Colorado River Aquatic Foodbase Studies at Tapeats Creek,

Grand Canyon National Park, Arizona: A Benthic 

Discontinuity L.E. Stevens, J.H. Holway, and C. Ellsworth (2020)

Stevens, L.E., J.H. Holway, and C. Ellsworth. 2020. Benthic discontinuity between an unregulated tributary and the 

dam-controlled Colorado River, Grand Canyon, Arizona, USA. Annals of Ecology and Environmental Science 4(1): 33-48. 
HOW AND TO WHAT EXTENT DOES GLEN CANYON DAM

AFFECT AQUATIC CRE BENTHIC COMPOSITION, STRUCTURE, AND FUNCTION

IN GLEN AND GRAND CANYONS? 

Dam Vars

Dam existence

Dam operations

Physical Habitat Vars

Sediment/turbidity

Nutrients

Flow variation

Substrata

Temperature

Disturbance regime

Production

Biofilm/epiphytes

Macrophytes

1° Consumers

Allo-, Auto-chthon E

External Vars

Climate variability

Geomorphology

Tributary FX

Channel orientation

Seasonality, PAR

BMI Life

Histories

Egg survival

Larvae

Pupae

Adults

Species diffs

FISH

Predation

Angling

AEM

Competition

Disease

Parasitism
HOW AND TO WHAT EXTENT DOES GLEN CANYON DAM

AFFECT AQUATIC CRE STRUCTURE AND FUNCTION

 Vars IN GLEN AND GRAND CANYONS? 

istence

Dam

Dam ex

Dam operations

Physical Habitat Vars

Sediment/turbidity BMI Life

Nutrients Production FISH Histories

Flow variation Biofilm/epiphytes Predation Egg survival Macrophytes Angling

Substrata Larvae 1° Consumers AEM

Temperature Pupae Allo-, Auto-chthon E Competition

Disturbance regime Adults Disease

Species diffs Parasitism

External Vars

Climate variability

Geomorphology

Tributary FX

Channel orientation

Seasonality, PAR
HOW AND TO WHAT EXTENT DOES GLEN CANYON DAM

AFFECT AQUATIC CRE STRUCTURE AND FUNCTION

Dam Vars AT TAPEATS CREEK? 

Dam existence

Dam operations

Physical Habitat Vars

Sediment/turbidity

Nutrients

Flow variation

Substrata

Temperature

Disturbance regime

Production

Biofilm/epiphytes

Macrophytes

1° Consumers

Allo-, Auto-chthon E

External Vars

Climate variability

Geomorphology

Tributary FX

Channel orientation

Seasonality, PAR

BMI Life

Histories

Egg survival

Larvae

Pupae

Adults

Species diffs

FISH

Predation

Angling

AEM

Competition

Disease

Parasitism
WHY TAPEATS CREEK?

Hofgnecht Benthic Discontinuity (1981): 

• Marked riverward decline in BMI (especially

EPT) richness at GC tributary confluences

Tapeats Creek: A WQ analogue to tailwaters

Measure and experimentally test factors 

affecting the tributary-mainstream BMI 

discontinuity

Why do EPT larvae from the tributary not 

survive in the adjacent mainstream?

Does this help inform dam management?

Hess, drift, and experimental basket 

sampling to evaluate seasonality, water 

quality, flow variation, sedimentation, 

and habitat on benthic 

macroinvertebrates (BMI), esp. EPT
0 30 60 90 120 150 180 210 240 270 300 330 360

0

50

Julian Day in 2017 * Sampling period

100

150

200

250

300

350

400

Modeled Daily Qmax (m

3

500

c)se

450

/

2 June

4 July

*

*

*

Mainstream Q at Tapeats Creek Confluence in 2017

*
June-July transition Upper Outflow Channel (2017)

Low (~8,000 cfs) vs High (~17,000 cfs) 

UOC 10:00 a.m. on 29 May 2017 UOC 17:00 on 3 June 2017
Tapeats Creek Benthos (TC/UVC/DOC):

Few fines, little embeddedness
Colorado River benthos (UCR/LCR):

