Entrainment

Entrainment of warmwater nonnative fish through Glen Canyon Dam The rate of entrainment is dependent on the season, reservoir elevation, and thermocline depth. Smallmouth bass are less susceptible to entrainment in the winter because they are largely absent from the forebay at that time of year. As Lake Powell warms in the spring and summer, smallmouth bass move from the shallow bays into the mainstem areas of the reservoir (1). Additionally, as the reservoir elevations decline and the thermocline reaches penstock elevation, fish are brought into closer proximity to the intakes of the dam which increases the risk of entrainment (2). Also, in years with large inflows, the thermocline can reach deeper into the reservoir which also brings fish into closer proximity to the intakes and increases the risk of entrainment (3), (4). How do you know if you are entraining smallmouth bass through the dam? Since smallmouth bass typically stay above the thermocline, any release when the thermocline is at or below penstock level is likely to be entraining bass and the warmer the release the higher the rate of entrainment. Recommendation: Keep reservoir elevations higher during the spring, summer, and fall when smallmouth bass are more susceptible to entrainment or find some way to cool the forebay to keep smallmouth bass away from the dam (see the Temperature Control page).

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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) 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 spring, summer, and fall when smallmouth bass are more susceptible to entrainment with a minimum target of 3543 ft and and an optimal target above 3621 ft. [1]

Links Temperature Control Page Smallmouth Bass Page Green Sunfish Page Striped Bass Page Projects 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 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 Presentations and Papers 2024 Glen Canyon Dam Turbine Outflow Tucker Trawl Netting - 2024 Progress Report Hydroacoustic Monitoring 2023 Characterizing the fish assemblage in the Lake Powell forebay and identifying the potential for nonnative fish escapement into the Lower Colorado River Hydroacoustic Fish Distribution Characterizing the fish assemblage in the Lake Powell forebay and identifying the potential for nonnative fish escapement into the Lower Colorado River Glen Canyon Dam Fish Exclusion: Project Overview 2022 Hydroacoustic Fish Distribution Glen Canyon Dam Fish Escapement Options Glen Canyon Dam Fish Escapement Report Characterizing the fish assemblage of the Lake Powell forebay: identifying the potential for nonnative fish escapement through Glen Canyon Dam and into the lower Colorado River Hydroacoustic Fish Distribution - Lake Powell 2022 Characterizing the fish assemblage of the Lake Powell forebay: identifying the potential for nonnative fish escapement through Glen Canyon Dam and into the lower Colorado River Glen Canyon Fish Escapement Options 2021 Existing and Planned Monitoring of Non-Native Aquatic Species in Lake Powell and Below Glen Canyon Dam Lake Powell Fish Monitoring Study 2021 -2024 Other Stuff