U.S. Department of the Interior

U.S. Geological Survey

Project F: Bug Flows 

and food base update

Theodore Kennedy1, Jeff Muehlbauer2, Cheyenne Szydlo1, Anya Metcalfe1

1-US Geological Survey, Southwest Biological Science Center, Grand Canyon 

Monitoring and Research Center

2-US Geological Survey, Alaska Cooperative Fish and Wildlife Research Unit

*This information is preliminary and is subject to revision. It is being provided to meet the need for timely best science. The 

information is provided on the condition that neither the U.S. Geological Survey nor the U.S. Government shall be held liablefor any 

damages resulting from the authorized or unauthorized use of the information.

Glen Canyon Dam Adaptive Management Program Annual Reporting Meeting 

January 24-25, 2023, Phoenix, AZ
Project F: Aquatic invertebrate ecology

 Project Elements and Objectives

 F.1 Aquatic invertebrate monitoring in Marble and Grand Canyons

 F.2 Aquatic invertebrate monitoring in Glen Canyon

 F.3 Aquatic invertebrate monitoring of Grand Canyon tributaries

 F.4 Fish diet studies

 Funding amount and source: $667,051 (AMP)

 Cooperators: Oregon State University

 LTEMP Resource goal: 

Natural Processes-Restore, to the extent practicable, ecological patterns and processes within their range of natural variability, including the natural 

abundance, diversity, and genetic and ecological integrity of the plant and 

animal species native to those ecosystems

This work also supports all LTEMP goals for fish
Goal 2. Natural Processes

Regulated Flows

Natural 

Processes?

Natural Flows

Annual

Seasonal

Daily pattern

all different

From Natural Processes-metrics draft, courtesy 

of Bridget Deemer & Emily Palmquist Preliminary data, subject to change, do not cite

Colorado River Food Webs

Flow regimes are a major driver of Natural Processes 

Damming and regulation of the Colorado River have 

fundamentally altered flow regimes and natural 

processes. 
Why Bug Flows?

 Insect production 

negatively related 

to tides in GC

 Insect diversity 

negatively related 

to tides across 

West

From Kennedy and others 2016 Insect diversity across 18 tailwaters in Western US. Tailwaters with large tides have low insect diversity (EPT)

Abundance of midges in Grand Canyon is predicted by timing of tides.

If timing aligns with egg laying (dusk) relatively high egg survival
Why Bug Flows?

Not enough insect prey for fish

Low diversity, inherently unstable

Food webs built on algae

Food webs of the Colorado River circa 2006-2009.

From Kennedy and others 2014 USGS Fact-Sheet. 

Colorado River downstream of Glen Canyon Dam 

only 1/3 the invertebrate genera of other tailwaters.

From Kennedy and others 2016 BioScience. 
Why Bug Flows?

 Daily hydropower 

flows create “tides”

 Insects lay eggs at 

water line

 When tide drops, 

eggs dry, die

Want to 

mitigate 

this

Illustration by Jeremy Monroe-Freshwaters Illustrated, from Kennedy and others 2016, Bioscience
Why Bug Flows?

Daily Tides

https://www.gcmrc.gov/discharge_qw_sediment/station/GCDAMP/09380000

Bug flows

Median value

over period

Lees Ferry gage

Load-following

Lees Ferry-discharge

Bug flows

Why?

Stabilize near-shore habitats that 

are critical to insect egg laying.
What is a Bug Flow?

 Give bugs the weekends off

 Enhance natural processes to support aquatic insects 

 Improve river health and food availability for fishes

 Weekend stable low flows from May-August

 Reduces impact to hydropower

 Experiment tested 2018-2020 & 2022

 Eggs laid on weekends never dry RM 61 Little Colorado River confluence

https://www.gcmrc.gov/discharge_qw_sediment/station/GCDAMP/09380000

Peak egg laying activity

occurs May-August

“Objectives of Bug Flow 

Experiment: Improve food 

base productivity and 

abundance or diversity of 

mayflies, stoneflies, and 

caddisflies” 

From 2016 Glen Canyon Dam 

EIS, Table 4.
Monitoring Bug Flows in 2022

How to monitor ecosystem 

response over 400 river 

kilometers in remote 

canyon?

