As Dams act as Obstruction to the Natural Flow of the River, they Prevent fish from moving along the River (i.e.) either from Sea to River or From River to Sea.
Answer: Dams can destroy the habitat's behind it. They can trap sediment and can bury rock riverbeds. This can harm the environment because fishes lives in the riverbeds being buried. River dams can flood the ecosystem which is also a bad thing.
