This document should be considered 

a “work in progress.” It has not gone 

through an external peer review 

process. It should not be cited in any 

reports until it has been completed 

and undergone a thorough peer 

review process. 
A DISCUSSION OF THE EVIDENCE FOR PERCEIVED 

FAILURE OF THE MODIFIED LOW FLUCTUATING FLOW 

ALTERNATIVE (MLFF) TO BENEFIT MOST ECOLOGICAL 

RESOURCES BELOW GLEN CANYON DAM, 1991 - 2001 

A White Paper Prepared Jointly By: 

Carl Walters, University of British Columbia, Fisheries Centre 

Josh Korman, Ecometric Research, Inc. 

Ted Melis, David Topping, Barbara Ralston, Lew Coggins and Scott Wright, 

U.S. Geological Survey, Grand Canyon Monitoring and Research Center 

Clark Burbidge and Clayton Palmer, Western Area Power Administration 

Stephen M. Wiele, U.S. Geological Survey, Water Resources Division 

As Documentation in Support of Multi-Attribute Trade Off Analysis (MATA) 

Discussions That Resulted in the Consequence Table Produced by Members of the 

Glen Canyon Adaptive Management Technical Workgroup at the December 2003 

Saguaro Lake Workshop 

July 1, 2004 
2

Introduction 

A critical transition from passive to active adaptive management occurred in the Glen 

Canyon Dam Adaptive Management Program (AMP) in 2003, with the implementation 

of a series of experimental treatments intended to be the start of a longer-term 

experimental design (see Walters and Holling 1990 for definitions of AM). Recently, a 

series of multi attribute utility analysis (MATA) workshops were used to elicit 

preferences of Glen Canyon Adaptive Management - Technical Working Group (TWG) 

members for policy options involving flow variation and control of exotic fishes in the 

Colorado River below Glen Canyon Dam (GCD). The objective of this effort was to 

better focus planning around long-term experimentation. A surprising result from the 

analysis was that many TWG members appear to prefer more variable diurnal flow 

policies than either the initial Low Fluctuating Flow policy (LFF) imposed in August 

1991, or eventually the Modified Low Fluctuating Flow (MLFF) policy that has been in 

place as the Record-of-Decision since 1996 (Figure 1). This is understandable in the case 

of stakeholders whose interest is primarily to optimize revenue associated with 

hydropower production at GCD, because the new policy has been quite costly (it has 

reduced potential value from power production by $50 million/year or more). However, 

there was also considerable support for more variable diurnal flows from stakeholders 

concerned about ecological performance measures, particularly recovery of the 

endangered humpback chub and management of sediment resources. These stakeholders 

were apparently responding to data and interpretations from scientists indicating that the 

MLFF policy has not provided benefits to humpback chub and sediment resources as 

predicted in the Operations of Glen Canyon Dam – Final Environmental Impact 

Statement (DOI 1995). 

Here we review the data presented to the TWG during the MATA process in December 

2003, to justify predictions of continued humpback chub decline, relatively poor quality 

of the Lees Ferry trout fishery (as measured by fish size), and relatively rapid sand loss if 

MLFF is continued (Rubin et al 2002). One of the justifications for the MLFF policy was 

apparently an assumption or prediction that the policy would have beneficial ecological 

effects relative to the more rapid and extreme diurnal flow variations that preceded it 

(operations termed, No Action that occurred from 1966-1990). We show that no such 

beneficial effects are evident in an ecological sense (except to abundance of exotic trout) 

and in fact the move to LFF in 1991 and then to MLFF in 1996, is correlated with a 

relatively sharp decline in humpback chub recruitment. We recognize that in the case of 

humpback chub, a move to summer-fall steady flows (as favored by some TWG members 

as one experimental treatment during the MATA workshops) could well produce 

beneficial effects on chub recruitment. We review evidence about the likely impacts of 

experimentally moving away from the MLFF policy on other Grand Canyon resources, 

including sediment, riparian vegetation and cultural resources. We also consider one 

specific tradeoff in flow policies with possible, but uncertain net benefits to sand 

conservation – the abandoning of MLFF for expanded fluctuating operations (increased 

sand export rates) simultaneously accompanied by a relaxation of current constraints 

(timing and frequency) associated with implementation of Beach/Habitat-Building Flows 

(BHBF). 
3

Changes in humpback chub recruitment and abundance 

The only remaining successfully reproducing population of humpback chub in Grand 

Canyon appears to be the population that spawns in the Little Colorado River (LCR). 

