Effects of the 2023 Spring HFE and Balancing 

Flows on Rainbow and Brown Trout 

in Glen Canyon 

Josh Korman, Ecometric Research 

Mariah Giardina, GCMRC 

November 7, 2023 
 

 

 

 

 

 

 

 

 

 

 

Components of LTEMP EIS based on Glen Canyon 

Trout Studies 

• LTEMP EIS imposed temporary ban on spring HFEs (through 2020) over 

concern that they could lead to high abundance of rainbow trout in Glen 

Canyon and ultimately at the LCR inflow reach. Spring 2023 HFE is the first 

since LTEMP EIS ban. 

• Adjustments in monthly volumes for reservoir Equalization/Balancing flows 

defined in the 2007 EIS were not in scope for LTEMP EIS, though effects of 

2011 equalization flows on trout were recognized and part of rationale for 

Trout Management flows (and relevant to upcoming EIS). 

• 2023 operations provide a great opportunity to evaluate the flow –trout 

assumptions behind LTEMP decisions: Do spring HFE’s and balancing

flows increase survival rates of young rainbow trout as they did in 2008 

and 2011? 
 

 

Hydrographs 

2008 

2023 

2011 

• 2008 ~ ROD operations during 

summer (~10-18 kcfs) 

• 2011 equalization: steady flows ~ 22-

25 kcfs 

• 2023 balancing: steadier flows of ~ 18 

kcfs 
 

Rainbow Trout Young-of-Year Catch Rates in Fall 

(Reach 1C – 4 mile bar) 

2023 HFE & 

Balancing 

Flows 
 

Brown Trout Young-of-Year Catch Rates in Fall 

(Reach 1C – 4 mile bar) 

2023 HFE & 

Balancing 

Flows 
 

 

 

 

 

 

 

 

 

So Why No response of Rainbows to HFE and 

Balancing Flows in 2023 Compared to 2008/2011? 

1. Only one spring HFE since high resolution monitoring began (2008). Other factors could have 

contributed to higher juvenile rainbow abundance in 2008 (e.g., phosphorous concentration, more 

spawners). 

2. 2023 HFE (Apr 24) implemented 1.5 months later than 2008 HFE (Mar 5). 

– Higher proportion of fry would be present during 2023 HFE (flushing effect) 

– Higher proportion of fry would be exposed to low prey conditions (due to bed scour) in the 1-1.5 

months following the 2023 HFE compared to 2008 HFE. 

3. Conditions in Glen Canyon in 2023 were very different compared to 2008 

– Brown trout are much more abundant (predation/competition) 

– Macrophytes are much more abundant 

– Algal community is much more diverse 

– Higher water temperature and lower dissolved oxygen concentrations(!) in summer of 2023. 

• Under current conditions, survival of young trout does not appear to be enhanced by latetimed spring HFEs or balancing flows (preliminary). Stay-tuned for population abundance 

and growth story at January 2023 ARM. 
Extra slides for potential questions 
Drivers of Long-Term Trend in Rainbow Trout 

Abundance in Glen Canyon 

 

Boom Bust Boom Bust Boom Bust 

2011 

Equalization 

flows 

Spring HFE& 

High flows 

in 1997 High condition factor 

and SRP concentrations 

2008 Low 

Reductions in 

1st drought 

Phase: 

Spring 

HFE 

2nd drought 

Phase 

Skinny/ 

recruitment 

large within-day 

variation in 

Skinny/Collapse 

(2005/06) 

Collapse 

(2014/15) 

flow (1991+) RTELSS Study NO/TRGD Study 

Rogowski et al. 2023 (AGF) 
 

 

 

 

 

 

 

 

Citations Supporting Hypotheses on Factors Driving 

Rainbow Trout Abundance in Glen Canyon and the LCR 

Inflow Reach 

Effects of 1991+ Steadier Flows and Other Factors on Rainbow Trout Abundance in Glen Canyon 

1. McKinney, T., Speas, D.W., Rogers, R.S., and W.R. Persons. 2001. Rainbow trout in a regulated river below Glen Canyon Dam, 

Arizona, following increased minimum flows and reduced discharge variability. Nor. Am. J. Fish. Manage. 21: 216 -222. 

2. Korman, J., M.D. Yard, and T.A. Kennedy. 2017. Trends in rainbow trout recruitment, abundance, survival, and growth during a 

boom -and -bust cycle in a tailwater fishery. Trans. Am. Fish. Soc. 146:1043 -1057. 

3. Korman, J., Deemer, B.R., Yackulic, C.B., Kennedy, T.A., and M. Giardina. 2022. Drought related changes in water quality surpass 

effects of experimental flows on trout growth downstream of Lake Powell reservoir. Can. J. Fish. Aquat. Sci. 80:424 -438. 

Effects of Flow and Density on Abundance Trends in Glen Canyon and the LCR Inflow Reach 

1. Korman, J., Martell, S.J.D., Walters, C.J., Makinster, A.S., Coggins, L.G., Yard, M.D., and W.R. Persons. 2012. Estimating 

recruitment dynamics and movement of rainbow trout (Oncorhynchus mykiss) in the Colorado River in Grand Canyon using an 

integrated assessment model. Can. J. Fish. Aquat. Sci. 69: 1827-1849. 

2. Korman, J, Yard, M.D., and C.B. Yackulic. 2016. Factors controlling the abundance of rainbow trout in the Colorado River in 

Grand Canyon in a reach utilized by endangered humpback chub. Can. J. Fish. Aquat. Sci. 73:105-124. 

