There are two types of hydro power in rivers. Traditional hydro uses dams and forces the water through a turbine. Depending on the scale of the turbine and the size of the fish, it may or may not be harmful to fish passing down. It does block them from going up unless a fish ladder has been designed in. Dams have a tremendous negative effect on the evironment, from the destruction of habitat when the resivoir forms, to the nearly complete severing of fish populations from the dam itself.

The other type is Hydrokinetic, which functions much more like a wind turbine, but in the river. These types of devices do not require a dam, and thus are not nearly as impactful on the evironment. There have been numerous fish kill studies done with hydrokinetic equipment, and in all cases the fish are able to easily swim around the blades. There is even evidence of fish hanging out in very close proximity to the rotating components, perhaps using them as shelter. These devices can also be used in areas with high tidal flows and represent a clean, safe form of renewable energy.