Based on recaptures of fish that have been PIT tagged since 1989, some 15-20% of the 

adult fish in this population die each year owing to natural causes. For a sustainable 

population, these losses must be replaced through recruitment, and the evidence now 

available from size composition and growth sampling in the LCR and Colorado River 

mainstem is that the recruitment now consists mainly of juveniles that have spent most of 

their pre-adult life (3-4 yrs) in the LCR. 

Mark-recapture estimates of population size and trend indicate that recruitment has not 

replaced natural losses from the LCR population for at least the last decade, and probably 

much longer (Figure 2). There has been a downward trend averaging around 14% per 

year in adult abundance since the initiation of intensive monitoring programs in the early 

1990s, indicating that recruitment has been only about half of that needed to sustain or 

stabilize adult population size. 

Hoop net catch-per-unit effort (CPUE) indices of juvenile abundance (recruitment 

measured at age 1), indicate that there was a relatively sudden decline in recruitment 

sometime around 1990 (Figure 3). The size composition of the population was sampled 

very intensively in the 1991-1993 period, and from this size composition we can assign 

probable ages to the fish that were present during that period and we can back-calculate 

how many recruits must have entered the population during the 1980s, to have produced 

these early 1990s survivors (Figure 3). This recruitment back-calculation indicates that 

recruitment was probably declining even before 1990, quite possibly following a period 

of strong recruitment during the early 1980s. 

We cannot say with any confidence that the decline in recruitment that apparently 

occurred in the early 1990s was caused by the move to MLFF flows; it could equally well 

have been caused by some unmonitored change in the carrying capacity or mortality 

conditions in the LCR. However, we can say with confidence that MLFF was not 

effective at reversing the decline or at providing sufficiently good habitat conditions in 

the CR mainstem to allow enough recruitment for the population to be sustained. That is, 

MLFF had either a negative effect or no effect at all, but it certainly did not have the 

hoped-for measurable beneficial effect on humpback chub required to increase chub 

population size (one obvious measure of ecosystem restoration identified by managers). 

There is a good chance that juveniles dispersing into the mainstem in summer and fall 

would be able to grow, survive, and return to the LCR for extended rearing if they were 

to encounter (1) reduced predation by exotic trout owing to mechanical removal 

treatments, and (2) relatively warm spatial refuges in near-shore locations, as would be 

created by steady flow conditions in late summer and fall. The Low Summer Steady 

Flow (LSSF) treatment of 2000, demonstrated that such lateral warming of backwater 

areas can be quite dramatic. A Summer-Fall Steady Flow (SFSF) experiment would need 

to maintain conditions for backwater warming from the time of the first summer freshet 

that disperses juveniles into the main channel (usually mid-to-late summer), until around 
4

November 1 when the equilibrium temperature in standing backwaters decreases (owing 

to nighttime cooling) to about the same as the main channel temperature. In contrast to 

this strategy, low, stable flows during the 2000 LSSF treatment were initiated in early 

June, before the first summer freshets in the LCR and stable flow releases were 

terminated in early September. 

Depending on how much it increases summer and fall temperatures in the main water 

flow, a temperature control device (TCD) operated at Glen Canyon Dam could 

considerably enhance the thermal effects of SFSF on backwaters - perhaps even to the 

degree that steady flows are no longer a requirement for successful recruitment of chub 

when combined with control of exotic fishes. Juvenile chub are unlikely to exhibit 

normal first-year growth unless water temperature is at least 18 degrees C in late summer. 

It is doubtful that such a large impact (4-5 degrees above the temperature now seen in the 

mainstem near the LCR at that time of year) would be achieved by a TCD alone, if that 

TCD were also planned to avoid GCD release temperatures high enough to cause 

negative impacts on the Lees Ferry trout population. 

Changes in abundance and size of trout (quality vs. quantity) 

Both rainbow and brown trout have increased dramatically under the MLFF policy. 

Densities of rainbow trout, as evidenced by both electrofishing monitoring data and catch 

per effort in the Lees Ferry trout fishery, have increased by at least 5-fold (Figures 4-5), 

while growth and average body size have decreased (Figure 5). Brown trout abundance 

outside the main population concentration near Bright Angel Creek has increased even 

more dramatically (Figure 6). 

Upstream of Lees Ferry, the rainbow trout population has benefited from two basic 

effects: 

(1) increases in primary production (and presumably insect food production), and 

(2) increases in spawning success and juvenile survival. 

Using the Grand Canyon Ecosystem Model (GCM), which does detailed predictions of 

primary production rate using diurnal stage variation and algal biomass development 

estimates (Walters et al., 2000), we estimate that primary production has increased by at 

least 30% since 1990. This increase in potential to support trout biomass has been 

dampened somewhat by shifts in community structure from algae (Cladophora) to 

macrophytes and from insect production to snail production, and by reduced availability 

of insects (increased diurnal flow fluctuation apparently promotes drift, i.e. dispersal, of 

insects, and trout feed mainly on such drifting organisms). 

