Difference between revisions of "Entrainment"

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! <h2 style="margin:0; background:#cedff2; font-size:120%; font-weight:bold; border:1px solid #a3bfb1; text-align:left; color:#000; padding:0.2em 0.4em;">Updates</h2>
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! <h2 style="margin:0; background:#cedff2; font-size:120%; font-weight:bold; border:1px solid #a3bfb1; text-align:left; color:#000; padding:0.2em 0.4em;">[[Media:Eppehimer_etal_2025_Declining_res_elevations_non-native_fish_passage_CJFAS.pdf|Eppehimer et al., 2025, Declining reservoir elevations following a two-decade drought increase water temperatures and non-native fish passage facilitating a downstream invasion: Canadian Journal of Fisheries and Aquatic Sciences]]
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When the reservoir was cold (<10 °C) and well-mixed, most fishes selected gently sloping habitat such as that found in Wahweap Bay (5 km from GCD), and thus winter entrainment risk is low for most species. When the reservoir was stratified (>20 °C surface temp.) fish were active in the forebay epilimnion within meters of the dam, increasing entrainment when the dam penstock is positioned within the epilimnion, as it was during our study, rather than below the thermocline when the reservoir water level is higher.
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Maintaining Lake Powell elevations above 3590 ft would likely minimize entrainment from the reservoir and create downstream conditions that minimize smallmouth bass population growth. <br>
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Smallmouth bass entrainment is predicted to increase with decreasing reservoir elevation until 3490 ft (minimum power pool), after which the dam switches to deeper river outlets which will lower entrainment. The rate of entrainment increases dramatically as reservoir elevation declines below 3530 ft.  
  
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Smallmouth Bass Micropterus dolomieu) may survive entrainment at rates above 50% at low water levels, with survival expected to decrease as the water level rises, but remaining problematically high across all water levels where entrainment is expected. Other fishes may survive entrainment at higher rates, depending on physiology.  
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Recommendation: Keep reservoir elevations above 3530 ft with an optimal target above 3590 ft. [[Media:Eppehimer_etal_2025_Declining_res_elevations_non-native_fish_passage_CJFAS.pdf|(1)]]
  
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Minimizing successful nonnative fish passage through GCD is best achieved through maintenance of water levels higher than those in 2022 and 2023, which would decrease both entrainment and survival of entrainment. [https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1304&context=etd2023]
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! <h2 style="margin:0; background:#cedff2; font-size:120%; font-weight:bold; border:1px solid #a3b0bf; text-align:left; color:#000; padding:0.2em 0.4em;"> [https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1304&context=etd2023 Friesen, Barrett T., 2024, "Invasion Potential of Nonnative Fishes Through a Large Western Dam Into a Prized and Vulnerable Ecosystem" . ]</h2>
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The entrainment risk is reduced in the winter because smallmouth bass are located in shallower warmer water like in Wahweap Bay. <br>
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The entrainment risk is higher in the summer because smallmouth bass are located in the forebay. <br>
  
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Lower reservoir elevations result in a higher entrainment risk because smallmouth bass are found nearer to the penstock withdrawals.
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*At elevations of 3543 ft, the risk of entrainment occurs in 95% of the traces evaluated 
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*At elevations of 3621 ft, the risk of entrainment occurs in only 5% of the traces evaluated
  
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The depth of the epilimnion in the Forebay varies annually as a function of spring runoff, a suite of environmental factors (e.g., Boehrer and Schultze 2008), and water management. Consequently, the minimum reservoir elevation necessary to prevent entrainment of epilimnetic water and associated biota will vary among years and will likely require an adaptive management framework (Williams 2011). Based on this study, minimum reservoir elevations necessary to limit epilimnetic entrainment in 2022 and 2023 were 1083 m (3552 ft) and 1096 m (3594 ft), respectively.
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Recommendation: Keep reservoir elevations higher during the summer when smallmouth bass are more susceptible to entrainment with a minimum target of 3543 ft and and an optimal target above 3621 ft. [https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1304&context=etd2023]
  
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Notably, in both years, the actual reservoir elevation was below these minimum thresholds, indicating that epilimnetic entrainment of some nonnative fishes was possible during portions of both years. A recent study suggested that a minimum water level of 1088 m (3570 ft) would limit Smallmouth Bass entrainment based on analysis of historical captures below GCD (Eppehimer et al. 2024).
 
