Electrical energy can be generated from the mechanical energy in a flow of water. Most hydroelectric power is created by the gravitational potential of water which is above sea level, and the tendency of that water to flow downhill. tidal currents are also used.

Generally this refers to generating electrical power by blocking up a river with a dam and then slapping a turbine generator on the resulting pressure differential.

Once, hydroelectric power was politically correct, because it is a renewable energy source. Environmentalists loved it. Now, it is politically incorrect, because the dams hurt fish, and environmentalists hate it.

Go figure.