Low flows in Glen Canyon: preliminary 

geomorphic analysis of the potential 

effects on fish and food base

T.S. Melisa, T. Gushuea, T.A. Kennedya, J.D. Muehlbauera, M.D. Yarda, P.E. 

Gramsa, J.B. Sankeya, K. Kohla, T. Andrewsa, J.E. Hazel Jr.b and J. Kormanc

a U.S. Geological Survey, Grand Canyon Monitoring and Research Center, Southwest Biological Science Center, 2255 N. 

Gemini Dr., Flagstaff, AZ 86001 Contact: tmelis@usgs.gov

b Northern Arizona University, cEcometric Research, 3650 W. 22nd Ave., Vancouver, B.C. V6S1J3

January 29, 2014

GCDAMP Annual Reporting Meeting, Phoenix, AZ
QUESTION:

HOW MUCH DO MINIMUM MLFF DAM 

RELEASES REDUCE AQUATIC HABITAT?

OR

How much shoreline area gets dewatered 

in the Glen Canyon Tailwater when dam 

releases are reduced from 

8,000 to 5,000 cfs?

Recall: Water Depth Limits Light 

Penetration to the Bed - influencing aquatic 

food production (from Yard, 2003) & that 

channel geometry controls depth, wetted 

area, etc.
Stakeholder Concern: about whether HFEs and low minimum flows 

under the 1996 ROD of 5,000 cfs (142 m3/s) between 07:00 p.m. and 

07:00 a.m. (versus 8,000 cfs (227 m3/s) might negatively affect the 

Lees Ferry fishery and foodbase?

Initially addressed by GCMRC (Kennedy & VanderKooi, 2012)

“It is our professional judgment that the effects on food base and rainbow trout of the 

two alternative flow regimes described above would be indistinguishable. Our 

judgment about minimal ecological effects is based largely on estimates of how much 

additional streambed is exposed when flows drop from 8,000 to 5,000 cfs. There are more 

than 20 cross-sections in Glen Canyon that have been monitored for geomorphic 

characteristics; the lower flows of 5,000 cfs only reduce the inundated area of the stream 

bed at these measured cross-section by an average of 5 percent (USGS unpublished data).”

-03.9-Left -15.2-Right -08.8-Right

PRELIMINARY DATA

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Stakeholder Concern: about whether HFEs and low minimum flows 

under the 1996 ROD of 5,000 cfs (142 m3/s) between 07:00 p.m. and 

07:00 a.m. (versus 8,000 cfs (227 m3/s) might negatively affect the 

Lees Ferry fishery and foodbase?

Initially addressed by GCMRC (Kennedy & VanderKooi, 2012)

“It is our professional judgment that the effects on food base and rainbow trout of the 

two alternative flow regimes described above would be indistinguishable. Our

judgment about minimal ecological effects is based largely on estimates of how much 

additional streambed is exposed when flows drop from 8,000 to 5,000 cfs. There are 

more than 20 cross-sections in Glen Canyon that have been monitored for geomorphic 

characteristics; the lower flows of 5,000 cfs only reduce the inundated area of the 

stream bed at these measured cross-section by an average of 10 percent (USGS 

unpublished data).”

-03.9-Left -15.2-Right -08.8-Right

So, What’s Changed Since October 2012... ?

...the minimum area estimated increase by 2X

PRELIMINARY DATA

DO NOT CITE
Only a limited subset of the existing channel cross sections are 

located across such low-angle shorelines as seen below, but those 

that do, such as at -14.4 (below Pumphouse Bar), show much greater 

than a 5% wetted width reduction between 8,000 and 5,000 cfs.

-14-Mile Bar (-14.4-Right), below Pumphouse

$

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#

$

$$ $$

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# $

$ $

N

Site Type # Intensive Method $ RAT Method

1000 0 1000 2000 Meters

Lees Ferry

Glen Canyon Dam

Paria River

Four Mile Bar

Ferry Swale

Powerline Bar

Pumphouse

Bar

R-17 & R-18

R-15

R-12 & R-11A

R-4

6 of the 24 Existing Cross Sections were

deemed to be representative of low-angle

channel habitats and were assessed for

habitat area dewatering in 5 study

segments between river miles -03.1 and -

14.4 [total subsample of 4 km] R-14 at this

RTELSS site

not available

Only 2 of 4 RTELSS

Study Sites Could be

Evaluated in this Study
Many Factors May Influence Aquatic 

Productivity in the Tailwater

 Canyon/Channel Geometry (Water Depth & Aspect [light])

 Lake Powell Quality of Water (nutrients, DO, Temp)

 Annual Thermal Regime (variations related to above)

 Turbidity (total suspended sediment & dissolved organics)

 Channel Bed Substrate (bedrock or gravel vs. sand or finer)