  
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Considering the full likely range of depth to thermocline (5th to 95th percentiles of USGS historical time series), we suggest avoiding epilimnetic entrainment requires a reservoir water level between 1080 m (3543 ft) at the 5th percentile and 1104 m (3621 ft) at the 95th percentile (note: as this range is based on historical context, it is valid for a stationary climate, and may shift or expand under nonstationarity).  These elevations translate to a substantial difference in water storage, with storage of 10.1 km3 (8.2 M acre-ft) at 1080 m elevation, increasing roughly 75% to 17.9 km3 (14.5 M acre-ft) at 1104 m elevation (Jones and Root 2022).  The degree of water storage required to maintain higher water elevations and limit epilimnetic entrainment has broadscale implications with respect to water supply and hydropower generation in the Colorado River Basin that are beyond the scope of this study. [https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1304&context=etd2023]
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[https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1304&context=etd2023 Friesen, Barrett T., "Invasion Potential of Nonnative Fishes Through a Large Western Dam Into a Prized and Vulnerable Ecosystem" (2024). All Graduate Theses and Dissertations, Fall 2023 to Present. 288. https://digitalcommons.usu.edu/etd2023/288 ]
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*[[Media:Eppehimer_etal_2025_Declining_res_elevations_non-native_fish_passage_CJFAS.pdf|Eppehimer et al., 2025, Declining reservoir elevations following a two-decade drought increase water temperatures and non-native fish passage facilitating a downstream invasion: Canadian Journal of Fisheries and Aquatic Sciences]]
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*[https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1304&context=etd2023 Friesen, Barrett T., "Invasion Potential of Nonnative Fishes Through a Large Western Dam Into a Prized and Vulnerable Ecosystem" (2024). All Graduate Theses and Dissertations, Fall 2023 to Present. 288. https://digitalcommons.usu.edu/etd2023/288 ]
  
 
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Revision as of 12:05, 31 October 2025
Entrainment.png

Entrainment of warmwater nonnative fish through Glen Canyon Dam

Winter: reduced entrainment risk, bass are in shallower warmer water like in Wahweap Bay.
Summer: higher entrainment risk, bass are found in the forebay

Lower reservoir elevations: higher entrainment risk, bass are found nearer to the penstock withdrawals

  • 3543 ft at the 5th percentile
  • 3621 ft at the 95th percentile

Recommendation: Keep reservoir elevations higher during the summer when bass are more susceptible to entrainment [1]

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Eppehimer et al., 2025, Declining reservoir elevations following a two-decade drought increase water temperatures and non-native fish passage facilitating a downstream invasion: Canadian Journal of Fisheries and Aquatic Sciences

Maintaining Lake Powell elevations above 3590 ft would likely minimize entrainment from the reservoir and create downstream conditions that minimize smallmouth bass population growth.

Smallmouth bass entrainment is predicted to increase with decreasing reservoir elevation until 3490 ft (minimum power pool), after which the dam switches to deeper river outlets which will lower entrainment. The rate of entrainment increases dramatically as reservoir elevation declines below 3530 ft.

Recommendation: Keep reservoir elevations above 3530 ft with an optimal target above 3590 ft. (1)

EntrainmentRate.jpg


Friesen, Barrett T., 2024, "Invasion Potential of Nonnative Fishes Through a Large Western Dam Into a Prized and Vulnerable Ecosystem" .

The entrainment risk is reduced in the winter because smallmouth bass are located in shallower warmer water like in Wahweap Bay.
The entrainment risk is higher in the summer because smallmouth bass are located in the forebay.

Lower reservoir elevations result in a higher entrainment risk because smallmouth bass are found nearer to the penstock withdrawals.

  • At elevations of 3543 ft, the risk of entrainment occurs in 95% of the traces evaluated
  • At elevations of 3621 ft, the risk of entrainment occurs in only 5% of the traces evaluated

Recommendation: Keep reservoir elevations higher during the summer when smallmouth bass are more susceptible to entrainment with a minimum target of 3543 ft and and an optimal target above 3621 ft. [2]


Forebay telemetry.png


Forebay sonor.png

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