Long before electricity was commercially available, water that would otherwise simply go over a waterfall was frequently diverted to provide mechanical energy in water wheels. That is why most of the early industrialization of the United States was along rivers with significant year-round flows. In hydroelectric generation, we use this same principle to turn a turbine, which is then connected to an electric generator.
The primary reason hydroelectric generating facilities were built throughout North America in the middle part of the 20th Century was that they were part of large flood control projects. The addition of electrical generation capability to these flood control facilities made obvious economic sense. History shows that some of the cheapest electricity generated in North America comes from some of these older hydroelectric facilities. Altogether, hydroelectric plants produce 10 percent of utilities' net generation in the United States.
Hydroelectric power requires either a constant source of water flow, or the damming of a stream or river to form a reservoir to provide for a controlled source of water flow. The amount of energy generated depends on the total amount of water flowing through the turbine and the height from which the water is falling before returning to the lower stream or reservoir. This vertical distance between the water level in the reservoir and the turbine below is called head. There are general two distinctions in head: high head hydro facilities have greater than 500 feet of head, and low head hydro facilities have less than 500 feet of head.
Another type of hydroelectric plant is called a pumped storage system. It allows the water in the reservoir to either flow through and force the turbine to spin the generator to make electricity or be reversed and use the generator as a motor to turn the turbine backwards and pump water back through the dam into the reservoir for storage. This type of system works well as a peaking plant where the water is pumped up into the reservoir at night when electricity costs are low and the water is used to generate electricity during the day when the need for electricity is highest.
Many consider hydroelectric facilities to be the most environmentally friendly power generating facilities. After all, the water resource is renewable, and no fuel is burned, thereby eliminating gaseous emissions into the atmosphere, and heat discharges into the atmosphere or water. However, environmental concerns over hydroelectric facilities' impact on fish populations, especially the salmon in the Pacific Northwest, may threaten the full operation of even the best of these facilities.