Rainbow trout populations typically show conservation of total biomass, meaning that a 

given area supports roughly the same total biomass whether that biomass consists of a 

few large fish or many small ones. This phenomenon creates a severe tradeoff in rainbow 

trout fisheries between quality and quantity of fish available to anglers, if quality is 

measured by availability of large fish. Increases in number of recruits to the Lees Ferry 

population, owing mainly to improved juvenile survival rates over the early life period 

when juveniles are restricted to using near-shore areas (much more stable environments 

under MLFF), have led to a dramatic increase in catch per effort measured in numbers of 
5

fish per angler day. However, there has been a considerable decrease in the average and 

maximum sizes of fish caught. It should be noted that the total trout biomass per area 

likely depends on total flows, with lower total biomass being supported in years of low 

flow (a more severe size vs. numbers tradeoff in low-flow years) like those seen recently 

in Glen Canyon. If low flows continue, improvements in fish size under policies aimed 

at reducing fish density may be considerably less than expected from growth and 

abundance data gathered during the higher flows of the late 1980s and 1990s. 

Experimental food for thought (an example of learning by doing) 

Since 2003, an experimental flow treatment has been implemented that consists of 

increased diurnal flow fluctuations of 5,000-20,000 cfs from January through March 

(about a factor of 2 increase in the daily stage-change over the maximum change allowed 

under MLFF), aimed at deliberately reducing rainbow trout recruitment so as to both 

improve fishing quality (fewer, larger fish) and reduce potential impact of rainbow trout 

on native fishes downstream. On the basis of preliminary experimental results, 

approximately 50% of the redds in 2003, in the Lees Ferry reach were excavated after 

March 31, when flows resumed to normal MLFF operations. The total egg deposition 

loss owing to Glen Canyon Dam operations in 2003, ranged from 30 to 40 percent in the 

Lees Ferry reach, with about half of this mortality being a direct consequence of the 

expanded daily-stage range of fluctuating flows in January through March (Korman et al. 

2004). Three flow recommendations for Glen Canyon Dam were made on the basis of 

results from a 2003 young-of-year (YoY) survey and analysis of otolith microstructure: 

1) Fluctuating flows targeting YoY rainbow trout should be implemented from April 

through July to coincide with the timing of the observed hatch; 2) Summer steady flows 

very likely improve the growth of YoY rainbow trout; and 3) Sudden reductions in the 

minimum daily flow have the potential to strand or displace many YoY rainbow trout in 

the Lees Ferry reach (Korman et al. 2004). 

The latter recommendation was based on an almost complete absence of fry from low 

angle shorelines after the reduction in the minimum flow from 10,000 to 5,000 cfs 

following the 2003, Labor Day weekend (September’s low-volume, MLFF operations). 

An event-based approach, where flows are increased to approximately 20,000 cfs for 2 

days, followed by a reduction to 5,000 cfs for one day, implemented on a monthly basis 

from May through September, might likely be more effective at reducing recruitment in 

the Lees Ferry Reach than the January - March experimental fluctuating-flow regime 

implemented in 2003-2004. Steady flows could be conducted between events to increase 

water temperatures for native fish downstream and would not have beneficial effects for 

YoY rainbow trout as their densities would be controlled through the temporary 

reductions in minimum flow. The effectiveness of the event-based approach on rainbow 

trout could also be easily monitored, with results available in the same year that it is 

implemented. 

Changes in sand storage and transport (beach-habitat loss) 

Recommendation for the MLFF policy in the Operations of Glen Canyon Dam - Final 

Environmental Impact Statement (DOI 1995), was in part based on the assumption that 

sufficient tributary-derived sand could be accumulated in lower eddy environments and 
6

the bed of the main channel over multi-year periods so as to be available for sand-bar 

restoration during occasional Beach/Habitat-Building Floods. Recent studies have 

demonstrated that this premise is false, and that the majority of fine sediment from the 

Paria River that is delivered during the warm-season runoff (July through October), is 

transported out of Marble Canyon in a few months to seasons under MLFF operations 

(Rubin and Topping 2001; Rubin et al. 1998; Topping et al. 2000a; 2000b). This has 

prompted sediment scientists to recommend testing of three alternate options to the EISderived MLFF policy for sand conservation and bar-habitat restoration (Rubin et al. 

