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. 
EVIDENCE FOR THE FAILURE OF THE MODIFIED LOW

FLUCTUATING FLOW ALTERNATIVE (MLFFA) TO

BENEFIT MOST ECOLOGICAL RESOURCES IN GRAND

CANYON

By:

Carl Walters, University of Bristish Columbia

Josh Korman, Ecometric Research Inc.

Ted Melis, USGS, Grand Canyon Monitoring and Research Center

David Topping, USGS, Grand Canyon Monitoring and Research Center

Lew Coggins, USGS, Grand Canyon Monitoring and Research Center

Barbara Ralston, USGS, Grand Canyon Monitoring and Research Center

Clark Burbidge, Western Area Power Administration

Clayton Palmer, Western Area Power Administration

DRAFT FINAL

June 22, 2004
Introduction

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

preferences of Grand Canyon 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). A surprising result from the analysis was that many TWG

members appear to prefer more variable diurnal flow policies than the Modified Low

Fluctuating Flow (MLFF) policy that has been in place since the early 1990s (Figure 1).

This is understandable in the case of stakeholders whose interest is primarily in efficient

power production from GCD, because the MLFF 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 comments from scientists that the MLFF policy has not

provided benefits to humpback chub and sediment resources as predicted in the EIS

(BOR 1995).

Here we review the data presented to TWG during the MATA process 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. One of the justifications for the MLFF policy was apparently an assumption

or prediction that MLFF would have beneficial ecological effects relative to the more

violent diurnal flow variations that preceded it. We show that no such beneficial effects

are evident in the ecological (except to abundance of exotic trout), and in fact the move to

MLFF is correlated with a relatively sharp decline in humpback chub recruitment. We

note that in the case of humpback chub, a move to summer-fall steady flows (as favored

by some TWG members during the MATA workshops) could well produce beneficial

effects on chub recruitment. We review evidence that there would likely be little impact

of moving away from MLFF on other Grand Canyon resources, including riparian

vegetation and cultural resources.

Changes in humpback chub recruitment and abundance

The only remaining reproducing population of humpback chub in Grand Canyon is

apparently 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 due 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-effort 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 in order 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 due to 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 a

measurable beneficial effect on humpback chub.

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 due to mechanical removal treatments,

and (2) relatively warm spatial refuges in nearshore locations, as would be created by

steady flow conditions in late summer and fall. The Low Summer Steady Flow LSSF

experiment 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 mainstem, until around November 1 when the equilibrium temperature

in standing backwaters decreases (due to nighttime cooling) to about the same as the

mainstem temperature.

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

flow, a temperature control device (TCD) could considerably enhance the thermal effects

of SFSF on backwaters (and perhaps even make steady flows unnecessary). 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 MLFF. 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)

(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, 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, due mainly to improved juvenile survival rates over the early life period when

juveniles are restricted to use near-shore areas (much more stable environments under

MLFF), have led to dramatic increase in catch per effort measured in numbers of fish per

angler day. But 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

(more severe size-numbers tradeoffs in low flow years) like those seen recently in GC. 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.

Since 2003, there has been an experimental flow treatment involving increased

diurnal flow fluctuations of 5-20 kcfs from January through March., 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. Approximately 50%

of the redds in 2003 in the Lee’s Ferry reach were excavated after March 31, when flows

resumed to normal ROD operations. The total egg deposition loss due to Glen Canyon

Dam operations in 2003 ranged from 30 - 40% in the Lee’s Ferry reach, with about half

of this mortality being a direct consequence of the enhanced fluctuating flows in January

through March (Korman et al. 2004). Three flow recommendations for Glen Canyon

Dam were made based on 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 Apr. through July to coincide with the timing of 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 Lee’s 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 to 5 kcfs following the labor-day weekend. An

event-based approach, where flows are increased to approximately 20 kcfs for 2 days,

followed by a reduction to 5 kcfs for one day, implemented on a monthly basis from May

through September, would almost certainly be much more effective at reducing

recruitment in the Lee’s Ferry Reach than the January - March experimental 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 sediment storage and transport (beach loss)

