Isolating the compounding effects of dam hydropeaking and climate change on freshwater mussel recruitment

Dam hydropeaking events, which are short-term variations in river flow because of turning turbines on and off to satisfy the sub-daily energy demand, affect riverine ecosystems. Our study area is the mostly forested Dan River in North Carolina where upstream dam operations affect streamflow and where the endangered James spinymussel (Parvaspina collina, JSM) has declined over time; conditions characteristic of many rivers. Wildlife managers across the Eastern U.S., and in North Carolina specifically, are seeking actionable knowledge to improve habitat conditions for freshwater mussels. Here, water is a key societal resource for power generation, but a risk factor during certain time periods of a freshwater mussel's life cycle. This study evaluates the relative contribution of hydropeaking and climate change in affecting freshwater mussel populations, in general, and freshwater mussel recruitment, specifically. We isolated hydropeaking events and calculated various hydrological metrics from the hydrograph (15-min interval) between 2001 and 2022 of the regulated Dan River and compared them to those from the adjacent unregulated Mayo River. Our major finding is that low-flow events, which are decreasing in part due to climate change, are important for freshwater mussel recruitment, and these low flows are being disrupted by dam releases, which are occurring more frequently, that together are correlated with a decrease in the JSM population in the Dan River. This study provides actionable knowledge for decision-makers and river managers to promote sustainable coexistence of human needs (e.g., hydropower generation) and ecological conservation.