Water used to produce hydroelectric power is classified as an "instream use," since the use occurs in the stream or reservoir. In this case, flowing water turns turbines located "instream" (generally inside of a dam) to produce power. So, unlike other categories of water use, very little water is used up and consumed.
By far, the largest use of water for hydroelectric power was in the Pacific Northwest, along the Columbia River, and in New York, on the Niagra and St. Lawrence River systems.