Recommendation for the MLFFA policy in the 1995 Environmental Impact Statement

(EIS) for Glen Canyon Dam (BOR 1995) was in part based on the assumption that

sufficient tributary-derived sediment could be stored in lower eddy environments and the

bed of the mainstem until a discharge beyond powerplant capacity could be initiated

between Jan. – June, when normal flood operations can legally be conducted. Recent

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

from the Paria River that is delivered during the monsoon season, exits Marble Canyon in

a few months, well before a controlled flood could be initiated. This has prompted

sediment scientists to recommend three alternate options to the ROD (Rubin et al. 2000):

 initiation of a controlled flood immediately after a tributary sediment input;

 initiation of reduced and steady flows immediately after a tributary sediment input

until such time when a controlled flood can be initiated; or

 operation of a sediment pipeline coupled with controlled floods during flood

control season.

There is currently little debate that ROD flows have failed to produce their intended

benefits for sediment resources in Grand Canyon. During discussions at the MATA

workshops, it was hypothesized that increased daily fluctuations in flow might actually

reduce sediment transport rates relative to MLFFA. 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. This prediction was

recognized as being quite uncertain as the discharge frequency over a day under a given

flow regime, and hence the relative sediment transport rates, would depend on the total

monthly volume from Glen Canyon Dam. In addition, the relative transport rates will

depend on the slope of the sediment transport-discharge relationship. An analysis, to

quantitatively examine this issue, was requested by the TWG and is reported here.
The 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 Mar, May, and July, 2003, were used to predict the

sand concentration as a function of water discharge for each hour (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 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 MLFFA (i.e. Y = 100 * (x-MLFFA)/MLFFA), where x

is the transport rate under any flow regime for a given GCD volume and MLFFA is the

corresponding rate under the MLFFA flow).

Table 1. Summary of flow regime characteristics for which sediment transport rates were

computed.

Regime Daily Variation

(kcfs)

Min/Max

(kcfs)

Upramp/Downramp

Rate (kcfs/hr)

MLFFA 5, 6, 8 (volume

dependent)

5/25 4/1.5

5-20 5-20 5/20 5/2.5

5-25 5-25 5/25 5/2.5

STEADY 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 R

2

March 6-11 5-20 Y=9.29e-16*q^4.08 0.97

May 5-11 7.5-13.5 Y=9.96*e-22*q^5.49 0.91

July 4-8 10.5-18.5 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 (Fig. 7).

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

than under MLFFA, even though the MLFFA regime had higher minimum flows (Fig. 8).

At lower monthly volumes of 400 and 500 TAF, the 5-20 and 5-25 kcfs regimes

produced similar transport rates, which were 40% and 140% higher than MLFFA rates,

respectively, based on the Mar 6-12, 2003 rating curve. Under higher volumes, sand

transport under the 5-25 kcfs regime was considerably higher than under the 5-20 kcfs

regime. Under the range of volumes that were examined, the steady flow alternative had

transport rates that were 50-80% less than those under MLFFA. Riverware predicts that
50% of the monthly release volumes from Glen Canyon Dam between 2004 and 2010

will be between 600 and 800 thousand Acre-Feet (Fig. 9). Under these volumes, the most

conservative sand transport relationship (lowest slope, Table 2) predicted that the 5-20

kcfs scenario will increase transport rates by 50% (800 TAF) to as much as 200% (600

TAF) relative to MLFFA. The relative differences among scenarios were also dependent

on the slope of the sand transport relationships (Table 3). The March rating curve had the

lowest slope and produced the smallest differences between scenarios (Table 2). The

basic conclusion drawn from the analysis was independent of the rating curves that were

used; increased daily fluctuations in flow increase the transport rate of sand past out of

Grand Canyon.

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

of sand in Grand Canyon, it does not necessarily imply that increased daily fluctuations

in flows should not be considered by the TWG. If increased daily fluctuations lead to an

increase in the probability of conducting sediment-retention flows or better-timed floods,

the net effect for the sediment resource in Grand Canyon could be beneficial. However,

in the absence of increased flexibility on the timing of controlled floods, increased daily

fluctuations in flow will certainly increase the rate at which sand resources in Grand

Canyon are being lost.

