began, with renewed emphasis on 

development of an approach that might allow for prediction of nearshore temperatures 

throughout the river ecosystem using existing channel geometry data. 

4.2 Hydropower 

1) What are the hydropower replacements costs of the MLFF (annually, since 1996)? 

2) What are the projected costs associated with the various alternative flow regimes 

being discussed for future experimental science (as defined in the next phase 

experimental design)? 

Following the 2005 workshop, experimental planning conducted by the Science Planning 

Group in cooperation with the GCMRC sought approval from the Technical Work Group 

to conduct an economic assessment of three experimental options being considered for 

implementation in FY2008 and beyond. This experimental support activity represents a 

new era for the GCMRC in attempting to evaluate a wider range of resource influence 

tied to future experimental designs. 

During external review of the draft Knowledge Assessment report and strategic 

questions, reviewers suggested that the Hydropower questions be reconsidered as part of 

a broader strategy for potential tradeoff analyses in support of assessing experimental 

policies: 

Science Advisors Recommended Question: “What are the most appropriate resource 

values and tradeoff methods for managers to use in evaluating the composite resource 

changes that occur under differing flow and nonflow treatments?” 

4.3 Food Base, Fish, and Lees Ferry Angling 

4.3.1 Food Base 

1) What are the important pathways, and the rate of flux along them, that link lower 

trophic levels with fish? 

2) How is invertebrate flux affected by water quality (e.g., temperature, nutrient 

concentrations, turbidity) and dam operations? 

3) Are trends in the abundance of fish populations, or indicators from fish such as 

growth, condition, and body composition (e.g., lipids), correlated with patterns in 

invertebrate flux? 

4.3.2 Native Fish 

1) To what extent are adult populations of native fish controlled by production of young 

fish from tributaries, spawning and incubation in the mainstem, survival of YoY and 

juvenile stages in the mainstem, or by changes in growth and maturation in the adult 

population as influenced by mainstem conditions? 
62

2) To what extent does temperature and fluctuations in flow limit spawning and 

incubation success for native fish? 

3) What is the relative importance of increased water temperature, shoreline stability, 

and food availability on the survival and growth of YoY and juvenile native fish? 

4) How important are backwaters and vegetated shoreline habitats to the overall growth 

and survival of YoY and juvenile native fish? Does the long-term benefit of 

increasing these habitats outweigh short-term potential costs (displacement and 

possibly mortality) associated with high flows? 

5) Will increased water temperatures increase the incidence of Asian Tapeworm in 

humpback chub or the magnitude of infestation, and if so, what is the impact on 

survival and growth rates? 

6) Do the potential benefits of improved rearing habitat (warmer, more stable, more 

backwater and vegetated shorelines, more food) outweigh negative impacts due to 

increases in nonnative fish abundance? To what extent could predation impacts by 

nonnative fish be mitigated by higher turbidities? 

Following review of the questions developed at the KAW, above, and discussions with 

TWG members, the following two questions were appended to the KAW list so that other 

pressing, relevant questions were considered: 

7) Which tributary and mainstem habitats are most important to native fishes and how 

can these habitats best be made useable and maintained? 

8) How can native and nonnative fishes best be monitored while minimizing impacts 

from capture and handling or sampling? 

Following review of earlier drafts of this document, the GCMRC Science Advisors 

proposed a synthetic approach to determining important questions to pursue in regards to 

native fishes in Grand Canyon. Their two questions are as follows: 

SA HBC 1: What are the most limiting factors to successful HBC adult recruitment in the 

mainstem: spawning success, predation on YoY and juveniles, habitat (water, 

temperature), pathogens, adult maturation, food availability, competition? 

SA HBC 2: What are the most probable positive and negative impacts of warming the 

Colorado River on HBC adults and juveniles? 

4.3.3 Rainbow Trout in Glen Canyon 

1) To what extent is the adult population of rainbow trout controlled by survival rates 

during incubation and YoY/juvenile rearing stages, or by changes in growth and 

maturation in the adult population influencing egg deposition? 

2) To what extent is the size of rainbow trout in Glen Canyon controlled by density and 

food availability? 

3) Does increased water temperature result in the occurrence of whirling disease in 

rainbow trout and if so, what affect will this have on population size and adult growth 

and condition? 

