ABSTRACT
The hydrology of the United States is discussed under a number of discrete regions that may be considered to serve, in part, as type areas for similar regions in other parts of the world. In general, in the United States, surface water furnishes the main supply of total fresh water. Instream use is the nonconsumptive, in-channel, or nonwithdrawal use of water. The largest instream use of water in the United States is for hydroelectric power generation. Offstream use pertains to water withdrawn or diverted from the stream channel. In many cases a high percentage of offstream water is returned to the stream, but with some aspect of its quality changed. Ground water is used to supplement surface-water supplies and is available mainly from three sources: areas of intersecting fractures in high-elevation mountain reaches; alluvial sediments in intermontane valleys; and alluvial basins.