 Dam Operations (monthly to seasonal high & low-flow patterns)

 Wetted Channel Area (and its variations related to above)
Many Factors May Influence Aquatic 

Productivity in the Tailwater

 Canyon/Channel Geometry (Water Depth & Aspect [light])

 Lake Powell Quality of Water (nutrients, DO, Temp)

 Annual Thermal Regime (variations related to above)

 Turbidity (total suspended sediment & dissolved organics)

 Channel Bed Substrate (bedrock or gravel vs. sand or finer)

 Dam Operations (monthly to seasonal high & low-flow patterns)

 Wetted Channel Area (and its variations related to above)

These three elements might be important, but not 

the most important parameters to consider

regarding questions about what primarily limits 

aquatic invertebrate diversity & abundance?
There are Clear Longitudinal Variations in Aquatic Resources

(likely related, in part, to channel characteristics)

Rainbow 

trout catch 

per 250-m 

shoreline 

segment

Pre- vs. Post2012 HFE

below Glen 

Canyon Dam

Data: Yard & Korman 

et al. unpublished

Drifting midge 

larvae 

concentrations 

~ 8,000 cfs, 

May 2013 

relative to 

channel-bed 

shear stress

Data: Muehlbauer & 

Kennedy et al. unpublished

Why are “Hot

Spots” hot...

...and why are 

others not?

Chironomidae

PRELIMINARY DATA

DO NOT CITE
Longitudinal Channel Depth Profile

[Glen Canyon Dam to Lees Ferry]

0

20

40

60

80

14 12 10 8 6 4 2 0

LONGITUDINAL PROFILE OF 

GLEN CANYON TAILWATER - MAY 2009

WATER DEPTH IN FEET AT 8,000 FT

3/S

DISTANCE ABOVE LEES FERRY IN MILES

Glen 

Canyon 

Dam

Lees Ferry

X-Section

Locations

APPROXIMATE 8,000 CFS WATER SURFACE

R-4

R-12 & R-11A

R-15

R-17 & R-18

PRELIMINARY DATA Recall that Depth Matters in Light Attenuation & GPP

DO NOT CITE
Longitudinal Channel Depth Profile

[Glen Canyon Dam to Lees Ferry]

0

20

40

60

80

14 12 10 8 6 4 2 0

LONGITUDINAL PROFILE OF 

GLEN CANYON TAILWATER - MAY 2009

WATER DEPTH IN FEET AT 8,000 FT

3/S

DISTANCE ABOVE LEES FERRY IN MILES

Glen 

Canyon 

Dam

Lees Ferry

X-Section

Locations

APPROXIMATE 8,000 CFS WATER SURFACE

R-4

R-12 & R-11A

R-15

R-17 & R-18

X-Sections that Would Ideally Represent some of 

the Other Low-Angle Habitats in Glen Canyon PRELIMINARY DATA

DO NOT CITE
Site Characteristics for Five Segments Evaluated

(Δ’s in shoreline area inundated from 8,000 to 5,000 cfs)

STUDY SITE

NAME

FOUR-MILE BAR 

(-04.1 to -03.1)

9-MILE DRAW & 

HORSESHOE 

BEND (-10.6 to -

09.1)

PROP BAR (-12.4 

to 11.8)

POWERLINE BAR 

(-13.9 to 13.5)

FOURTEEN-MILE

BAR (-14.5 to -

14.0)

Approximate Site 

Length (m) 1000 1500 600 400 500

X-Section(s) & 

(River Miles) for 

study of lowangle shorelines

R-5 (-04.4)

R-4 (-03.2)

R-13 (-10.6)

R-12 (-10.2)

R-11A (-09.6) 

R-11 (-09.1)

R-15 (-12.4) R-17 (-13.8) R-18 (-14.4)

Vert. Elev. Δ from 

227 to 142 m3/s 

(2000 era) in 

meters

R-5 (0.39 m)

R-4 (0.20m)

R-13 (0.30 m)

R-12 (0.30 m)

R-11A (0.34 m)

R-11 (0.33 m)

R-15 (0.36 m) R-17 (0.38 m) R-18 (0.41 m)

Historic Vertical

Scour 1965-2000 

(m) ~ 150 m3/s 

stage

R-5 (0.70 m)

R-4 (0.60 m)

R-13 (2.20 m)

R-12 (2.15 m)

R-11A (1.30 m)

R-11 (2.0 m)

R-15 (2.05 m) R-17 (2.20 m) R-18 (2.25 m)

Study Segment 

Aspect (annual 

solar insolation)

NW-SE & E-W

(less variable)

E-W & N-S

(less variable)

E-W

(more variable)

E-W

(more variable)

NE-SW

(less variable)

~49% (12 km) of tailwater has “low-angle” habitat – (4 km of that was subsampled)

The 6 

X-Sections

used in the 

assessment

were not 

optiamally

located over

“low-angle”

[11 percent

slopes] 

habitats

of most 

interest, but 

were all that 

exist at present
Preliminary Low-Flow Habitat Area Assessment Results

(Δ’s in shoreline area inundated from 8,000 to 5,000 cfs)

This Estimate is about 10X larger than one made by Yard Initially - 1994

16 hectars is an Area 

Equal to ~35 NFL 

Football Fields...