M







N

s







o





 Preliminary data, subject to change, do not cite

ark-Recapture studies

Estimate growth rates 

at seasonal intervals

Estimate marginal 

effect of Bug Flow

New: Diet studies in Night fish sampling

2022

etwork of community 

cientists

Light trap 

monitoring of 

aquatic insects

Robust scope

New: Bat monitoring 

added in 2017 Night bug sampling

Network of dissolved 

xygen sensors

Model gross primary 

production in entire 

river

Key ecosystem 

process Dissolved oxygen

Daily time step sensor
Monitoring Bug Flows in 2022

Network of dissolved 

oxygen sensors

 Model gross primary

production in entire

river

 Key ecosystem

process

 Daily time step

How to monitor ecosystem 

response over 400 river 

kilometers in remote 

canyon?

Network of community 

scientists

 Light trap

monitoring of

aquatic insects

 Robust scope

 New: Bat monitoring

added in 2017

Mark-Recapture studies

 Estimate growth rates 

at seasonal intervals

 Estimate marginal

effect of Bug Flow

 New: Diet studies in

2022

Night fish sampling

Night bug sampling

Dissolved oxygen

sensor Preliminary data, subject to change, do not cite

Focus of talk

today.

See also poster

by Megan 

Starbuck

See poster by

Lindsay 

Hansen

For Bats, paper accepted at Journal of 

Wildlife Management pending minor

revision.

For trout, paper accepted at Canadian

Journal of Fisheries & Aq. Sciences
Community science light trapping

 600+ samples 

per year, 

throughout 

Canyon

 Robust dataset 

for tracking 

aquatic insect 

response

From Kennedy and others 2016, Bioscience
Insect response

12

No Bug Flows

2012-2017

Bug Flows

2018-2020

Unpublish Estimates of annual average from mixed effects model ed data, subject to change, do not cite.

 2018-2020 Bug Flows 

 Midges: no change

 Caddisflies: 400% increase

in two of three years

2018-2020 had low phosphorus concentrations,

which is a driver of food base production
Insect response

 2018-2020 Bug Flows 

 Midges: no change

 Caddisflies: 400% increase

in two of three years

2019-high sediment + fall HFE in prio

year = poor growing

conditions?

Unpublished data, subject to change, do not cite. Late Early Emergence

Low High

Sediment

Graph showing relation 

between spring sediment 

and caddisfly emergence; 

emergence timing indicator

of growing conditions

r 

13

No Bug Flows

2012-2017

Bug Flows

2018-2020

Estimates of annual average from mixed effects model
Insect response

 2021 cessation of Bug Flows

 ~50% decline in midges

 no statistical difference in 

caddisflies

Unpublished data, subject to change, 

do not cite.

Decline in midges consistent with hypothesis that 

Bug Flows was supporting midge populations

14

No Bug Flows

2012-2017

Bug Flows

2018-2020

No 

Bug 

Flows

2021 

Estimates of annual average from mixed effects model
2022 Caveats

 Only 50% of samples processed (320 completed out of 600)

 But samples have been randomly pulled for processing

 And models account for variation in sampling effort across years

 Remaining samples would need to have extremely low 

abundance to dramatically decrease numbers that you are 

about to see...

Unpublished data, subject to change, 

do not cite.
Insect response

 2022 Bug Flows 

 80% increase in midges*

 120% increase in caddisflies*

Consistent with hypothesis

that Bug Flows supporting 

aquatic insect populations

16

s)

No Bug Flows

2012-2017

Bug Flows

2018-2020

No 

Bug 

Flows

2021 

Estimates of annual average from mixed effects model

Bug 

Flows

2022 

Unpublished data, subject to change, do not cite.

Note that 2021 & 2022 had similar environmental 

conditions

-high suspended sediment (enough to trigger HFE

-warm water (1st and 2nd warmest years since dam

filling in 1960s) *only 50% of samples processed,

numbers will change
Smallmouth bass

2000-LSSF

• Water temperatures below critical thresholds for HBC growth 

(12C) and spawning (16C). 

• Modest warming, if drives temps above thresholds, may be 

ecologically meaningful

Max temp:

11C

https://www.gcmrc.gov/discharge_qw_sediment/station/GCD

2022-Bug Flows

• Release temperatures near optima for SMB

• Biggest risk factors are high rates of passage through dam and warm, 

canyonwide temperatures

• Modest nearshore warming (2d/week, 4 mo/yr) does not represent meaningful 

risk factor in comparison

Max temp:20+C

AMP/09380000
Conclusions

 Bug Flows appears to be a useful 

tool for enhancing natural 

processes that sustain aquatic 

insect populations and the 

Colorado River ecosystem

No Bug Flows

2012-2017

Bug 

Flows

Bug 

Flows

Kennedy’s professional opinion: SMB represent far greater threat to 

native fish conservation than low diversity/production of prey base. 

SMB Flows take precedence over Bug Flows. 

Unpublished data, subject to change, do not cite.