<P> Hydroelectric power stations in the United States are currently the largest renewable source of energy, but the second for capacity (behind Wind power in the United States). Hydroelectric power produced 35% of the total renewable electricity in the U.S. in 2015, and 6.1% of the total U.S. electricity . </P> <P> According to IEA the United States was the 4th largest producer of hydroelectric power in the world in 2008 after China, Canada and Brazil . Produced hydroelectricity was 282 TWh (2008). It was 8.6% of the world's total hydropower . The installed capacity was 100 GW in 2007 . The amount of hydroelectric power generated is strongly affected by changes in precipitation and surface runoff . </P> <P> Hydroelectric stations exist in at least 34 US states . The largest concentration of hydroelectric generation in the US is in the Columbia River basin, which in 2012 was the source of 44% of the nation's hydroelectricity . Hydroelectricity projects such as Hoover Dam, Grand Coulee Dam, and the Tennessee Valley Authority have become iconic large construction projects . </P> <P> Of note, however, is that California does not consider power generated from large hydroelectric facilities (facilities greater than 30 megawatts) to meet its strictest definition of "renewable", due to concerns over the environmental impact of large hydroelectric projects . As such, electricity generated from large hydroelectric facilities does not count toward California's strict Renewable Portfolio Standards . Roughly about 10 to 15 percent of California's energy generation is from large hydroelectric generation that is not RPS - eligible . </P>

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