Experimental Bug Flows Enhance Natural Processes That 

Sustain The Colorado River Ecosystem 

*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.

Ted Kennedy1, Jeff Muehlbauer2, Anya Metcalfe1, Bridget Deemer1, 

Morgan Ford1

, Cheyenne Szydlo1, Kate Behn1, Charles Yackulic1

U.S, Department of the Interior

U.S. Geological Survey

1-U.S. Geological Survey,

Southwest Biological Science

Center, Flagstaff, AZ

2-U.S. Geological Survey,

Alaska Cooperative Fish and

Wildlife Research Unit,

Fairbanks, AK
 Outline 

Background 

Lees Ferry fishery 





 Grand Canyon 

From Ellsworth 2023, 3 minutes ago... 

My talk will also cover 
-

 

 

 

 

 

Why Bug Flows? 

 Fish are food 

limited 

 Very few insects 

 Food webs built 

upon algae 

From Kennedy and others 2016, Bioscience 

Food webs of the Colorado 

River circa 2006 2009. 

Modified from Cross and 

others 2013, Ecological 

Monographs 

Humpback chub Insects play critical role in river food webs; Baxter and others 

2005, Freshwater Biology 
 

 

 

 

 

 

 

Why Bug Flows?

Because load following... 

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. 

High and low 

flows eliminated 

Load-following 

causes 

daily tides 

From Fairly and others, Metrics draft dated March 2023, 

Figure courtesy Bridget Deemer & Emily Palmquist 

Conceptual model of select Natural Processes 

at the Little Colorado River confluence 

Unpublished data, subject to change, Figure courtesy of Diana Valentine 

do not cite. 
 

 

 

Why Bug Flows?

Because Load Following... 

Bug flows 

Flow epochs 

Median value 

over epoch 

Daily stage change (ft) 

From Fairly and others, Metrics draft dated March 2023, figure courtesy of Anya Metcalfe 

Unpublished data, subject to change, do not cite. 

Figure courtesy of Diana Valentine 

Bug Flows restores discharge 

to natural range of variability 

(i.e., no/minimal tides) 
 

Why Bug Flows?

Because Load Following... 

Unpublished data, subject to change, do not cite. 
Why Bug Flows?

Because Load-Following... 

 Insect diversity

negatively related 

to tides across 

western US C 

From Kennedy and others 2016, Bioscience 
 

 

 

 

 

What Is A Bug Flow? 

 Give bugs the weekends off 

 Weekend stable low flows from May-August 

 Minimizes impact to hydropower 

 Experiment tested 2018-2020 & 2022 

 paused in 2021 for Science Advisor review 

Adult caddisfly abundance 

May-August 

 Restores discharge to natural range of variability (no tide) 

“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. 

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

Lees Ferry Fishery 

 Long-term Invertebrate Drift 

 Monthly since 2008 

 Long-term Trout Growth Studies 

 Seasonal since 2012 

Humans collecting invertebrate drift 

Rainbow trout collecting invertebrate drift 
 

 

 

Lees Ferry Fishery 

Caveats 

-No increase in blackflies was predicted 

-Drift is imperfect measure of food availability in 

Lees Ferry (next slide) 

-Yard et al. 2022 (next slides) demonstrates trout 

consumption has huge impact on invertebrate 

drift concentrations 

-Therefore, to evaluate Bug Flows in Lees Ferry 

focus on trout growth and angling 

“Annual average drift concentrations for midges aad blackflies during Bug Flows 

are the three lowest years on record...” From Bug Flow synthesis report (2021) 

Unpublished data, subject to change, do not cite. Humans collecting invertebrate drift 
 

 

Drift Nets Are Imperfect Predictor Of Diet 

Humans collecting invertebrate drift Rainbow trout collecting invertebrate drift 

Unpublished data, subject to change, do not cite. 
 