2002): 

• Experimentally test implementation of a controlled flood (BHBF) immediately 

after significant tributary sediment inputs from the Paria River (and if that is not 

possible, then), 

• Test initiation of low, steady flows immediately after a tributary sediment input 

until such time that a controlled flood can be initiated, or (if the first two options 

are not successful), 

• Consider future operation of a system to achieve sediment augmentation coupled 

with controlled floods. 

There is currently little debate over whether MLFF operations have failed to produce 

their intended benefits for sediment resources in Grand Canyon. During discussions at 

the 2003, MATA workshops, it was hypothesized that relaxation of some constraints on 

MLFF daily fluctuations might actually increase net conservation of sand, despite 

contrary predictions made in the EIS. Because the sediment transport-water discharge 

relationship is non-linear, the notion here was that reduced sediment transport associated 

with longer periods of low flow under higher daily fluctuations would more than 

compensate for increased transport during the higher flows. It was also hypothesized that 

such a benefit might only occur under higher monthly release volumes associated with 

wetter hydrology when MLFF daily range constraints (no more than 8,000 cfs per day) 

limit daily flow minimums to relatively high discharges. This sand-transport prediction 

was recognized to be quite uncertain, as the discharge frequency over a day under a given 

flow regime would depend on the monthly volume from Glen Canyon Dam, as well as 

optimized release patterns that would be prescribed for hydropower generation and 

maximum revenue. In addition, it was recognized that the relative transport rates will 

depend on the slope of the sediment transport-discharge relationship. Another possibility 

for enhanced sand-bar conservation under alternative fluctuations was identified to be 

related to whether or not higher fluctuations could temporarily increase sand volumes 

stored within recirculating eddies throughout Grand Canyon. An analysis, to 

quantitatively examine these issues is reported here. 

The Western Area Power Administration’s (WAPA) Hydro LP model was run under 

monthly volumes ranging from 400-1,000 thousand acre-feet (TAF) for four flow 

scenarios (Table 1). The model predicted discharge every hour for a one-week period for 

each volume and flow regime combination. Three alternate sediment rating curves at the 

Grand Canyon gage, developed over 3 one-week periods in March, May, and July, 2003, 

were used to predict the sand concentration as a function of water discharge for each hour 
7

(Table 2). Total sand transport for the week under reach rating curve was then computed 

as the sum of products between predicted hourly sand concentrations and volumes. It is 

important to note that the sand transport for a given discharge will depend on the grain 

size and quantity of sand on the bed (Rubin and Topping 2001) and predictions should 

not be used to forecast future sand transport rates. We therefore standardized the 

transport predictions for each flow scenario by the corresponding value under MLFF (i.e. 

Y = 100 * (x-MLFF)/MLFF), where x is the transport rate under any flow regime for a 

given GCD volume and MLFF is the corresponding rate under the MLFF flow). 

Table 1. Summary of flow regime characteristics for which suspended-sediment 

transport rates were computed. 

Regime Daily Variation 

(kcfs) 

 Total Allowed Range 

Min/Max 

(kcfs) 

Upramp/Downramp 

Rate (kcfs/hr) 

MLFF (CURRENT 

POLICY) 

5, 6 or 8 (volume 

dependent) 

5/25 4/1.5 

5-20 (EXP) 5-20 5/20 5/2.5 

5-25 (EXP) 5-25 5/25 5/2.5 

STEADY (EXP) 0 Volume dependent 0 

Table 2. Sand concentration (mg/l) vs. water discharge (cfs) relationships at the Grand 

Canyon gage, 2003 (D. Topping, GCMRC, unpublished data). 

Period Daily Range (kcfs) Equation R2

March 6-11, 2003 5-20 (EXP) Y=9.29e-16*q^4.08 0.97 

May 5-11, 2003 7.5-13.5 (MLFF) Y=9.96*e-22*q^5.49 0.91 

July 4-8, 2003 10.5-18.5 (MLFF) Y=4.80e-20*q^5.07 0.93 

The Hydro LP model predicted that maximum flows under each scenario would be 

maintained for at least 12 hrs of each day during normal weekday operations (Figure 7). 

Scenarios that allowed for higher maximum flows therefore had higher transport rates 

than under MLFF, even though the MLFF regime had higher minimum flows (Figure 8). 

At lower monthly volumes of 400 and 500 TAF, the 5,000 – 20,000 and 5,000 – 25,000 

cfs regimes produced similar transport rates, which were 40 and 140 percent higher than 

MLFF rates, respectively, based on the March 6-12, 2003 rating curve (Figure 8b). 

