U.S. Department of the Interior

U.S. Geological Survey

Floods, flows, and the aquatic foodbase

Presenter: Ted Kennedy

Contributors: 

Mike Dodrill, Charles Yackulic, Jeff Muehlbauer
Conceptual Model

Unpublished data, 

subject to change, 

do not cite
Monitoring the foodbase

Invertebrate Drift Insect emergence

Plankton nets

Direct measure of prey availability for drift feeding fishes

Captures EDIBLE, high quality prey 

(midges, blackflies, and Gammarus)

and

INEDIBLE, low quality prey

(worms, mudsnails)

Sticky traps Light traps

Captures adult midges and blackflies,

high quality prey items for fishes

Unpublished data, 

subject to change, 

do not cite
Foodbase Knowledge Assessment

Unpublished data, 

subject to change, 

do not cite

Resource 

Characteristic Specific Measure Status Trend Confidence Rationale: status &

trend

Rationale:

confidence

Recommendations

Invertebrate

Production

Drift and Emergence 

Monitoring

Significant 

Concern Deteriorating High

Condition of HBC, 

and annual number 

of HBC spawners, is 

down and correlated 

with declining food 

base availability. Drift 

and emergence 

monitoring both large 

declines in 

abundance of key 

prey species. 

Declines in key prey 

evident in both drift 

and emergence 

monitoring, and 

across all sites 

(Glen, Marble, and 

Grand Canyon) 

indicating a systemwide constraint on 

aquatic insects. 

Test Spring 

HFEs + 

Bug Flows 

Insect diversity EPT abundance (%) Moderate 

Concern Unchanging High

The Colorado River 

downstream of Glen 

Canyon Dam is one 

of the few 

streams/rivers in the 

western US that 

doesn’t have 

populations of EPT. 

Food base efforts, 

including intensive 

research and

monitoring since 

~2006, indicate 

EPT% is zero/low 

and unchanging 

under MLFF.

Test Bug Flows

Subsequent slides present ratings and 

terminology from the Food Base 

Knowledge Assessment
Lees Ferry Drift Monitoring:

Edible vs. Inedible Prey

Unpublished data, 

subject to change, 

do not cite

EDIBLE, 

HIGH QUALITY

INEDIBLE,

LOW QUALITY 
Spring HFE:

Improving Food Base, Good Condition

Unpublished data, 

subject to change, 

do not cite

Spring HFE

Inedible species declined

All edible species increased
Fall HFEs:

Deteriorating Food Base, Moderate Concern

Unpublished data, 

subject to change, 

do not cite

Spring HFE

Fall HFE

Inedible species abundant

2 of 3 edible species 

extremely rare 
LCR Confluence

Midges and blackflies dominate

Unpublished data, 

subject to change, 

do not cite
LCR confluence: 

Deteriorating Food Base, Significant Concern Average drift biomass (mg/m

3

)

Spring and summer only

Unpublished data, 

subject to change, 

do not cite

Drift biomass decreased by >50% over period of record

Correlates strongly with drop in native fish condition (Yackulic presentation)
Citizen science in Grand Canyon 

2012-2015

Distance from dam (km)

N >3000

From Kennedy and 

others (2016) 

BioScience
Marble and Grand Canyon: 

Deteriorating Food Base, Significant Concern

Unpublished data, 

subject to change, 

do not cite

Midge catch rates decreased by >50% over period of record

Correlates strongly with drop in native fish condition (Yackulic presentation)
Foodbase Knowledge Assessment

Unpublished data, 

subject to change, 

do not cite

Resource 

Characteristic Specific Measure Status Trend Confidence Rationale: status &

trend

Rationale:

confidence

Recommendations

Invertebrate

Production

Drift and Emergence 

Monitoring

Significant 

Concern Deteriorating High

Condition of HBC, 

and annual number 

of HBC spawners, is 

down and correlated 

with declining food 

base availability. Drift 

and emergence 

monitoring both large 

declines in 

abundance of key 

prey species. 

Declines in key prey 

evident in both drift 

and emergence 

monitoring, and 

across all sites 

(Glen, Marble, and 

Grand Canyon) 

indicating a systemwide constraint on 

aquatic insects. 

Test Spring 

HFEs + 

Bug Flows 

Insect diversity EPT abundance (%) Moderate 

Concern Unchanging High

The Colorado River 

downstream of Glen 

Canyon Dam is one 

of the few 

streams/rivers in the 

western US that 

doesn’t have 

populations of EPT. 

Food base efforts, 

including intensive 

research and

monitoring since 

~2006, indicate 

EPT% is zero/low 

and unchanging 

under MLFF.

Test Bug Flows
“The fundamental fish-related 

science question is:

Does the seasonal timing of 

controlled floods influence 

rainbow trout response?”

-From Wright and Kennedy, 

2011, Science-based strategies 

for future high flow experiment

ANSWER: Yes, flood timing influences 

rainbow trout response. Fall-timed floods 

are bad-to-neutral for foodbase.
“The annual status check 

would consist of reviewing 

available monitoring data and 

scientific interpretations. If 

any resource trends are 

deemed to be unacceptable 

by natural-resource managers 

(for example, rainbow trout 

response), other HFE 

implementation options could 

be considered (some 

examples are presented in 

table 1).”

-From Wright and Kennedy, 

2011, HFE Circular

Resource Trends Unacceptable?
HFE Menu

Current HFE 

implementation strategy 

based on option #2.

-From Wright and Kennedy, 2011, HFE 

Circular

But other HFE 

implementation options 

exist, including ones 

that would lead to 

primarily spring-timed 

HFEs
But we’re not playing with a full deck

EPT = 

# oooo EE,PP,TT

tt tttttt # oooo iiiiiiiiii iiiiii iiii

= 

C

Hydropeaking index = Daily CV of discharge over 5+ yrs

From Kennedy and others 

(2016) BioScience
Recruitment limitation

From Kennedy and others 

(2016) BioScience
Hydropeaking strands a lot of midge eggs

Dusk

>1,000,000

eggs

Dawn

>10,000

eggs

Unpublished data, 

subject to change, 

do not cite
Consistent timing of daily tides

400

Distance from dam (km)

200 300

Relative egg survival

0 100

From Kennedy and others 

(2016) BioScience
Spatial periodicity in midge abundance

Midges: 3X greater at nodes

Relative egg survival

From Kennedy and others 

(2016) BioScience
Give bugs the weekend off!!

 Steady/low flows weekends May-Aug (36-38d/yr)

 Periodically create ideal egg-laying conditions

Eggs laid here will never be 

desiccated

Unpublished data, 

subject to change, 

do not cite
Conclusions

1) Foodbase was already bad

-No EPT species for >30years

2) And it’s getting worse

-Midges and blackflies, declining or scarce

-NZMS, increasing or abundant

3) To improve Food Base:

-consider Bug Flows 

-consider Spring HFEs

Unpublished data, 

subject to change, 

do not cite