Hydroelectric energy is generated by water flowing over a turbine, which turns an electric generator and creates current. The force that moves the turbine comes from the flowing or falling water, and the turbine continues to generate electricity as long as it remains in motion.
Most hydroelectric plants are associated with dams because a steady, controlled flow of water works best for generating electricity. A dam lets the operators build up a reservoir of water and allow it through the turbine at controlled speeds. Water flow from a dam can also be adjusted to meet demand. Hydroelectric plants can also take advantage of low demand by pumping water back upstream, storing it in the reservoir for use during peak power demand periods.