 

Trout Consumption Estimates Derived From 

Growth Measurements 

Rainbow trout collecting invertebrate drift 

Yard, Michael D., Charles B. Yackulic, Josh Korman, Michael J. Dodrill, and Bridget R. Deemer. "Declines in prey production 

during the collapse of a tailwater Rainbow Trout population are associated with changing reservoir conditions." Transactions of 

the American Fisheries Society 152, no. 1 (2023): 35-50. 
 

 

 

 

Trout Are Way Better At Sampling Drift Than Humans 

Model estimates of drift that 

trout might collect in their mouths 

Model estimates of drift that 

we might collect in our nets 

Yard, Michael D., Charles B. Yackulic, Josh Korman, Michael J. Dodrill, and Bridget R. Deemer. "Declines in prey production 

during the collapse of a tailwater Rainbow Trout population are associated with changing reservoir conditions." Transactions of 

the American Fisheries Society 152, no. 1 (2023): 35-50. 
 

 

Bug Flows and Trout Growth 

 Based on NO/TRGD

mark/recap studies

spanning 2012-2022

 51 seasonal growth

intervals, 5 of which

include Bug Flows

 Estimate marginal

effect of:

 Bug Flow, fall HFE,

competition, discharge,

phosphorus, P*light,

and temperature

Korman, J., Deemer, B. R., Yackulic, C. B., Kennedy, T. A., & Giardina, M. (2022). Drought related 

changes in water quality surpass effects of experimental flows on trout growth downstream of Lake Powell 

reservoir. Canadian Journal of Fisheries and Aquatic Sciences, (ja). 
 

 

 

 

 

 

 

Bug Flows Increased Trout Growth 

Growth in length Growth in weight 

Bug Flow 

Korman, J., and others. (2022). 

Significant 

positive effect on 

growth in length 

Estimate of growth in weight 

had positive sign but 

overlapped zero (not 

statistically significant) 

“In our study, [Bug Flows] only had the potential to affect growth rates in 5 of 51 trip intervals clustered near 

the end of our 10 year study when spring and summer SRP levels were consistently low due to effects of a 

persistent drought. The resulting unbalanced design matrix led to partial confounding of SRP and SFE effects, 

which increased uncertainty in the SFE effect size.” 
Conclusions 

Rainbow Trout Fishery 

 Results consistent with LTEMP goal 

 “Achieve a healthy high-quality recreational rainbow trout fishery in 

GCNRA and reduce or eliminate downstream trout migration 

consistent with NPS fish management and ESA compliance.” 

Bug Flows helps achieve fishery goals by: 

 Improving angling 

 Supporting higher growth in trout length (and possibly weight) 

 But over range of variability (~10C!), warm water decreased growth 

dramatically, and Bug Flows are unlikely to offset negative effects of 

sustained 20+C water 
 

 

 

 

 

 

 

Part II: Grand Canyon

Bug Flows Increased Gross Primary Production

~58% higher GPP on Bug Flow 

weekends relative to 

hydropeaking weekday 

“If increased native fish production is desired in 

Marble and Grand Canyons, other management 

actions could be considered. For example, 

hydroelectric power generation causes large daily 

changes to the Colorado River’s discharge and 

lowers algae production relative to more stable 

discharges (Robert Hall, Jr., and others, unpub. 

data, 2013). Thus, stabilizing the discharge 

regime could lead to increased algae 

production at downstream sites, which may in 

turn have positive effects on invertebrate and fish 

production” 

-From Kennedy and others 2013, Fact-Sheet 

Deemer and others, PNAS-Nexus 2022 
 

 

 

 

Community Science Insect Monitoring 

Figure courtesy of Diana Valentine 

Thank you guides and GCRG!! 

Kennedy and others 2016, Bioscience 

Community science monitoring started in 2012 

~750 samples of adult aquatic insects per year 

Robust dataset for quantifying insect population response to Bug Flows 
 

 

Insect Response 

 2018-2020 Bug Flows 

 Midges: no change 

 Caddisflies: 400% increase 

in two of three years 

 2021 cessation of Bug Flows 

 ~50% decline in midges 

 no statistical difference in 

caddisflies 

No 

Bug 

No Bug Flows Bug Flows Flows 

2012-2017 2018-2020 2021 

Unpublished data, subject to change, 

do not cite. Estimates of annual average from mixed effects model 

19 
 

 

 

 

Science Advisor Review, Jan 2022 

 Dr. A. Ruhi: “...Bug Flows were successful, overall, in 

enhancing natural processes...” 