3. Yackulic, C.B., Korman, J., Yard, M.D., and J. Dzul. 2018. Inferring species interactions through joint mark-recapture analysis. 

Ecology 99(4): 812-821. 

2008 HFE Effect 

1. Korman, J. Kaplinski, M. and T.S. Melis. 2011. Effects of Fluctuating Flows and a Controlled Flood on Incubation Success and 

Early Survival Rates and Growth of Age -0 Rainbow Trout in a Large Regulated River. Transactions of the American Fisheries 

Society 140: 487 -505 

Equalization Flow Effect and Carry-on Effect Leading to RBT Population Collapse in 2014/15 

1. Avery L.A., Korman, J., and W.R. Persons. 2015. Effects of increased discharge on spawning and age-0 recruitment of rainbow 

trout in the Colorado River at Lees Ferry, Arizona. N. Am. J. Fish. Manage. 35:671-680. 

2. Korman, J., M.D. Yard, Dzul, M.C., Yackulic, C.B., Dodrill, M.J., Deemer, B.R., and T.A. Kennedy. 2021. Changes in prey, 

turbidity, and competition reduce somatic growth and cause the collapse of a fish population. Ecol. Monog. 91(1). 
HFE

Proportion 

0.10 

0.08 

0.06 

0.04 

0.02 

0.00 

Dec Feb Apr Jun Aug 

backcalculated 

from age-0 catch 

predicted fom 

spawn timing 

relatively poor survival relatively good survival 

Flood 

Scour and Increase in gravel pore space 

burial of redds (habitat) and prey 

Hatch Date 

Effect of 2008 HFE on Seasonal Variation in Early 

Survival Rates 
Considerable Variation in Early Survival Rates Among 

Weekly Cohorts in 2008 Only (spring HFE) 

0.00 

0.02 

0.04 

0.06 

0.08 

0.10 

Dec Feb Apr Jun Aug 

Proportion 

2003 

0.00 

0.02 

0.04 

0.06 

0.08 

0.10 

Dec Feb Apr Jun Aug 

2007 

2008 

HFE

0.00 

0.02 

0.04 

0.06 

0.08 

0.10 

Dec Feb Apr Jun Aug 

Proportion 

2004 

0.00 

0.02 

0.04 

0.06 

0.08 

0.10 

Dec Feb Apr Jun Aug 

back 

calculated 

predicted 

(flow effect) 

Flood 

0.00 

0.02 

0.04 

0.06 

0.08 

0.10 

Dec Feb Apr Jun Aug 

Hatch Date 

Proportion 

2006 

0.00 

0.02 

0.04 

0.06 

0.08 

0.10 

Dec Feb Apr Jun Aug 

Hatch Date 

2009 
Effects of Non-native Fish Suppression Flows and 

the 2008 spring HFE on Early Survival Rates 
0.00 

0.02 

0.04 

0.06 

0.08 

0.10 

Proportion 

Predicted from spawn 

Backcalculated 

Equal survival among weekly cohorts 

Hatch Date Analysis 

• Objective is to relate variation 

in early survival rates among 

weekly cohorts to changes in 

flow between spawning and 

capture of age-0 fish 

Dec Feb Apr Jun Aug 

0.00 

0.02 

0.04 

0.06 

0.08 

0.10 

Proportion 

Unequal survival among weekly cohorts 

Less fish than expected 

(i.e., low survival) 

More fish than expected 

(i.e., high survival) 

Dec Feb Apr Jun Aug 

Hatch Date 
 

High Flows Inundate Vegetated Areas 

Creating ideal habitat for juvenile trout if flows are steady 
 

 

 

Current State of Rainbow Trout 

(Condition Factor) 

Current condition factor 

(Apr ‘23) similar to Apr 

’16, which resulted in good 

recruitment in 2017 under 

normal operations 

What will happen in 2023 

with an HFE and balancing 

flows? 
 

Redd-Based Fry-based Drift 

0.1 

0.3 

0.5 

1.0 

2.0 

4.0 

8.0 

16.0 

0.00 

0.01 

0.02 

0.03 

0.04 

0.05 

0.06 

0.07 

0.08 

0.09 

Dec Feb Apr Jun Aug 

Blackfly and Midge Drift Rate (#/m3 ) 

Proportion of Age-0 Population 

Hatch Date 

spawn pre-HFE 

hatch pre-HFE 

low drift 

spawn pre-HFE, hatch post-HFE, low drift 

Ted Kennedy, USGS, 

Flagstaff, unpublished data 

Effects of 2008 HFE on Food Base for Age-0 Trout 
Effects of 2008 HFE on Food Base for Age-0 Trout 

Ted Kennedy, USGS, 

Flagstaff, unpublished data 
 

 

 

 

Current State of Rainbow Trout 

(Spawning Capability) 

% of RBT <150 mm that 

express gametes when squeezed 

• All observations lead to a prediction of a strong recruitment year for RBT in Glen Canyon in 

2023, and thus possibility of increase RBT abundance at the LCR in later years 

• Uncertainties: 1) later-timed HFE in 2023; 2) much higher BNT abundance; 3) higher 

macrophyte abundance; 4) very different algal community 
 

 

 

 

 

Phosphorous 

• Higher concentrations of soluble reactive phosphorous (SRP) in 2008 and 2011 

may have contributed to food base and trout fry response. 

• SRP is a better/equal predictor of annual rainbow trout recruitment than flowbased metrics (e.g., high flows, fluctuations in flow). 

• But SRP levels partially confounded with higher flows 

C. Yackulic & B. Deemer, USGS, 

Flagstaff, unpublished data