Hydropower
Hydropower is energy made from water flowing downhill through a turbine that spins a generator. Most conventional hydroelectric facilities employ dams to block the flow of a river so that the amount of water released and energy generated can be controlled to meet demand. Hydro plants also can use water diverted from the flow of a river.

Dams pose several environmental problems, such as the destruction of ecosystems and disruption of fish and eel migrations. Run-of-river plants like the one shown below are less harmful, but they can still prevent a barrier to migrating wildlife.

Emerging in-stream hydro technologies offer reduced ecological impacts. They employ modified turbines or other spinning structures to capture energy without disrupting the water flow. In local environments, similar technologies situated in tidal currents offer some potential.

A small-scale hydroelectric facility can make electricity without blocking the flow of water.

(Photo courtesy of the National Renewable Energy Lab)