4) Do rainbow trout immigrate from Glen to Marble and eastern Grand Canyons, and if 

so, during what life stages? To what extent to Glen Canyon immigrants support the 

population in Marble and eastern Grand Canyons? 
63

4.3.4 NonNative Fish in Marble and Eastern Grand Canyons 

1) Does a decrease in the abundance of rainbow trout and other coldwater and 

warmwater nonnatives in Marble and eastern Grand Canyons result in an 

improvement in the recruitment rate of juvenile humpback chub to the adult 

population? 

2) Will a limited number of years of mechanical removal of rainbow trout in Marble and 

eastern Grand Canyons result in a long-term decrease in abundance or will recolonization from tributaries and from below and above the removal reach require 

that mechanical removal be an ongoing management action? This question also 

applies to future removal programs targeting other nonnative species. 

4.3.5 Lees Ferry Angling 

1) Assuming a trade-off between trout density and size, what is the preferred 

combination for anglers? 

2) What GCD flow constraints (ramping rates, daily flow range, etc.) maximize fishing 

opportunities and catchability? 

4.4 Riparian Habitat 

1) How do processes occurring at a variety of spatial scales (i.e., population level to 

community to landscape scales) interface to influence riparian habitat? 

2) What is the nature and timing of terrestrial — aquatic linkages and what is their 

influence on the recipient habitat? 

3) How do terrestrial habitat and cultural/recreation resources interface? 

i. What are the rates of vegetation encroachment (trees vs. shrubs) on 

camp sites? 

4) How do flows, including the absence of flows (e.g., predam high water zone), affect 

productivity and decomposition rates of riparian vegetation including the absence of 

flows (e.g., OHWZ)? 

5) How do warmer releases affect viability and productivity of native/nonnative 

vegetation? 

6) To what extent and in what respects can BHBF’s (magnitude and frequency) achieve 

reduction of exotic species? 

7) How could monthly volumes be changed to beneficially affect riparian habitat? 

4.5 Recreation 

1) How do dam controlled flows affect visitors’ recreational experiences, and what is/are 

the optimal flows for maintaining a high quality recreational experience in the CRE? 

2) What are the drivers for recreational experience in the CRE, and how important are 

flows relative to other drivers in shaping recreational experience outcomes? 

3) How do varying flows positively or negatively affect campsite attributes that are 

important to visitor experience? 
64

4) What are the minimum size, quantity, distribution and quality of campsites to meet 

NPS goals for visitor experience? 

5) Can changes in quality of recreational experience be quantified for single event 

opportunities (e.g., white water rafting, angling, and camping) vs. multi-opportunity 

experiences (e.g. white water rafting with overnight camping)? 

6) How can safety & navigability be reliably measured relative to flows? 

7) How do varying flows positively or negatively affect visitor safety, health, and 

navigability of the rapids? 

8) How do varying flows positively or negatively affect group encounter rates, campsite 

competition, and other social parameters that are known to be important variables of 

visitor experience? 

Following a review of an earlier draft of this document, the GCDAMP Science Advisors 

proposed a synthetic approach to determining important science questions for GCMRC to 

pursue in regards to recreation in Grand Canyon. They recommended that the above 

questions be distilled and subsumed within two broad questions, as follows: 

R1.0 What are the drivers for recreational experience in the CRE, and how important 

are flows and campsite beaches relative to other drivers in shaping this experience? 

R2.0 How do dam controlled flows affect visitors’ recreational experience, and what 

are optimal flows for maintaining a high quality recreational experience? 

4.6 Cultural Resources 

1) Do dam controlled flows affect (increase or decrease) rates of erosion and vegetation 

growth at arch sites and Traditional Cultural Property (TCP) sites, and if so, how? 

2) How do flows impact the sedimentary matrix of the higher terrace deposits, and what 

kinds of important historical/legacy information about the CRE ecosystem is being 

lost due to ongoing erosion of these older Holocene sedimentary deposits? 

3) If dam controlled flows are contributing to (influencing rates of) arch site/TCP 

erosion, what are the optimal flows for minimizing future impacts to historic 

properties? 

4) How effective are check dams in slowing rates of erosion at archaeological sites over 

the long term? 

