Much of the water lost by the Snake River as it transects the plain issues back into the river at its western end, by way of many artesian springs. Watershed The Snake River is the thirteenth longest river in the United States. Its watershed is the 10th largest among North American rivers, and covers almost 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— compared to —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, receives less than 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 , although the average snowfall is . 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 average discharge is significantly less. However, in the high Rockies of Wyoming, in the upper Jackson Hole area, the average precipitation is over , and snowfall averages . 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