The Snake is a major river in the greater Pacific Northwest region of 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. 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, with the Missoula Floods carving out Hells Canyon and other features along the middle and lower Snake. 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 and into the Snake, which were central to the lives of those along the Snake below Shoshone Falls. Contact with Europeans introduced horses to these tribes, reshaping their lifestyles 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 Shoshones representing fish was misinterpreted to represent a snake, giving the Snake River its name. Steamboats and later railroads moved agricultural products and minerals along the lower Snake throughout the 19th century and early 20th century. The powerful flow and steep gradient of the Snake has been utilized since the early 20th century to generate hydroelectricity, enhance navigation and provide irrigation water from fifteen major dams that have transformed the lower river into a series of reservoirs, several of which have been proposed for removal to restore some of the river's once tremendous salmon runs.