Effects of Bug Flows, Fall HFEs, Compe

Water Quality on Growth Rate

Rainbow Trout in Glen Canyo

Josh KormanEcometric ResearchBridget DeemerGrand Canyon Monitoring &

Research Center, SouthwestBiological Science Center, US

Geological SurveyCharles Yackulic, Ted Kennedy,Mariah GiardinaGrand Canyon Monitoring &

Research Center, SouthwestBiological Science Center, US

Geological Survey

Photo Credit. David Herasimtschuk, ©Freshwaters Illustrated

Funded through Project H. LTEMP Goals: Rainbow Trout Fishery, Humpback Chub,

Recreational Experience, and Western Area Power Administration
Background

• A major objective of GCD AMP is to understand the effects Glen Canyon Dam operations on 

the downstream environment

• Two experimental flows, fall HFEs (5 years) and bug flows (3 years), have been 

implemented repeatedly over the last decade

• These flows are hypothesized to influence growth rates of rainbow trout in Glen Canyon

• Fall HFEs could reduce growth during late fall/winter by scouring the prey base when subsequent 

recovery is expected to be slow

• Bug flows could increase growth could during spring and summer by improving prey base and 

feeding conditions

• Changes in water quality resulting from drought are expected to have negative effects on 

rainbow trout growth

• High water temperatures and low dissolved oxygen levels reduce growth due to negative 

metabolic effects

• Low phosphorous concentrations are expected to reduce benthic production and hence prey 

availability

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Outline of Talk

Part I (Korman)

• Identify the dominant drivers of growth rates of rainbow trout in Glen 

Canyon

• Compare effects of experimental flows with effects of water quality 

• Predict future growth and population/fishery persistence under droughtdriven water quality conditions for 2022+

Part II (Deemer)

• Describe controls on phosphorus export from Lake Powell and 

expectations of future conditions

• Discuss potential confounding between high phosphorus and low 

dissolved oxygen under drought conditions

3
Why Study Growth? 

Growth is an important determinant of rainbow trout abundance in Glen and 

Marble Canyons through effects on survival, maturation, and recruitment

(it also effects the size of fish caught in the fishery)

Prey, Competition, Predation Risk, Flow, 

Water Temperature, Phosphorous, Light

Korman et al. 2021. Changes in prey, turbidity, and competition reduce somatic 

growth and cause the collapse of a fish population. Ecological Monographs.

Crossman et al. 2022. Population reproductive structure of rainbow trout determined 

by histology and advancing methods to assign sex and assess spawning capability. 

Transactions of the American Fisheries Society.

4
Mark-Recapture (TRGD program) Provides Direct 

Observations of Growth in Length and Weight

5

Unpublished 

data, subject 

to change, do 

not cite.

Model predicts variation in growth in length and weight across 51 1-3 month intervals 

over 10 years based on fish size, flow treatment effects (bug flows or fall HFEs), 

competition, average flow, water temperature, phosphorous concentration, and sunlight
Considerable Seasonal and Interannual Variation in Growth

Potential confounding of flow treatments with other factors that change over time

6 Preliminary Data, Do Not Cite

2012 | 2013 | 2014 | 2015 |2016 | 2017 |2018 | 2019 |2020|2021

Monthly Growth in Weight for 300 mm Rainbow Trout
Examples of Trends in Confounding Factors

7 Preliminary Data, Do Not Cite Soluble Reactive Phosphorous Concentration (mg/l)
Very Limited Effects of Annual Implementation 

of Bug Flows or Fall HFEs on Lifetime Growth

2012 2018 2012 2018

Preliminary Data, Do Not Cite 8
no bug flows for

both scenarios

drought SRP (2018-2021) 

for both scenarios

Water Quality:

Phosphorous (P) additions produce twice the growth benefit

relative to bug flows if baseline P is low (drought)

Preliminary Data, Do Not Cite 9
Water Quality: 

Very large negative water temperature effect 

under drought (2022) conditions

345 g

195 g

10

Preliminary Data, Do Not Cite
current condition considerably 

higher than in 2013

Current Rainbow Population is Dominated by Larger Fish that are More Sensitive 

to Temperature Increases, but their Condition is Good

Preliminary Data, Do Not Cite 11

2012 | 2013 | 2014 | 2015 |2016 | 2017|2018 | 2019 |2020| 2021
Conclusions

• Separating effects of flow treatments (bug flows or HFEs) on growth, from other 

temporally varying factors, like competition and water temperature, is possible if 

they are jointly modelled, time series is long, and sample size is large

• Highlights benefit of long-term monitoring of vital rates via mark-recapture

• Simple year-by-year comparisons, which don’t account for non-treatment effects, are not informative

• Effects of bug flows and fall HFEs on rainbow trout growth were small, not 

consistently observed in both length and weight, and may be spurious

• Effects of competition, water temperature, and phosphorous concentration on 

growth were larger, and competition and temperature effects were more certain 

and consistently observed for length and weight

• Higher water temperatures for summer and fall of 2022+ are predicted to result 

in substantive weight loss for larger rainbow trout, leading to poor condition and 

a die-off by late 2023

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Is the Dire Prediction of Severe Weight Loss for 

Larger Rainbow Trout in 2022/2023 Correct?

• Most likely answer is Yes

• Model calibrated to ~10,000 growth observations from Glen Canyon across 

51 one-three month intervals over the last 10 years (2012-2021)

• Caveat: Model predictions are never perfect!

• E.g., observed growth of larger fish between Sep-Nov ‘21 (at 16 0C) was 

higher than predicted by the model. But this anomaly is not unusual (there 

is equivalent lack of fit in some other intervals)

• Projected average water temperatures in late summer and fall of 2022 will 

be much higher (18-19 0C) than ever observed (predicting outside of 

observed range)

13
What Happens if Larger Rainbow Trout Die 

Due to Poor Growth and Condition?