5) What are the TCPs in the CRE, and where are they located? 

6) How can tribal values/data/analyses be appropriately incorporated into a western 

science-driven adaptive management process in order to evaluate the effects of flow 

operations and management actions on TCPs? 

7) Are dam controlled flows affecting TCPs and other tribally-valued resources in the 

CRE, and if so, in what respects are they being affected, and are those effects 

considered positive or negative by the tribes who value these resources? 
65

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Yard, M.D., Bennett, G.E., Mietz, S.N., Coggins, L.G., Jr., Stevens, L.E., Hueftle, S., and 

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

estimates for the Colorado River, Grand Canyon, AZ. Ecological Modeling, 183 

(2005) 157–172. 

Yard, M.D., 2003, Light availability and aquatic primary production: Colorado River, 

Glen and Grand Canyons, AZ: Flagstaff, Northern Arizona University, Ph.D. 

Yeatts, Michael. 1997. High Elevation Sand Retention Following the 1996 Spike Flow. 

Hopi Cultural Preservation Office. Report prepared for Grand Canyon Monitoring 

and Research Center. Dated December 1997. 15 Pages. Electronic Access: 

http://www.gcmrc.gov/library/reports/cultural/Hopi/Yeatts1997.pdf

Young, J.A., and Clements, C.D., 2003. Seed germination of willow species from a desert 

riparian system. J. Range Manage. 56:496–500. 

Young, J.A., Clements, C.D. and Harmon, D. 2004. Germination of seeds of Tamarix 

ramosissima. J. Range Manage. 57:475–481. 
79

Table 3.1. Knowledge assessment matrix for the physical resources sub-model. ‘+’, ‘-‘, and ‘0’ indicate positive, negative, and no effec

yellow) or unknown (if color is red) responses. Dark green, light green, yellow, and red denote increasing uncertainty in the predicted rTable 2.1 for definitions). A positive response of a performance measure corresponds with the direction of AMP goals. 

Performance Measure 

Increase in 

GCD Release 

Water 

Temperature 

Overall 

Effect of 

Increased 

Fluctuations 

Relative to 

MLFF 

Reduce 

Variation 

in 

Monthly 

Volume 

BHBF 

with 

adequate 

sand 

supply 

BHBF 

without 

adequate 

sand 

supply 

HMF with 

adequate 

sand 

supply 

HMF 

without 

adequate 

sand 

supply 

Sustained 

Low 

Steady 

Flow 

(summerfall) 

Hig

Sustai

Flow 

(pondi

sprin

Fine- Sediment above 25 kcfs + - + + 0/+ 0 0 + - Fine-Sediment between 8 and 25 kcfs + - + + - 0/+ - + - Fine-Sediment below 8 kcfs + - + + - - - + - Mainstem Water Temperature + - + - - - - + - Nearshore Water Temperature + - + - - - - + - 
80

Table 3.2. Knowledge assessment matrix for the power sub-model. ‘+’, ‘-‘, and ‘0’ indicate positive, negative, and no effect (if color is unknown (if color is red) responses. Dark green, light green, yellow, and red denote increasing uncertainty in the predicted response dirdefinitions). A positive response of a performance measure corresponds with the direction of AMP goals. Performance Measure 

Increase in 

GCD Release 

Water 

Temperature 

Overall 

Effect of 

Increased 

Fluctuations 

Relative to 

MLFF 

Reduce 

Variation 

in 

Monthly 

Volume 

BHBF 

with 

adequate 

sand 

supply 

BHBF 

without 

adequate 

sand 

supply 

HMF with 

adequate 

sand 

supply 

HMF 

without 

adequate 

sand 

supply 

Sustained 

Low 

Steady 

Flow 

(summerfall) 

Hig

Susta

Flo

(pond

sprin

Hydro Power LoadFollowing Capacity - + - - - - - - - Hydro Power Replacement Costs - + - - - - - - - Note: Although information about hydro power capacity and replacement power cost impacts are known by some stakeholders (and as known with high certainty in the above matrix (highlighted as GREEN in all cells), this information was not readily accessible for use dParticipants identified that improved approaches for assessing economic resources and data sharing were needed in the future to supporthe KAW. Such activities were initiated by the Science Planning Group and Technical Work Group in summer 2006.
81

Table 3.3. Knowledge assessment matrix for food base and fish sub models. ‘+’, ‘-‘, and ‘0’ indicate positive, negative, and no effect (i

or unknown (if color is red) responses. Dark green, light green, yellow, and red denote increasing uncertainty in the predicted response 

for definitions). A positive response of a performance measure corresponds with the direction of AMP goals with the exception of perfoinvasive fish species, disease, and rainbow trout abundance in Marble Canyon.

