largest tributary of the Columbia River and one of the most important streams in the Pacific Northwest section of the United States. It rises in the mountains of the Continental Divide near the southeastern corner of Yellowstone National Park in Wyoming and flows south through Jackson Lake along the eastern base of the Teton Range in Grand Teton National Park. Swinging northwest near the mouth of Greys River, it enters Idaho through the Palisades Reservoir. Near Heise the river leaves the mountains and crosses the broad Snake River Plain of southern Idaho, an area covered by lava beds. On the western edge of the state, it is joined by the Boise River. Turning north, it forms the Oregon–Idaho boundary for 216 mi (348 km). From the northeastern corner of Oregon it forms the Washington-Idaho boundary to Lewiston, Idaho, and then turns west to join the Columbia just south of Pasco, Wash., after a course of 1,040 miles (1,670 km).

The lower Snake River, from Weiser to the mouth, flows through a gorge 1 mile (1.6 km) deep known as Hells Canyon (q.v.), the deepest river gorge in North America (see photograph-->). The Salmon River, its largest tributary, joins the Snake near the downstream end of the canyon section.