Under higher volumes, sand transport under the 5,000 - 25,000 cfs regime was 

considerably higher than under the 5,000 - 20,000 cfs regime. Under the range of 

volumes that were examined, the steady flow alternative had transport rates that were 50 

to 80 percent less than those under MLFF; information that was similarly reported in the 

EIS (DOI 1995). Recent Riverware simulations (operations software used by BuRec for 

managing storage in the Colorado River basin), predict that 50 percent of the monthly 

release volumes from Glen Canyon Dam between 2004 and 2010, will be between 600 

and 800 thousand acre-feet (Figure 9). Under these volumes, the most conservative sand 

transport relationship (lowest slope, Table 2) predicted that the 5,000 - 20,000 cfs 

scenario will increase transport rates by approximately 50 percent to as much as 200 
8

percent, relative to MLFF. The relative differences among the hypothetical fluctuation 

scenarios were also dependent on the slope of the sand transport relationships (Table 3). 

The March 2003 rating curve, having the lowest slope, produced the smallest differences 

between scenarios (Table 2). However, the results shown in Table 3 also suggest that 

some benefit may be derived from the 5,000 – 20,000 cfs operation (a variation of the 

originally proposed Low Fluctuating Flow (LFF) alternative in the EIS with relaxed 

ramping and daily range constraints) over MLFF in higher-volume release months 

associated with wetter hydrology in the future (as originally hypothesized). 

The basic conclusion drawn from the analysis, independent of choice of which rating 

curve was used; is that daily fluctuations in flow that are reduced in total allowable range, 

but less constrained than those of MLFF with respect to daily range and ramping rates, 

will increase the transport rate of sand past the Grand Canyon gage (RM 87) and promote 

export from the upper third of the ecosystem. This is predicted to be true for most 

monthly release volumes predicted by Riverware for Glen Canyon Dam under the 

current, protracted drought impacting the Upper Colorado River basin. It is also likely 

that this sediment-transport estimate would also apply to the lower two-thirds of the 

ecosystem as well (below RM 87). 

While this analysis has demonstrated that the steadier the flows, the higher the retention 

of sand below Glen Canyon Dam, it does not necessarily imply that increased daily 

fluctuations in flows should not be considered as part of long-term experimental 

treatments by the TWG. This owes to the possibility that such experimental operations 

might provide both economic benefit, as well as benefit to sand-bar morphologies that 

support habitat, such as more abundant backwaters following periods of sand-bar 

restoration that might still be achieved after implementation of options recommended 

above by Rubin et al. (2002). If an experimental policy that relaxes rules on daily 

fluctuations is implemented simultaneously with more strategically timed controlled 

floods, then the overall effect for the sediment resources in Marble and Grand Canyons 

could turn out to be beneficial overall. It is also possible that strategic implementation of 

fluctuations after sand-bar restoration floods and prior to periods of warmer, stable 

releases might provide optimal benefits for promoting chub recruitment if such combined 

“designer” flows result in increased near-shore nursery habitats. However, in the absence 

of increased flexibility in the timing of controlled floods, increased daily fluctuations in 

flow alone will certainly increase the rate at which sand resources in Grand Canyon are 

eroded in low to medium volume release months. 

 

Table 3. Change in the sand transport rate at the Grand Canyon gage under 3 flow 

scenarios (see Table 1) relative to the transport rate under MLFF. Relative rates are 

expressed as a percentage. 

 Monthly Vol. 5-20 kcfs 5-25 kcfs Steady 

March 6-12, 

2003 curve 

400 42 42 -81

500 140 140 -74

600 216 287 -72
9

700 147 351 -65

800 48 188 -66

900 -1 138 -60

1000 -19 81 -56

 

May 5-11, 2003 curve 

400 63 63 -89

500 234 234 -84

600 390 562 -83

700 241 719 -76

800 64 323 -78

900 -4 219 -73

1000 -29 120 -69

 

July 4-8, 2003 

curve 

400 57 57 -87

500 203 203 -81

600 330 465 -80

700 210 585 -73

800 59 278 -75

900 -3 193 -70

1000 -26 108 -65

Variability in predicted influence of higher fluctuations on eddy 

sand bars and related habitats

While there is a high degree of certainty associated with the suspended-sediment 

transport response below Glen Canyon Dam under MLFF versus experimental fluctuating 

flows, there is relatively less certainty about short-term sand habitat responses under such 

proposed alternative operations (see Wiele and Franseen 2001, for more details). 