Performance Measure 

Location and/or Species 

Increase in 

GCD 

Release 

Water 

Temp. 

Overall 

Effect of 

Increased 

Fluctuations 

Relative to 

MLFF 

Reduce 

Variation 

in 

Monthly 

Volume 

BHBF 

with 

adequate 

sand 

supply 

BHBF 

without 

adequate 

sand 

supply 

HMF 

with 

adequate 

sand 

supply 

HMF 

without 

adequate 

sand 

supply 

Sustained 

Low 

Steady 

Flow 

(summerfall) 

High 

Sustained 

Flow 

(pondingspring) 

Food base Glen 

+

+ - + 

Grand + - - + 

Mainstem spawning & incubation 

HUMPBACK CHUB + + FMS + - + RBT-Glen 0 - + + + RBT-Marble 0 - + + 

YOY/Juvenile nearshore rearing 

HUMPBACK CHUB + - + - - + FMS + - + - - + RBT-Glen + - + - - - - + + RBT-Marble - + - - - - + + 
82

Table 3.3. Con’t. Performance Measure 

Location or Species 

Increase in 

GCD 

Release 

Water 

Temp. 

Overall 

Effect of 

Increased 

Fluctuations 

Relative to 

MLFF 

Reduce 

Variation 

in 

Monthly 

Volume 

BHBF 

with 

adequate 

sand 

supply 

BHBF 

without 

adequate 

sand 

supply 

HMF 

with 

adequate 

sand 

supply 

HMF 

without 

adequate 

sand 

supply 

Sustained 

Low 

Steady 

Flow 

(summerfall) 

High 

Sustained 

Flow 

(pondingspring) 

Invasive Fish Species Coldwater + - 0 0 0 0 + 

Warmwater + - + 

Disease 

Asian Fish Tapeworm + Whirling Disease + 

Adult Population 

HUMPBACK CHUB FMS + + RBT #s - Glen + - 0 0 0 0 + 

 RBT Size - Glen + + 0 0 0 0 +  RBT #s- Marble - 0 0 0 0 + Angling Opportunity and Quality Glen + - + - - - - - - 
83

Table 3.4. Detail of food base and fish sub models KAW matrix describing elements of ROD flow constraints that contribute to the ovefluctuations in daily flow. Also shown are responses to humpback chub translocation and hatchery supplementation options. ‘+’, ‘-‘, an

negative, and no effect (if color is green or yellow) or unknown (if color is red) responses. Dark green, light green, yellow, and red denin the predicted response direction (see Table 2.1 for definitions). A positive response of a performance measure corresponds with the dwith the exception of performance measures for invasive fish species, disease, and rainbow trout abundance in Marble Canyon. 

Performance Measure 

Location or Species 

Overall 

Effect of 

Increased 

Fluctuations 

Relative to 

MLFF 

Increase 

Upramp 

Rate 

Increase 

Downramp 

Rate 

Increase 

and/or 

Lengthen 

Maximum 

Daily Flow 

Increase 

Daily 

Variation 

in Flow 

Decrease 

Minimum 

Daily 

Flow 

Supplementatiofrom Hatchery 

Food base Glen 

Grand - - - 

Mainstem spawning & incubation HBC 

FMS - - RBTGlen - - - - RBTMarble 0 - - - 

YOY/Juvenile nearshore rearing HBC - - + 

FMS - - + RBTGlen - - RBTMarble - - 
84

Table 3.4. Con’t. 

