Hells Canyon is a stretch of canyon on the Snake River between Idaho and Oregon. This canyon, deeper than the Grand Canyon and formed in ancient basalt flows, contains some of the United States' wildest rapids and has provided extensive recreational and scenic boating since the 1920s. The narrow canyon has also provided outstanding dam sites. Hells Canyon became the subject of nationwide controversy between 1967 and 1975, when environmentalists challenged hydroelectric developers over the last stretch of free-flowing water in the Snake River from the border of Wyoming to the Pacific.

Historically Hells Canyon, over 100 mi (161 km) long, filled with rapids, and averaging 6,500 ft (1,983 m) deep, presented a major obstacle to travelers and explorers crossing the mountains and deserts of southern Idaho and eastern Oregon. Nez Percé, Paiute, Cayuse, and other Native American groups of the region had long used the area as a mild wintering ground...