Snake River - Watershed

Watershed

By length the Snake River is the thirteenth largest river in the United States. Its watershed is the tenth largest among North American rivers, and covers almost 108,000 square miles (280,000 km2) in portions of six U.S. states: Wyoming, Idaho, Nevada, Utah, Oregon, and Washington, with the largest portion in Idaho. Most of the Snake River watershed lies between the Rocky Mountains on the east and the Columbia Plateau on the northwest. The largest tributary of the Columbia River, the Snake River watershed makes up about 41% of the entire Columbia River Basin. Its average discharge at the mouth constitutes 31% of the Columbia's flow at that point. Above the confluence, the Snake is slightly longer than the Columbia—1,078 miles (1,735 km) compared to 928 miles (1,493 km)—and its drainage basin is slightly larger—4% bigger than the upstream Columbia River watershed.

The mostly semi-arid, even desert climate of the Snake River watershed on average, as a whole, typically receives less than 12 inches (300 mm) of precipitation per year. However, precipitation in the Snake River watershed varies widely. At Twin Falls, in the center of the Snake River Plain, the climate is nearly desert, with an annual rainfall of just 9.24 inches (235 mm), although the average snowfall is 13.1 inches (330 mm). This desert climate occupies the majority of the basin of the Snake River, so although it is longer than the Columbia River above the Tri-Cities, its discharge is on average significantly less. However, in the high Rockies of Wyoming, in the upper Jackson Hole area, the average precipitation is over 30 inches (760 mm), and snowfall averages 252 inches (6,400 mm). Most of the Snake River basin consists of wide, arid plains and rolling hills, bordered by high mountains. In the upper parts of the watershed, however, the river flows through an area with a distinct alpine climate. There are also stretches where the river and its tributaries have incised themselves into tight gorges. The Snake River watershed includes parts of Yellowstone National Park, Grand Teton National Park, Hells Canyon National Recreation Area, and many other national and state parks.

Much of the area along the river, within a few miles of its banks, is irrigated farmland, especially in its middle and lower course. Irrigation dams include American Falls Dam, Minidoka Dam, and C.J. Strike Dam. Aside from water from the river, water is also pulled from the Snake River Aquifer for irrigation. Major cities along the river include Jackson in Wyoming, Twin Falls, Idaho Falls, Boise, and Lewiston in Idaho, and the Tri-Cities in Washington (Kennewick, Pasco and Richland). There are fifteen dams in total along the Snake River, which aside from irrigation, also produce electricity, maintain a navigation channel along part of the river's route, and provide flood control. However, fish passage is limited to the stretch below Hells Canyon.

The Snake River watershed is bounded by several other major North American watersheds, which drain both to the Atlantic or the Pacific, or into endorheic basins. On the southwest side a divide separates the Snake watershed from Oregon's Harney Basin, which is endorheic. On the south, the Snake watershed borders that of the Humboldt River in Nevada, and the watershed of the Great Salt Lake (the Bear, Jordan and Weber rivers) on the south. The Snake River also shares a boundary with the Green River to the southeast; the Green River drains parts of Wyoming and Utah and is the largest tributary of the Colorado River. On the western extremity for a short stretch the Continental Divide separates the Snake watershed from the Bighorn River, a tributary of the Yellowstone River, which the Snake begins near. On the north the Snake River watershed is bounded by the Red Rock River, a tributary of the Beaverhead River, which flows into the Jefferson River and into the Missouri River, part of the Gulf of Mexico drainage basin.

The rest of the Snake River watershed borders on several other major Columbia River tributaries - mostly the Spokane River to the north, but also Clark Fork in Montana to the northeast and the John Day River to the west. Of these, the Clark Fork (via the Pend Oreille River) and the Spokane join the Columbia above the Snake, while the John Day joins downstream of the Snake, in the Columbia River Gorge. It is of note that the northeastern divide of the Snake River watershed forms the Idaho-Montana boundary, so the Snake River watershed does not extend into Montana.

Mountain ranges in the Snake watershed include the Teton Range, Bitterroot Range, Clearwater Mountains, Seven Devils Mountains, and the extreme northwestern end of the Wind River Range. Grand Teton is the highest point in the Snake River watershed, reaching 13,775 feet (4,199 m) in elevation. The elevation of the Snake River is 358 feet (109 m) when it joins the Columbia River.