Hydroelectric power, the use of water to power machinery or produce electricity, provides the most renewable energy in the United States and uses alternating current in most modern systems ("hydroelectric power ..."). 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, costing half as much as using nuclear energy, two-fifths the cost. using fossil fuels and a quarter of the cost of using natural gas (“Wind and Water…”). Furthermore, hydroelectric energy is not subject to market fluctuations or embargoes and the average life of a plant 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 disruption of marine life (“hydropower…”). But how exactly does hydroelectricity work? Dams are the main source of energy harvesting through hydroelectricity. There are approximately 80,000 dams in the United States, although not all are active energy producers. There are four main types of hydroelectric plants, all of which require turbines: reservoir, diversion, operation. of the river and storage by pumping. Furthermore, there are two main types of hydroelectric turbines: impulse turbines and reaction turbines ("hydropower..."). In a containment structure, 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, spinning it and then activating a generator that converts...... middle of paper......wer.inel.gov/techtransfer/pdfs/34916 . pdf>."Sustainable energy for your home: small-scale hydroelectric systems." The Environment Center: Promotion of sustainable lifestyles in the South. Network. 04 October 2011. "The future of hydroelectric energy". Macalester College: Private liberal arts college. Network. 03 October 2011. "Which countries use hydroelectric energy?" World Resources Institute. Network. October 4, 2011. “Wind and Water Energy Program: Hydroelectric Resource Potential.” EERE: EERE server maintenance. U.S. Department of Energy, 13 December 2010. Web. 04 October. 2011. .