 Dr. B. Downes: “Experiment successfully met proximate and 

ultimate objectives” 

 Dr. S. Kroll: “...high likelihood the experiment has worked...” 

 Dr. M. Colvin: “The Bug Flows are meeting primary and 

proximate objectives and the science being conducted is 

cutting edge.” 
 

 

 

 

Insect Response 

 2022 Bug Flows 

 137% increase in midges 

 125% increase in caddisflies 

Consistent with hypothesis 

that Bug Flows supporting 

aquatic insect populations 

No 

Bug Bug 

No Bug Flows Bug Flows Flows Flows 

2012-2017 2018-2020 2021

21 

Estimates of annual average from mixed effects model 

75% of samples processed (n = 457) 

Unpublished data, subject to change, do not cite. 

Midges significantly more abundant during Bug Flow years 

Bug Flows marginal effects: z = 23.85, p < 0.001. 

Estimate with Bug Flows = 220 midges/light trap 

Estimate without Bug Flows = 211midges/light trap 
 

Bug Flows Increase EPT% 

EPT% = EPT in sample/Total aquatic insects in sample 

Unpublished data, subject to change, 

do not cite. 

P<0.001 

Significantly higher EPT% in Bug Flow years 

EPT% graph shown in June 2022 & March 2023 Metrics report 
 

 

 

 

 

 

 

 

 

Pre-Bug Flows, Caddisflies Tied To Tributaries 

“The abundance of microcaddisflies was generally low 

throughout the Grand Canyon and declined precipitously with 

distance from tributaries...This suggests that microcaddisflies 

are not well established in the mainstem Colorado River and 

that the majority of adult microcaddisflies captured in light 

traps actually dispersed from tributaries that do support diverse 

aquatic-insect populations (Oberlin et al. 1999).” 

-Kennedy and others 2016, Bioscience 

Unpublished data, subject to change, 

do not cite. 
 

 

Caddisflies Increase With Bug Flows, No 

Longer Tied To Tributaries 

p < 0.001 

Unpublished data, subject to change, 

do not cite. 

Maps showing 

caddisfly 

distribution from 

Jan 2021 ARM 

presentation 
 

 

 

 

 

 

Bug & Bat sampling 2017-2020 

• 1,428 paired bug and bat samples 

between 2017-2020 

• 611 unique sampling dates 

• 46+ participants 

• modeled 12 different physical and 

temporal variables 

• modeled 7 different prey categories 

Metcalfe, Anya N., Carol A. Fritzinger, Theodore J. Weller, Michael J. Dodrill, Jeffrey D. 

Muehlbauer, Charles B. Yackulic, P. Brandon Holton et al. "Insectivorous bat foraging 

tracks the availability of aquatic flies (Diptera)." The Journal of Wildlife 

Management (2023): e22414. 
 

 

 

Aquatic Flies (midges) Best Predictor Of Bat Activity 

Insects play critical role in river food webs; Baxter and others 2005 

Metcalfe, Anya N., Carol A. Fritzinger, Theodore J. Weller, Michael J. Dodrill, Jeffrey D. 

Muehlbauer, Charles B. Yackulic, P. Brandon Holton et al. "Insectivorous bat foraging 

tracks the availability of aquatic flies (Diptera)." The Journal of Wildlife 

Management (2023): e22414. 
 

 

 

Conclusions 

 Bug Flows temporarily restores 

discharge to natural range of

variability (no tides) thereby

enhancing natural processes that 

sustain aquatic insect populations 

and the Colorado River ecosystem 

Unpublished data, subject to change, do not cite. 

Conceptual model of select Natural Processes 

at the Little Colorado River confluence 

Figure courtesy of Diana Valentine