Performance Measure 

Location or Species 

Overall 

Effect of 

Increased 

Fluctuations 

Relative to 

MLFF 

Increase 

Upramp 

Rate 

Increase 

Downramp 

Rate 

Increase 

and/or 

Lengthen 

Maximum 

Daily Flow 

Increase 

Daily 

Variation 

in Flow 

Decrease 

Minimum 

Daily 

Flow 

Supplementatiofrom Hatchery 

Invasive Fish Species Coldwater - - + 

Warmwater - - + 

Disease 

Asian Tapeworm Whirling Disease 

Adult Population HBC 

FMS RBT #s - Glen - - - RBT Size - Glen + + + RBT #sMarble - - - 

Angling Opportunity and Quality Glen - - - - - 
85

Table 3.5. Knowledge assessment matrix for the riparian habitat sub-model. ‘+’, ‘-‘, and ‘0’ indicate positive, ne

(if color is green or yellow) or unknown (if color is red) responses. Dark green, light green, yellow, and red deno

uncertainty in the predicted response direction (see Table 2.1 for definitions). A positive response of a performancorresponds with the direction of AMP goals. 

Performance Measure

Location and/or Species

Increase 

in GCD 

Release 

Water 

Temp.

Overall 

Effect of 

Increased 

Fluctuations 

Relative to 

MLFFA

Reduce 

Variation 

in 

Monthly 

Volume

BHBF 

with 

adequate 

sand 

supply

BHBF 

without 

adequate 

sand 

supply

HMF 

with 

adequate 

sand 

supply

HMF 

without 

adequate 

sand 

supply

Sustained 

Low 

Steady 

Flow 

(summerfall)

High 

Sustaine

Flow 

(ponding

spring)

Fluctuation zone <25 kcfs

Glen & Grand Canyon + + 0/+ 0/+ 0/+ - +

Lower Riparian 25-45 kcfs + + 0/+ 0/+ 0/+ 0/- 0/+Upper riparian zone 45-60 cfs> 60 kcfs
86

Table 3.6. Knowledge assessment matrix for the recreation sub-model. ‘+’, ‘-‘, and ‘0’ indicate positive, negativ

color is green or yellow) or unknown (if color is red) responses. Dark green, light green, yellow, and red denote i

in the predicted response direction (see Table 2.1 for definitions). A positive response of a performance measure 

direction of AMP goals. 

Performance Measure 

Location and/or Species 

Increase 

in GCD 

Release 

Water 

Temp. 

Overall 

Effect of 

Increased 

Fluctuations 

Relative to 

MLFF 

Reduce 

Variation 

in 

Monthly 

Volume 

BHBF 

with 

adequate 

sand 

supply 

BHBF 

without 

adequate 

sand 

supply 

HMF 

with 

adequate 

sand 

supply 

HMF 

without 

adequate 

sand 

supply 

Sustained 

Low 

Steady 

Flow 

(summerfall) 

High 

Sustained 

Flow 

(pondingspring) 

Campsites (available campable area) 

+ - 

+ + + - + - 

Access to attraction sites 0 - 0 0 0 0 + 0 Rafting navigability + + 0 0 0/- 0/+ qw and human health - + + + + + - + safety + + - Recreational experience + - 0/- + 
87

Table 3.7. Knowledge assessment matrix for the cultural sub-model. ‘+’, ‘-‘, and ‘0’ indicate positive, negative, is green or yellow) or unknown (if color is red) responses. Dark green, light green, yellow, and red denote increapredicted response direction (see Table 2.1 for definitions). A positive response of a performance measure corresdirection of AMP goals. Performance Measure 

Location and/or Species 

Increase 

in GCD 

Release 

Water 

Temp. 

Overall 

Effect of 

Increased 

Fluctuations 

Relative to 

MLFF 

Reduce 

Variation 

in 

Monthly 

Volume 

BHBF 

with 

adequate 

sand 

supply 

BHBF 

without 

adequate 

sand 

supply 

HMF 

with 

adequate 

sand 

supply 

HMF 

without 

adequate 

sand 

supply 

Sustained 

Low 

Steady 

Flow 

(summerfall) 

High 

Sustained 

Flow 

(pondingspring) 

 Conservation> 45 kcfs stage arch. sites + - + 0/+ 0 0 0/+ - Conservation <45 kcfs + - + 0/+ 0 0 0 -  Conservation> 45 kcfs stage TCPs Conservation <45 kcfs