Primary point sources of pollutants in the basin are industrial discharges, municipal wastewater-treatment facilities, and fish farms. Permitted discharges from these point sources during 1985-90 were about 269 billion gallons per year. Although point sources account for only a small percentage of the total nitrogen and phosphorus introduced annually to the upper Snake River Basin, they discharge directly to streams, where nitrogen and phosphorus become immediately available for uptake. During the irrigation season, diversions and irrigation returns reduce streamflows and degrade stream-water quality. Storage reservoirs for irrigation, flood control, and hydroelectric power reduce streamflow velocities and stream habitat to the detriment of native biological communities.
Irrigated agriculture is the primary consumptive use of water in the basin.
About half of these diversions are from five canals at Lake Walcott and Milner Dam. About 7.1 million acre-feet, or 47 percent, of the total irrigation use was withdrawn from ground water. Some water applied to fields for irrigation returns to streams or infiltrates back to the ground water, transporting nutrients, pesticides, and sediment. Fish farms and hydroelectric power facilities are the primary nonconsumptive water uses in the basin. Many fish farms, primarily along the Snake River between Milner Dam and King Hill, use ground water discharging from springs as a source of water.