|Fig. 1: Folsom Dam power plant. (Source: Wikimedia Commons)|
The largest source of emissions-free electricity in the United States is hydroelectricity.  Hydroelectric power is generated by the conservation of free-falling water to electricity. In California, hydroelectric power is a major source of electricity - producing 14,000 megawatts. More specifically in 2014, 6 percent of the total in-state electricity was hydroelectricity.  This number unfortunately went down from 12 percent in 2013 because of the California drought. Hydroelectricity depends heavily on rainfall. 
There are 287 hydro generation plants in California. 4 percent of the state's hydroelectricity comes from the Pacific Northwest. Most of the plants in California are located in the eastern mountain ranges where there is more rainfall. There are three larger plants on dams in California called Shasta, Folsom (see Fig. 1) and Oroville that are operated by the states Department of Water Resources and the U.S. Bureau of Reclamation. There are multiple smaller plants that are just operated by utilities. Hydroelectricity. Facilities that are larger than 30 MW of generation capacity are considered large hydros, the rest being small hydros. 
Hydroelectricity can be derived from flowing water from rivers or man-made installations like dams. Kinetic energy is extracted from the flow of water through turbines and turned into electrical energy. High-speed turbines drive the generators that converts the mechanical energy to electrical energy. The water flow and the vertical distance the water falls determine the amount of hydroelectricity that is produced. 
© Sydney Shaw. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 A. Rosenkranz, Hydropower in the United States," Physics 240, Stanford University, Fall 2015.
 "Renewable Energy Essentials: Hydropower," International Energy Agency, 2010.