The Snake River is a major river in the greater Pacific Northwest in the United States. It is the largest and longest tributary of the Columbia River, which is the largest North American river that empties into the Pacific Ocean. Rising in western Wyoming, the river flows westwards through the Snake River Plain, and turns north to empty into the Columbia at the Tri-Cities area of the state of Washington, draining 108,000 square miles (280,000 km2) in parts of six U.S. states. The river is 1,040 miles (1,670 km) long, and its average flow is 56,900 cubic feet per second (1,610 m3/s).
Steep mountains, low hills, deep canyons and predominantly, the flat alluvium of the Snake River Plain characterize the geologically diverse and active watershed of the Snake River. The plain originates from a large volcanic hotspot below the North American Plate, which now lies underneath Yellowstone National Park, the headwaters of the Snake River.
Gigantic flooding episodes that occurred during the previous Ice Age, involving glacially formed lakes spilling into the middle and lower Snake River, carved out Hells Canyon, the Palouse Hills, and many other topographical features along the middle and lower Snake. Two of these catastrophic flooding events affected the river. Pluvial Lake Bonneville spilled catastrophically into the Snake River Plain about 14,500 years ago. The Missoula Floods also played a role in the shaping of the lower Snake's watershed, although its effects were more pronounced along the Columbia River. As far back as 11,000 years, tribes of prehistoric Native Americans lived along the length of the Snake. Salmon from the Pacific Ocean traveled up the Columbia River and into the Snake River, often numbering in the millions. These fish were central to the lives of the people that lived along the Snake below Shoshone Falls.
By the time the Lewis and Clark Expedition crested the Continental Divide and sighted the valley of the Snake's major tributary, the Salmon River, the Nez Perce and Shoshone were the most powerful tribes along the Snake River. At this time, contact with Europeans introduced horses to Snake River Plain tribes, reshaping their lifestyles for the next few hundred years before American settlement of the area. Later American explorers and British fur trappers from the Hudson's Bay Company further changed and utilized the resources of the Snake River basin. At one point, a hand sign made by the Shoshone Indians representing fish was misinterpreted to represent a snake, giving the Snake River its name.