Power Production

So how do hydroelectric dams work? Generally, flowing water is used to turn a turbine, which turns a shaft in the electric generator, producing electricity.

In some cases water flows down into a reservoir to be pumped back up for later use.

However, in this case, about 10% of the water that flows downhill will be used in household applications which requires a clean source of water.
Source: http://ga.water.usgs.gov/edu/hyhowworks.html