Preliminary simulation results for three study sites located within the upper third of the 

ecosystem suggest that variations in channel morphology and sediment supply will result 

in highly variable sand-bar responses under alternative fluctuating flows (Figure 10). Of 

the three sites where the alternative 5,000 – 25,000 cfs fluctuations were modeled, only 

one site – RM 22 – showed a net gain for sand volume stored in the eddy after four 

weekly cycles of diurnal operations (Figure 10a). Of the other two sites (both 

downstream where sand supply was assumed to be relatively greater), one was a clear 

loser at the end of the experimental simulation (RM 30 shown in Figure 10b), while the 

most-downstream site showed relatively no change in net volume of sand storage despite 

small daily changes that occurred around the diurnal release pattern. Interestingly, this 

third site recently showed a dramatic increase in available backwater habitat following 

experimental 5,000 – 20,000 cfs fluctuating flow operations of winter 2003-04; an 

experimental period without a BHBF that followed only moderate sand production from 

the Paria River in August 2003 (about 30 percent of the average annual sand input). 

In conclusion, while the transport estimates described above indicate increased export of 

sand under the experimental diurnal releases, there is potential for benefit to native fish 
10

from maintaining or even creating nursery habitats for YoY chubs in the main channel 

(e.g. return current channels associated with sand bars) at select sites under limitedduration, but higher fluctuating flows. Any such habitat maintenance to bars derived 

from expanded fluctuating flows would obviously come at the expense of elevated sand 

export, but such losses might turn out to be offset by repeated release of controlled floods 

implemented under relaxed constraints when Paria River sand production is significant. 

The basic question that remains to be answered at present is whether or not the remaining 

6 to 7 percent of the pre-dam sand supply below the dam can be managed with flows 

alone to restore and maintain sand bars in a sustainable manner, or even to arrest 

historical patterns of post-dam erosion? The high degree of uncertainty associated with 

the above question, as well as mixed levels of certainty related to suspended-sediment 

transport and sand bar responses under diurnal flow patterns suggests that additional field 

experiments are required before managers can effectively consider all available options 

for achieving desired outcomes. 

Changes in the terrestrial riparian community (vegetation and 

animals) 

As would be expected with flow regulation, the pre-dam flood frequency has been 

compressed by closure of Glen Canyon Dam in 1963 (Figure 1a). Additionally, the 

timing of annual high flows has also been delayed from spring to summer, a key finding 

reported by Topping et al. (2000a; 2000b). Compared with the “No Action” period of 

1966-1990, the LFF and MLFF policies between 1991 and 2003, have resulted in further 

decreases in maximum daily flows as well as increases in minimum daily flows (Figure 

1b). These changes have allowed terrestrial vegetation like tamarisk and willow to 

invade more beach and cobble area, and have probably reduced the recharge rate of 

groundwater used by plants like mesquite that were naturally restricted to above the 

natural seasonal high water line. The LSSF experiment of 2000, was associated with 

formation of a strong cohort of tamarisk within what is normally the MLFF varial zone, 

and this cohort has been able to survive diurnal flooding for three years now. Most likely 

a return to higher daily maximum flows would be accompanied by a similar lack of, or 

very slow, response by the vegetation that has developed within the MLFF varial zone. 

The LSSF experiment also demonstrated that the timing of planned beach habitat 

building flows and other flow variations can be critical in establishing or preventing good 

seeding conditions for species like tamarisk. 

Under MLFF, some backwater areas initially developed quite diverse and productive 

plant and animal communities due to protection from diurnal flooding and scouring. But 

without natural scouring and renewal events, these areas are undergoing vegetation 

succession and gradual accumulation of sediments and will likely eventually lose their 

diverse character. 

Changes in cultural resources 

Mass storage and transport models are not yet available to accurately predict the net total 

impact, through combined water and aeolian processes, of altered diurnal flow regimes 

on amount and distribution of sand available to protect cultural resources. One 

complicating factor in such modeling is that much of the sand now subject to aeolian 
11

movement is from large sand deposits that occurred during major floods well before 

GCD; such deposits are likely losing mass slowly, independent of managed water flows, 

and hence would result in exposure of some cultural sites no matter how the water is 

managed. 

Discussion: should MLFF be abandoned? 

