Basinwide, nonpoint sources account for about 98 and 99 percent of the total nitrogen and phosphorus, respectively, introduced annually to the upper Snake River Basin. Nonpoint sources account for most of the sediment reaching streams in the basin. Primary nonpoint sources of pollutants in the basin are agricultural activities, confined-animal feeding operations, rangeland grazing, recreational activities, logging, and atmospheric deposition. Agricultural areas, which account for 21 percent of the basin area, are concentrated along the main stem of the Snake River in the downstream parts of the basin.
Agricultural areas contribute pesticides, nitrogen, and phosphorus to streams and ground water.
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.