• A large proportion of population will be lost

• Owing to limited recruitment (new births) after 2017, current population is dominated by 

older/larger fish which are more sensitive to high water temperatures and low dissolved 

oxygen

• Recovery of fishery will be slow

• Few young and small (less temperature-sensitive) fish are present to grow into larger size 

classes that support the fishery

• Recovery of catch rates in fishery from 2014 collapse took ~ four years

• Higher brown trout abundance may impede recovery

• Rainbow trout population recovered from collapses in 2006 and 2014, but brown trout 

abundance was much lower

14
Water Quality of GCD Releases 

Under Drought

Gregory Natural Bridge- March 10, 2022

Last above water in ~1969

Escalante arm of Lake Powell 

Campsite for Lake Powell Quarterly trip- March 9, 2022 15
We are drawing increasingly from 

the reservoir surface waters

In summer: 

Warmer water

Less SRP rich water

Higher pH

Figure from Vernieu et al. 2005 

(second version manually 

modified) 

16
Warmer summer temps reflected 

in this year’s WQ predictions

17

From BOR- Glen Canyon Monthly Operations Call– Basin Hydrology 

and Operations. May 25, 2022
How Do Warm Temperatures Overlay 

with Other Water Quality Conditions?

Modified from Dodrill et al. 2016

Average

Cool Warm

Predicted 

Summer 

2022

Modified from Bear et al. 2005

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Lower water levels (and warmer 

temperatures) will generally mean 

less SRP

Average SRP gradient of 2 

μg L-1 per 10 m drop below 

the penstock 

Mechanisms:

Plankton in surface water 

take up phosphorus so less 

is available

Surface waters have higher 

pH/temp so free SRP is 

bound to calcite

Preliminary Data, Do Not Cite 19
Depth-related differences in SRP 

are biologically meaningful

2 μg SRP L-1

20
Bypass release will most-always lead to 

higher SRP availability in Glen Canyon

Bypass Tubes

Penstocks

Preliminary Data, Do Not Cite

• Elevated SRP in Glen Canyon 

due to bypass releases during 

2018 Fall HFE

Detection limit

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How Do Warm Temperatures Overlay 

with Other Water Quality Conditions?

Graphic created by Three Rivers 

Ranch

Based on values in Chapman 1966, 

Raleigh et al. 1964, and Raleigh et 

al. 1966

Predicted 

Summer 

2022

Bear et al. 2005

22
Deltaic Sediment Remobilization: 

low DO events

Plot courtesy of Robert Radtke, Preliminary Data Do Not Cite

• Resuspension 

events generally 

occur during spring 

inflows (May and 

June)

• More recently low 

DO plumes 

developed due to 

large monsoon

View from Highway 95 Near Hite Marina March 2002 (left) and March 2003 (right). Photos 

by John Dohrenwend

0.5

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mg/ Penstock

River Outlet

Km from Dam

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Deltaic Sediment Remobilization: 

low DO events

Graphic created by Three Rivers 

Ranch

Based on values in Chapman 1966, 

Raleigh et al. 1964, and Raleigh et 

al. 1966

Wahweap Profile from 10/20/21

Dissolved Oxygen

Unpublished data, subject to change, 

do not cite.

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Improving our modeling capabilities 

for dissolved oxygen in Lake Powell

• Sediment incubations this summer will quantify 

chemical and biological dissolved oxygen demand 

and phosphorus availability under

• 3 different temperature treatments

• 3 different elevations of Colorado inflow deposits

Anna Fatta measures dissolved 

oxygen and pH in a closed 

bottle incubation.

Bryce Mihalevich will be joining 

USGS to help update the Lake 

Powell CE-QUAL model 25
Low Dissolved Oxygen Will Have 

Largest Impact on Trout Near Dam

 Increase in

maximum DO 

owing to algae 

production

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Same parcel

of water

26

Site Max. DO (mg/L)

Dam (RM-16) 5.65

RM -7.6 7.50

Lees Ferry (RM 

0)

7.74

Slide courtesy of Ted 

Kennedy

Unpublished data, subject

to change, do not cite.
Deltaic Sediment Remobilization: 

another source of SRP?

• Lower pH in plume may also 

elevate SRP

• Associated low dissolved 

oxygen might trump any 

benefit to trout

Wahweap Profile from 10/20/21

pH

Unpublished data, subject to change, 

do not cite.

27
New Era for Water Quality 

May 16, 2022 Profile at Wahweap

28

Unpublished data, 

subject to change, 

do not cite.
Actions to Avoid Population Collapse

Due to Drought Conditions

• Lower temperature and increase oxygen in GCD outflows

• Release some water through jet tubes (which would also elevate SRP)

• Oxygenate water immediately upstream of dam

• Increase prey supply

• Nutrient supplementation

• Implement a spring HFE (uses water, less certain)

• Purpose of these actions would be to avoid ‘catastrophic failure of 

the trout population’

• Objective 4 of Lees Ferry fisheries management plan (AGF-Rogers 2015)

• Under current circumstances these are not ‘enhancement’ actions

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Questions?

Gregory Natural Bridge- March 10, 2022

Last above water in ~1969

Escalante arm of Lake Powell 

Campsite for Lake Powell Quarterly trip- March 9, 2022

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Acknowledgements

• Administrative and logistical support: Carol Fritzinger, Seth Felder, Dave Foster, Scott 

Vanderkooi, Dave Ward

• Field work: Thanks to the many boatmen, biologists, and technicians!

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