passage: Snake River -- 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 108,000 square miles (280,000 km) 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.