An assessment of run-of-river hydroelectric dams on mountain stream ecosystems using the American dipper as an avian indicator

Rivers provide numerous ecological and cultural services for humans and wildlife that are vulnerable to increasing demands for freshwater and renewable energy production. Run-of-river (RoR) dams are becoming a common energy production alternative on coastal mountain streams throughout the world, but their ecological impacts remain poorly understood. A well-described indicator of water quality sharing steep mountain stream habitat with RoR developments, the American dipper (Cinclus mexicanus) river bird is an ideal species for studying potential eco-hydrological impacts of RoR dams. We monitored American dipper populations over two years at stream sites with and without RoR dams across three watersheds in coastal British Columbia, Canada. We found that dippers selected regulated streams across seasons, with the headpond and diversion reach frequently used for foraging. Regulated streams also had higher dipper densities in the slow-flowing habitats, a higher occupancy rate of banded dippers, a higher rate of confirmed year-round residents, and a greater proportion of adults; however, fewer hatch-year dippers were recorded on regulated streams, which may indicate lower reproductive potential or juvenile survival. RoR reaches modified by flow abstraction may offer an opportunity for altitudinal migrant dippers to exploit a consistent food resource closer to their high elevation breeding territory, enabling them to take on a “resident strategy” that is more typical of low elevation river habitats. However, the long-term consequences of year-round occupancy and foraging in headponds requires further investigation, as it could have positive or negative impacts on dippers and their associated foodweb. Collectively, our data suggest that high elevation, RoR-regulated streams provide stable year-round habitat for the American dipper and that presence and density of this species can be used to accurately track changes in stream flow and condition following dam installation.