I don't think anyone opposes hydropower due to reservoirs, the concern is lifecycle species like salmon that are born at the headwaters of rivers, live their adult life in the ocean, and return to headwaters to reproduce. Dams get in the way, and the kill rate of dams even with ladders is enormous. The other big concern I see is access, e.g. canoeing or kayaking rivers, destroying fishing spots, culturally significant sites, etc.