Declining reservoir elevations following a two-decade drought increase water temperatures and non-native fish passage facilitating a downstream invasion

Abstract River ecosystems are threatened by interactions among river regulation, non-native species, and climate change. Water use has exceeded supply for two decades in the USA’s Colorado River basin draining its two largest storage reservoirs (Lake Powell and Lake Mead). In 2022, after more than two decades of declining reservoir levels and warming downstream river water temperatures, Lake Powell began releasing water from its lower epilimnion into the Grand Canyon segment of the Colorado River. While managers were concerned about the risk of non-native, predatory smallmouth bass (Micropterus dolomieu) entrainment and reproduction, they lacked a quantitative tool to understand entrainment rates and population growth potential under different reservoir management strategies. To fill this void, we developed models in spring 2022 that: 1) predicted propagule pressure at different reservoir elevations, and 2) linked reservoir storage/operations, water temperatures, and smallmouth bass population dynamics to forecast population growth rates under different hydrologic and management scenarios. In the summers of 2022 and 2023, smallmouth bass were documented reproducing in the lower Colorado River for the first time. Our models accurately forecasted adult catch of smallmouth bass in 2022 and 2023 and forecasted that reproduction would occur in both years for the first time in the history of this river segment. Above average runoff in 2023 increased reservoir elevations, however the potential for smallmouth bass establishment remains high because of long-term forecasts of reduced reservoir inflows and lake levels significantly below full pool. Maintaining Lake Powell elevations above 1,088 m (3,570 ft) would likely minimize propagule pressure from the reservoir and would likely create downstream conditions that minimize smallmouth bass population growth. Competing Interest Statement The authors have declared no competing interest.