The Continental Divide, which separates streams draining to the Pacific Ocean from those draining to the Gulf of Mexico and the Atlantic Ocean, follows part of the Idaho-Montana state line in the southern part of the Bitterroot Mountains. Consequently, nearly all the rivers in the state drain toward the Pacific. Most of Idaho lies within the drainage basin of the Columbia River system. The Snake River, which is the chief river in southern and central Idaho, follows a crescent-shaped course for about 790 km (about 490 mi) across southern Idaho. It then swings northward along the Idaho state line and joins the Columbia River in Washington. Major tributaries of the Snake River in Idaho are the Boise, Payette, Clearwater, and Salmon rivers. The main tributaries of the Columbia River in northern Idaho are the Saint Joe and Coeur d’Alene rivers, which form the Spokane River at Coeur d’Alene Lake. The Kootenai, Clark Fork, and Pend Oreille rivers flow for part of their length in Idaho. Small areas in the south drain southward into the Great Basin.
The rivers of Idaho are widely used for irrigation and hydroelectric power production, especially in the south. However, the Snake River is used for navigation as far upstream as Lewiston. Major road and railroad routes follow the principal valleys, although canyons along the Snake, Salmon, and other rivers pose major problems for road builders. The deepest gorge is Hells Canyon, also called Seven Devils Canyon, Box Canyon, Grand Canyon of the Snake River, or Snake River Canyon. The canyon, the deepest in the United States, has a maximum depth of 2,400 m (about 7,900 ft). The river at the point it leaves the state is the lowest point in Idaho, at 216 m (710 ft) above sea level. Lakes are abundant in Idaho, and most of the large ones are located in the Panhandle. The state’s largest lake is Pend Oreille Lake, which covers 344 sq km (133 sq mi). It is fed by the waters of Clark Fork and is drained by the Pend Oreille River.
Other large natural lakes are Coeur d’Alene Lake, Priest Lake, and the Upper and Lower Payette lakes. Reservoirs created by irrigation and power dams include the Dworshak, American Falls, Cascade, Palisades, Brownlee, Blackfoot, Lucky Peak, Island Park, Anderson Ranch, and Arrowrock.
Idaho’s abundant water resources play a key role in the state’s economy because much of the state’s agricultural production is dependent on irrigation. About 1.2 million hectares (about 3.1 million acres) of cropland in Idaho are irrigated. This irrigated area covers 53 percent of all cropland and accounts for an even larger share of Idaho’s income from agricultural production. The Snake River and its tributaries are the chief source of surface water. Dams and reservoirs even out the irregular flow of the rivers by holding back snow meltwater in early spring and summer and by releasing it for use in the dry summer months. Underground water from wells is increasingly being used for irrigation projects in the Snake River Plain. "Idaho" © Emmanuel BUCHOT, Encarta, Wikipedia
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