Hells Canyon is a ten-mile-wide (16 km) canyon in the Western United States, located along the border of eastern Oregon, western Idaho, and a small section of eastern Washington. It is part of the Hells Canyon National Recreation Area which is also located in part of the Wallowa-Whitman National Forest. It is North America's deepest river gorge at 7,993 feet (2,436 m), running deeper than the Grand Canyon in Arizona.

A canyon, gorge or chasm, is a deep cleft between escarpments or cliffs resulting from weathering and the erosive activity of a river over geologic time scales. Rivers have a natural tendency to cut through underlying surfaces, eventually wearing away rock layers as sediments are removed downstream. A river bed will gradually reach a baseline elevation, which is the same elevation as the body of water into which the river drains. The processes of weathering and erosion will form canyons when the river's headwaters and estuary are at significantly different elevations, particularly through regions where softer rock layers are intermingled with harder layers more resistant to weathering.