Hydropower, the use of water to power machines or generate electricity, provides the most renewable energy in the United States and uses alternating current in most modern power plants (“Hydroelectricity…”). water cycle and is a clean fuel source; it does not pollute the environment like power plants that burn fossil fuels. It is by far the most efficient way to generate electricity, being half the cost of nuclear power, or two-fifths the cost of nuclear power. using fossil fuels, and a quarter of the cost of using natural gas (“Wind and Water…”). Additionally, hydropower is not subject to market fluctuations or embargoes, and the average lifespan of a facility is 100 years. Hydropower also has many non-energy benefits such as water supply, flood control, navigation, irrigation and recreation. However, it faces many environmental challenges such as impacts on aquatic habitats, aesthetic alterations of landscapes, changes in water quality and disruptions to marine life (“Hydropower…”). But how exactly does hydropower work? Dams are the main source of hydroelectric power. There are approximately 80,000 dams in the United States, although not all of them are active producers of energy. There are four main types of hydroelectric installations, all of which require turbines: impoundment, diversion, exploitation. river and pumped storage. Furthermore, there are two main types of hydroelectric turbines: impulse turbines and reaction turbines (“Hydroelectricity…”). Reservoir is the most common type of hydroelectric plant. In a reservoir facility, a large hydroelectric system uses a dam to store river water in a reservoir. When water is released from this reservoir, it flows through a turbine, spins it, then activates a generator that converts...... middle of paper ......wer.inel.gov/ techtransfer/pdfs/34916. pdf>."Sustainable energy for your home: small-scale hydroelectric systems." The Center for the Environment: Promoting sustainable lifestyles in the South. Internet. October 4, 2011. “The Future of Hydropower.” Macalester College: Private liberal arts college. Internet. October 3, 2011. “Which countries use hydroelectric power? » World Resources Institute. Internet. October 4, 2011. “Wind and Water Power Program: Hydroelectric Resource Potential.” » EERE: Maintenance of the EERE server. US Department of Energy, December 13, 2010. Web. October 4. 2011. .