The MLFF provides an excellent example and warning to practitioners of adaptive 

management about the difficulties that scientists encounter in trying to determine the 

ecological impact of a policy change when only before-after monitoring data are 

available (Walters and Holling 1990). We can develop suspicions about possible 

unintended side effects of the change (e.g. on native fishes), but it is always possible to 

explain away any such apparent effects as owing to some other factor that happened to 

change at the same time. Had at least one alternative flow operating regime, for instance, 

one with more violent diurnal fluctuations, been implemented for a few years during the 

1990s as an alternative experimental treatment for comparison to MLFF, we would be in 

a much better position today to rule out alternative explanations. So from the standpoint 

of adaptive management, MLFF should not be abandoned entirely but rather treated as 

just one of several flow management options to be compared to others within a longerterm experimental plan. The MLFF policy has obviously achieved some stakeholder 

objectives, particularly those related to interests of the recreational rafting community 

and trout sport anglers who may care more about catching lots of fish rather than catching 

bigger fish. Such benefits will certainly lead to pressure from those stakeholders to at 

least include the policy among future experimental treatments and to seek other ways to 

mitigate any negative ecological impacts that it may have (e.g. by continuing mechanical 

removal of exotic predators as well as movement toward more flexibility in strategic 

implementation of controlled floods to conserve sediment resources). 

However, because MLFF does not appear to be a win-win option for all stakeholders, it 

raises an important ethical issue about who should bear the costs of continuing it 

exclusively as a management policy. At present, power utilities and their ratepayers are 

essentially subsidizing, at considerable cost, improvement in the quality of Grand Canyon 

for some, but not all of its designated recreational uses. It is one thing to impose such 

cost to protect against the loss of an endangered species, but quite another to do so for the 

benefit of a limited few stakeholders. Schmidt et al. (1998), suggests that these valuebased issues must be addressed and resolved as part of the AMP process to avoid a 

breakdown in the collaboration among stakeholders that has made adaptive management 

possible and relatively successful in the Colorado River ecosystem in the first place. 
12

References 

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Impact Statement, Colorado River Storage Project, Arizona: Salt Lake City, 

Bureau of Reclamation, Upper Colorado River Regional Office. 

Korman, J., M. Kaplinski, J. Hazel, T. Melis, J. Sneep, and C. Magirl. 2004. Effects of 

fluctuating flows from Glen Canyon Dam on the early life history stages of 

rainbow trout in the Lee’s Ferry reach of the Colorado River, FY2003, Draft 

report prepared by Ecometric Research for GCMRC. 

Rubin, D.M., D.J. Topping, J.C. Schmidt, J. Hazel, M. Kaplinski and T.S. Melis. 2002. 

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Geophysical Union, v. 83, n. 25, p. 273, 277-278. 

Rubin, D.M., J.M. Nelson and D.J. Topping. 1998. Relation of inversely graded deposits 

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Grand Canyon, Geology, v. 26, p. 99-102. 

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Walters, C.J. and C.S. Holling. 1990. Large-scale management experiments and learning 

by doing. Ecology, v. 71, n. 6, p. 2060-2068. 

Walters, C., J. Korman, L.E. Stevens and B. Gold. 2000. Ecosystem Modeling for 

Evaluation of Adaptive Management Policies in the Grand Canyon. Conservation 

Ecology, [online] URL: http//www.conseco.org/vol4/iss2/art1, 35 p. 

Wiele, S.M. and M.A. Franseen. 2001. Sand transport and bed evolution modeling 

applications in the Colorado River, Grand Canyon. Proc. 7th Fed. Interagency Sed. 

Conf., Reno, I-115 – I-119. 
13

Figure 1. A) Pre-dam versus the post-dam monthly flow at Lees Ferry, B) changes in daily 

maximum and minimum flow below Glen Canyon Dam since 1948. Low Fluctuating Flow 

regime (LFF, with maximum daily range of 8,000 cfs and a total fluctuating range of 5,000 – 

20,000 cfs) appears after 1990 as the preliminary change in operations, later designated as 

Modified Low Fluctuating Flow (MLFF, with maximum daily range of 8,000 cfs and a total 

fluctuating range of 5,000 – 25,000 cfs) in the Record-of-Decision of 1996.

A.

B.

0

10,000

20,000

30,000

40,000

50,000

60,000

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

MONTH

MEAN DISCHARGE (cfs)

1923-1965

1966-2003

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

49 53 58 62 67 72 76 81 85 90 95 99

Year

Flow (CFS) 

Daily Minimum 

Daily Maximum
14

Figure 2. Trends in humpback chub population size in Grand Canyon as evidenced by various estimation 

procedures based on PIT tagging data.

0

2000

4000

6000

8000

10000

12000

14000

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

Population size (N)

4+ Abundance ASMR1

4+ Abundance ASMR2

4+ Abundance ASMR3

4+ Abundance Jolly-Age
15

Figure 3. Trends in humpback chub recruitment estimated by A) analysis of PIT tags and from, B) hoop 

net catch rates near the mouth of the Little Colorado River. 

A. 

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

1999

Year

Number of recruits

Age 2 Recruitment ASMR1

Age 2 Recruitment ASMR2

Age 2 Recruitment ASMR3

B. 

