The process of generating electricity by harnessing the power of moving water is called hydroelectricity. Hydroelectric power (hydropower) is generated by forcing water that is flowing downstream, often from behind a dam, through a hydraulic turbine that is connected to a generator. The water exits the turbine and is returned to the stream or riverbed. Much of the hydroelectricity in the United States is generated at large facilities and in the Pacific Northwest, where it meets about two-thirds of the electricity demand. In the U. S., hydroelectricity contributes about 10 percent of the total electricity supply.