High embeddeness

Boulders embedded in sand
Mean Velocity and Substrata Among Zones

(Mainstream Habitats were Near-shore (<1.5 m depth)

TC UVC DOC UOC LCR UCR

Microhabitat

TC UVC DOC LCR UCR

Microhabitat

UOC
BMI Composition and Abundance Decreases Strongly 

from Tapeats Creek to the Mainstream

Circle size reflects density/m2
Spatio-Temporal 

Variation of BMI Among 

Zones
BASKET SAMPLER RESULTS

• BMI and EPT abundance and 

richness/L differed only slightly 

between the LCR and TC

• Embeddedness excludes EPT 

larvae from occupying the 

mainstream
Fluctuating Flow Experiment

in Tapeats Creek 

Methods

Basket samplers

21 day trial duration, 6 reps each

Flow changes:

Fixed position (unmoved)

Move 0.5 m vertically and replace

Day up (0.5 m up to near surface)

Night up (0.5 m up to near surface)

Measured BMI, EPT Abundnc and S / L gravel

Results

Qflux: Slight increase in control BMI, but nsd

and EPT nsd

nsd on S for BMI and NSD

Qflux did not appear to affect colonization
0

500

1000

1500

2000

2500

3000

3500

Discharge (m

3/sec)

Date

Discharge (m3/sec) Beg. of LV Beg. of UV Beg. of TC

Pre-dam flows inundated lower Tapeats Creek <25% of time

(1957 modeled inundation based on historical Lees Ferry data)
1965-1991 highly varying flows inundated lower Tapeats

Creek ca. 25% of the time, but daily inundation

(Modeled 1978 inundation, based on historical flow data at Lees Ferry)

0

100

200

300

400

500

600

700

800

900

Discharge (m

3/sec)

Date

Discharge (m3/sec) Beg. Of LV Beg. Of UV Beg. Of TC
0

200

400

600

800

1000

1200

Discharge (m

3/sec)

Date

Discharge(m3/sec) Beg. of LV Beg. of UV Beg. of TC

150

200

250

300

350

400

450

500

550

600

Q m3/sec)

Date

Discharge(m3/sec) Beg. Of LV

Beg. Of UV Beg. Of TC

2017 WY MLFF inundated lower Tapeats Creek ca. <1% of time

(Modeled based on historical flow data at Lees Ferry)

Pre-dam and 1965-1991 ponding likely limited flow direction and 

increased in-mouth sedimentation. Contemporary flows likely 

enhance tributary mouth BMI P and drift into the mainstream.
CONCLUSIONS

• Water quality conditions ~similar to mainstream in Glen Canyon tailwaters, but

sediment grain size patterns and extent of embeddedness differed greatly.

• High densities of in-creek EPT and other BMI decreased markedly in the adjacent

mainstream in this tributary-mainstream discontinuity (TMD).

• Experimental habitat placed in the mainstream allowed larval EPT colonization

~equivalent to that in the adjacent creek within three weeks in July.

• Absence of larval EPT in the mainstream due to limited larval habitat quality

(sedimentation, embeddedness, flow direction, velocity, and life history).

• Decreased BMI in creek occurs independent of daily 0.5 m stage fluctuations

that characterize the mainstream. Out-drift did not increase with 1m stage flux.

• BMI and EPT P and out-drift is likely considerably greater under contemporary

flows than it was either in pre-dam time or from 1965-1991.

• Simultaneous management for a positive sand balance and EPT is unlikely:

managing for aquatic Diptera, amphipods may be the most practicable strategy.

• The intensity of the TMD should be investigated at other CRE tributary mouths.
Acknowledgements

Thanks to WAPA, Argonne Nat’l Lab, 

especially Kirk Lagory and Mario 

Salerno, to MNA, and the NPS 

Permitting Office. We could not 

have accomplished this project 

without the logistical support of the 

commercial recreational 

community, particularly Arizona 

Raft Adventures, Canyoneers Inc., 

Hatch River Runners, OARS Inc., 

and Tour West Inc., and their 

extraordinary staff. Special thanks 

to Louise Fuchs, Sam Goddard, and 

David Sinclair for their assistance, 

athleticism, and enthusiasm for this Questions?

project.