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 About 7.1 million acre-feet, or 47 percent, of the total irrigation use was withdrawn from ground water.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.
Fish farms contribute nitrogen, phosphorus, and organic wastes to streams.
There is a large amount of variability in the health of fish communities in the upper Snake River Basin. In some streams of the basin, cutthroat, rainbow, and brown trout fisheries represent some of the best fishing in the Nation. In most of the headwater streams assessed, only two to four different fish species were found, primarily trout and sculpin. Excellent water-quality conditions, in combination with stocking of gamefish, make the upper reaches of the Henrys Fork and the Snake River upstream from its confluence with the Henrys Fork some of the best trout fishing streams in the Nation. These streams support healthy populations of cutthroat, rainbow, and brown trout, but are dependent on stocking.