Predicting an invasion: Smallmouth bass near-term risk below Glen Canyon Dam

Authors: Drew Eppehimer, Charles Yackulic, Lindsey Bruckerhoff, Jian Wang, Kirk Young, Kevin Bestgen, John Schmidt

Year: 2022

Abstract

Invasive fishes are a primary cause of declines in native fish species throughout the United States including the Colorado River Basin. Although dams and reservoirs have dramatically altered the natural ecosystems of regulated rivers, they also serve as barriers to the movement of invasive fishes. Warmwater invasive fishes, such as the smallmouth bass (Micropterus dolomieu), have invaded much of the Upper Colorado River Basin and are abundant in Lake Powell. Historically, Glen Canyon Dam limited propagule pressure of smallmouth bass from the Upper Basin and created thermally unfavorable conditions downriver. Currently, use and management of the Colorado River is facing a paradigm shift due to long-term drought that has decreased storage in the river’s reservoirs creating great uncertainty about critical water supply and limiting efforts to manage the river’s natural resources. Lower elevations in Lake Powell are expected to increase rates of fish passage through the dam and create thermally suitable conditions in Lees Ferry for smallmouth bass. To quantify the risk of smallmouth bass establishment downstream from Glen Canyon Dam, we developed predictive entrainment and thermal suitability models. These models are relatively simple tools based primarily on Lake Powell elevation, which pairs the depth of dam’s intakes with fish depth distributions and thermal profiles. Our model results show that risk of establishment increases with decreasing elevation, and downriver risk increases over time as individuals accumulate at rates that outpace annual mortality. Our models also illustrate that under current and projected elevations, temperatures in Lees Ferry are and will be suitable for smallmouth bass reproduction. Water storage decisions that maintain or increase Lake Powell elevation reduce the potential for smallmouth bass establishment in the Grand Canyon in the short term. Potential responses to an invasion also include deep water, selective withdrawal dam operations to cool the river.