Hydropower Operations Reduce Alluvial Nesting Habitat and Alter Riverine Turtle Population Demographics

ABSTRACT Hydropower management has altered discharge regimes of large rivers worldwide, reducing sediment mobilization and early‐seral conditions essential for many riverine species. Spiny softshell turtles ( Apalone spinifera ) rely on alluvial habitats for nesting and may serve as sentinel species to assess the effects of regulated flow regimes and sediment dynamics. We compared nesting habitat availability, nest site characteristics, and population structure of A. spinifera in the free‐flowing Yellowstone River and the hydropower‐regulated Bighorn River in Montana. Supervised land‐cover classification of Sentinel‐2A satellite imagery was integrated with field surveys of 504 nests and demographic sampling of 496 turtles across two 85.5 river‐kilometer study reaches. The Yellowstone River supported up to 7.6 times more potential nesting habitat than the Bighorn River, where nearly 80% of nests were concentrated at only nine aggregation sites. Nests were positively associated with gravel substrates (87%), large woody material (70%), and side channel accessibility (65%), which were all prevalent on the Yellowstone River. We calculated minimum nesting areas and quantified a range of nesting habitat availability that reflects differences in geomorphic complexity and sediment supply in each river (112–626 ha Yellowstone River, 50–82 ha Bighorn River). Demographic measures, including sex ratios, size classes, and catch‐per‐unit‐effort, indicate recruitment challenges for the Bighorn River population. Remote‐sensing of alluvial habitats combined with demographic assessments can help guide discharge strategies to restore conditions that support a diverse group of riverine species.