Hoopnet CPUE for Age 1 fish in the LCR (AGFD Data)

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

Year

CPUE (fish/hour
16

Figure 4. Trends in abundance of rainbow trout in Grand Canyon and Glen Canyon since initiation of 

MLFF flows. 

Rainbow Trout CPE

(# fish per 10 Hrs Effort)

0

100

200

300

400

500

600

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Grand Canyon Near LCR 

0

500

1,000

1,500

2,000

2,500

3,000

3,500

Glen Canyon

Grand Canyon

Glen Canyon
17

Figure 5. Trends in abundance and mean size of rainbow trout in the Lees Ferry fishery, comparing 

population model reconstructions to observed trend index data prior to 2000. 

250

270

290

310

330

350

370

390

410

430

450

1965 1970 1975 1980 1985 1990 1995 2000 2005

Mean vul fish lengt

h

Model mean length (mm)

Survey Mean length

0

0.5

1

1.5

2

2.5

3

1965 1970 1975 1980 1985 1990 1995 2000 2005

Predicted angler CPUE

Observed angler CPUE
18

Figure 6. Trends in relative abundance of brown trout (catch per effort from electrofishing monitoring 

program) in Grand Canyon. Major population concentration is in the region of Bright Angel Creek 

(spawning area), but dramatic increases have been seen in recent years both upstream and downstream of 

this concentration. The increase upstream of Bright Angel Creek (river miles RM<80) represents a 

growing threat to native fishes that spawn in the Little Colorado River (RM 61). 

Brown Trout Catch Per Effort (cpue)

0

0.2

0.4

0.6

0.8

1

1.2

1980 1985 1990 1995 2000

Year

Upstream/downstream cpue 

0

5

10

15

20

25

30

Bright Angel Reach cpue 

Average cpue,

RM<80

Average RM>130

Average RM 80-130

(Bright Angel)
19

Figure 7. Hourly discharge pattern from Glen Canyon Dam for a one-week period under a release volume 

of 800 thousand acre-feet for 4 flow scanrios predicted by the WAPA Hydro LP model. See Table 1 for 

description of scenarios. 

0

5

10

15

20

25

Sunday Monday Tuesday Wednesday Thursday Friday Saturday

GCD Discharge (kcfs)

MLFFA 5 - 20 5 - 25 Steady
20

Figure 8. A) shows relative sand transport rates at the Grand Canyon gage (based on March 2003, rating 

curve shown in B) as a function of the monthly release volume from Glen Canyon Dam under 3 alternate 

flow scenarios. See Table 1 for a description of scenarios. 

A. 

B. 

Sand Transport at Grand Canyon

-100

0

100

200

300

400

200 400 600 800 1000 1200

Monthly Volume from GCD 

(Ac-Ft * 1000)

% Change Relative to MLFF

A

5-20

5-25

Steady

1

10

100

1,000

6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000 22,000

MARCH 6 - MARCH 12, 2003 5,000-20,000 cfs DAILY RANGE

MAY 5 - MAY 11, 2003 7,500-13,500 cfs DAILY RANGE

JULY 4 - JULY 8, 2003 10,500-18,500 cfs DAILY RANGE

SAND CONCENTRATION (mg/l)

WATER DISCHARGE (ft3/s)

y = 9.28e-16 * x^(4.08) R2= 0.966

y = 9.96e-22 * x^(5.49) R2= 0.912

y = 4.80e-20 * x^(5.07) R2= 0.930
21

Figure 9. Frequency of monthly volumes released from Glen Canyon Dam predicted by Riveware for the 

period 2004-2010. 

Projected GCD Monthly Volumes, 2004-2010

0

5

10

15

20

25

30

35

40

400 500 600 700 800 900 1000 1100

Monthly GCD Volume (Ac-Ft * 1000)

% of Month

s
22

Figure 10. Preliminary numerical simulations of estimated sand-storage responses at three eddies in 

Marble Canyon under hypothetical experimental fluctuating operations of 5,000 to 25,000 cfs. Part A, is 

the estimated response at the 22-Mile study site. 

A.
23

Figure 10. Preliminary numerical simulations of estimated sand-storage responses at three eddies in 

Marble Canyon under hypothetical experimental fluctuating operations of 5,000 to 25,000 cfs. Part B is the 

estimated response at the 30-Mile study site. 

B. 
24

Figure 10. Preliminary numerical simulations of estimated sand-storage responses at three eddies in 

Marble Canyon under hypothetical experimental fluctuating operations of 5,000 to 25,000 cfs. Part C is the 

estimated response at the 43-Mile study site (Eminance Break). 

C.