Small dams, hydropwer and fish can co exist, though there is a loss in 'efficiency' in extracting the free power embodyed in flowing water. Undershot wheels need to be relativly large and also leave room for fish to swim underneath them, but thats it, the wheel turns, the fish swim on by. The real problem are 'mid sized' hydropower turbines which cut of many large water sheds to fish migration, and I have been involved with water protectors in various ongoing attempts to remediate or remove varios dams and 'water control' aperatus, and the push back from entrenched intrests is rabid, anti everything but wealth and privlage, and the 'legal rights' to water sheds underpins many an unearned fortune, and perfectly illustrate the very ancient concept of Hydraulic Despotism.