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 MLFFA. Relative rates are

expressed as a percentage.

5-20 5-25 Steady

Mar 6-12

400 42 42 -81

500 140 140 -74

600 216 287 -72

700 147 351 -65

800 48 188 -66

900 -1 138 -60

1000 -19 81 -56

May 5-11

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

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

Changes in the riparian community (vegetation and animals)

MLFF led to decreases in maximum daily flows as well as increases in minimum daily

flows. These decreases 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 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

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. 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 being due to some other factor that happened to change at the same time. Had

at least one alternative flow regime, 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 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 in a long-term experimental plan.

MLFF has obviously benefited some stakeholders, particularly the recreational rafting

community and trout fishers who care more about catching lots of fish than catching big

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 mechanical removal of

exotic predators, use of beach habitat building flows to conserve sediment resources).

But 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. At present,

power utilities and their ratepayers are essentially subsidizing, at considerable cost,

improvement in the quality of Grand Canyon for some recreational uses. It is one thing

to impose such cost to deal with some broad public interest such as protecting an

endangered species, but quite another one to impose it for the benefit of particular

stakeholders. This issue needs to be addressed openly and quickly as part of the overall

adaptive management planning process, before it leads to a breakdown in the

collaboration among stakeholders that has made adaptive management possible in Grand

Canyon in the first place.
References

Bureau of Reclamation. 1995. Operation of Glen Canyon Dam, Colorado River Storage

Project, Arizona: Environmental Impact Satement.

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. 2000. Summary and discussion of

recent research findings related to dam operations and sand bar resources of the

Colorado River ecosystem. Memo to Barry Gold, GCMRC, Aug. 29, 2000.
Figure 1. Changes in daily maximum and minimum flow below Glen Canyon

Dam since 1948. Modified Low Fluctuating Flow regime (MLFF) appears after

1990.

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 (C

F

S) .

Daily Maximum

Daily Minimum
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

1989199019911992199319941995199619971998199920002001

P

o

p

ulatio

n

siz

e (N)

4+ Abundance ASMR1

4+ Abundance ASMR2

4+ Abundance ASMR3

4+ Abundance Jolly-Age

Figure 3. Trends in humpback chub recruitment estimated by analysis of PIT tags and

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

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

1975197719791981198319851987198919911993199519971999

Year

N

u

m

b

er of re

cruits

Age 2 Recruitment ASMR1

Age 2 Recruitment ASMR2

Age 2 Recruitment ASMR3

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)

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

Cany

on Near L

C

R

0

500

1,000

1,500

2,000

2,500

3,000

3,500

Glen Cany

on

Grand Canyon

Glen Canyon
Figure 5. Trends in abundance and mean size of rainbow trout in the Lee’s 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

Me

a

n

v

ul fis

h le

n

gth

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
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 (river miles RM<80) represents a growing threat to native

fishes that spawn in the Little Colorado River.

Brown Trout Catch Per Effort (cpue)

0

0.2

0.4

0.6

0.8

1

1.2

1980 1985 1990 1995 2000

Year

U

p

strea

m/d

o

w

n

strea

m

c

p

u

e .

0

5

10

15

20

25

30

Brig

ht A

n

g

el R

ea

c

h

c

p

u

e .

Average cpue,

RM<80

Average RM>130

Average RM 80-130

(Bright Angel)
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

G

C

D

Disch

arge (kcfs)

MLFFA 5 - 20 5 - 25 Steady
Figure 8. Relative sand transport rates at the Grand Canyon gage (based on March 2003

rating curve) 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.

Sand Transport at Grand Canyon

-100

0

100

200

300

400

200 400 600 800 1000 1200

Monthly Volume from GCD

(Ac-Ft * 1000)

%

C

h

a

n

g

e

R

elativ

e to

M

L

F

F

A

5-20

5-25

Steady
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 M

o

nth

s