Again, this is about 2X 

the area initially 

estimated by Kennedy & 

VanderKooi in 2012...

It is still not possible to 

estimate what low-flows 

to 5,000 cfs mean relative 

to fish and food base...

Operations in fall 

months have already 

been as low as 6,000 cfs...

Summer 2013 sand 

inputs & NOV HFE 

might also be 

confounding influences

PRELIMINARY DATA

DO NOT CITE
Low-Flow Inundation - Site Responses

(Δ’s in area/unit length (100 m) - 8,000 to 5,000 cfs)

Responses Vary by Site - Most Low-Flow Sensitive Sites Upstream – Most Robust Downstream

DOWNSTREAM

UPSTREAM

Lees Ferry

Dam

1/7th

Effect

7X

Effect

PRELIMINARY DATA

DO NOT CITE
Historical Channel Incision in Response to Dam 

Operations Increased Low-Flow Areas Upstream

Cross-Section Just Below Duck Island – 9.9-I

Vertical Δ

from 5-8

kcfs is a

2X larger

near dam

than at

FourMile

Bar

(from Grams et 

al. 2007)

Fourteen Mile Bar (14.4-R)

CHANGE IN STAGE (m) AT ~150 

m3/s

FLOW ELEVATION [1965 vs.2000]

X-Section R-11A

(river mile -9.6)

 

Vertical & Lateral Erosion Expanded Width of

X-Sectional Low-Flow Area
Consider Implications of Higher Flows on Habitat

(Δ’s in shoreline area inundated Above 8,000 cfs)

Perhaps Aquatic Habitats (cobble bars) could also be 

Individually Evaluated for Flows above 8,000 cfs?

WY 2008 Spring HFE & Equalization Operations in WY 

2011 Increased Trout Survival (after Korman et al. 2011)

5.5

6

6.5

7

7.5

8

8.5

200 400 600 800 1000 1200

PUMPHOUSE BAR (river miles -14.5 to -14.0) 

Low-Angle Habitat Area Changes with Increasing 

Glen Canyon Dam Releases [X-Section R-18]

y = 3.3956 * x^(0.12725) R= 0.83532 

CUMULATIVE WETTED CHANNEL AREA (ha)

DISCHARGE (m3/s)

25,000 CFS

8,000 CFS

6,000 CFS

5,000 CFS

7,000 CFS

HISTORIC VERTICAL SCOUR OF R-18 

[1965-2000] ~ 2.25 METERS

14

14.5

15

15.5

16

16.5

17

17.5

18

200 400 600 800 1000 1200

FOUR-MILE BAR (river miles -4.2 to -3.2) 

Low-Angle Habitat Area Changes with Increasing 

Glen Canyon Dam Releases [X-Section R-4]

y = 8.2299 * x^(0.10344) R= 0.95438 

CUMULATIVE WETTED CHANNEL AREA (ha)

DISCHARGE (m3/s)

HISTORIC VERTICAL SCOUR OF R-14

[1965-2000] ~ 0.60 METERS

Four-Mile Bar

X-Section R-4

River mile -3.2

Fourteen Mile Bar

X-Section R-18

river mile -14.4

IS THIS

LUNCH?

PRELIMINARY DATA

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Estimating Shorelines w/o a Flow Model

8,000 to 45,000 cfs

Higher Elevation Bars have become More Vegetated under Lower Volumes?

FOUR-MILE 

BAR 

(-4.1 to -3.1)

Estimating

shorelines

over range

of releases

may be 

critical to 

planning

future 

managed

flows &

experiments?

FOURTEEN MILE 

 BAR (-14.4-R)

PRELIMINARY DATA

DO NOT CITE
Preliminary Concluding Thoughts

 Flows at 5,000 cfs have rarely occurred since 1996 (mostly 2003-6 testing)

 No food base monitoring data were collected when they did occur (ugh)

 Wet channel area is reduced in low-angle habitats by ~10% (8 to 5 kcfs)

 This area reduction consists of about 16 hectares or 1,700,000 sq. ft.

 Habitat sensitivity to low-flow changes is highly site dependent

 Sites nearer to the dam appear to be most sensitive to these changes

 More detailed channel geometry data is needed for areas not assessed

 Scientists unlikely to detect effects of daily flows at 8,000 vs. 5,000 cfs

Perhaps Seasonal Timing of Low vs. Higher Flows & Temperature is the ???