Date of Award:

8-2024

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Watershed Sciences

Committee Chair(s)

Phaedra Budy

Committee

Phaedra Budy

Committee

Casey Pennock

Committee

David Rosenberg

Abstract

Native river fish face threats from habitat fragmentation, water overallocation, invasive species establishment, and powerful synergies amongst these factors. In the Colorado River Basin, USA, these threats coalesce at Glen Canyon Dam (GCD), which impounds the Colorado River to create Lake Powell, a reservoir inhabited by nonnative fish species. There is widespread concern low water level in Lake Powell may allow nonnative fish to escape through the dam, adversely affecting native fish species downstream in the Grand Canyon. In this study we characterized the seasonal distribution of nonnative fishes in the vicinity of GCD and identified environmental conditions that may lead to increased entrainment and survival of such entrainment, to inform invasion risk into the lower Colorado River (entrainment refers to water and potentially fish drawn into and passed through the GCD hydroelectric infrastructure). When the reservoir was cold, most fishes selected gently sloping habitat such as that found in Wahweap Bay (5 km from GCD), and thus winter entrainment risk is likely minimal for most species. During the summer and fall, fish were found in the forebay surface waters within meters of the dam, increasing entrainment when the dam penstock (fixed withdrawal structure) is positioned near the surface, as it was during our study, rather than lower as it is at higher water levels.

To evaluate potential injuries to fish during entrainment, we measured conditions of passage through GCD using a pressure sensor and compared our results to previous studies. We estimated members of Centrarchidae (e.g., Smallmouth Bass) 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. Fish of other families may survive entrainment at higher rates, depending on their physiology. Maintaining water levels higher than those in 2022 and 2023 would decrease both entrainment and survival of entrainment. Under a changing climate the current overallocation of Colorado River water will likely continue to result in conditions suitable for nonnative escapement through CGD, unless and until operational and engineering solutions are implemented.

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