The Snake River is a major river of the greater Pacific Northwest in the United States.
At 1,078 miles (1,735 km) long, it is the largest tributary of the Columbia River, the largest North American river that empties into the Pacific Ocean. Rising in western Wyoming, the river flows through the Snake River Plain of southern Idaho, then through the rugged Hells Canyon area via northeastern Oregon and the rolling Palouse Hills, to reach its mouth near the Washington's Tri-Cities area, where it enters the Columbia. Its drainage basin encompasses parts of six U.S. states, and its average discharge is over 54,000 cubic feet per second (1,500 m3/s).
Rugged mountains divided by rolling plains characterize the physiographically diverse watershed of the Snake River. The Snake River Plain was created by a volcanic hotspot which now lies underneath Yellowstone National Park, where the headwaters of the Snake River arise. Gigantic glacial-retreat flooding episodes that occurred during the previous Ice Age carved out many topographical features, including various canyons and ridges along the middle and lower Snake River. Two of these catastrophic flooding events significantly affected the river and its surrounds.