Hells Gate State Recreation Area near Lewiston, Idaho, marks the farthest point upstream that the Missoula Floods' backwater reached on the Snake River -- the eastern edge of an inland sea that briefly stretched from here to the Pacific....
Hells Gate State Recreation Area near Lewiston, Idaho, marks the farthest point upstream that the Missoula Floods' backwater reached on the Snake River -- the eastern edge of an inland sea that briefly stretched from here to the Pacific. When the floods poured through the Columbia Basin and hit the Wallula Gap bottleneck, water backed up the Snake River for over 100 miles, temporarily reversing the river's flow and pushing floodwaters upstream to this point. The Snake River canyon walls near Lewiston show high-water marks and sediment deposits at elevations hundreds of feet above the modern river level, evidence of the extraordinary volume of water that was forced up this normally fast-flowing canyon. The area is named for the narrow, rugged canyon of the Snake River, which even today is one of the deepest gorges in North America. At the confluence of the Snake and Clearwater Rivers near the park, sediment layers record the repeated backwater flooding events. Hells Gate offers a unique perspective on the floods: not the raging torrent downstream, but the eerie spectacle of a major river running backward and rising hundreds of feet in a matter of hours.
Open year-round. Idaho State Parks Motor Vehicle Entry Fee ($7/day non-resident, $5/day resident in 2026; $40 annual Idaho passport). Camping fees additional. Located 4 miles south of Lewiston, Idaho on the Snake River. Includes the Lewis & Clark Discovery Center (free with park entry, summer hours Wed–Sun 9 a.m.–5 p.m.).
Hells Gate is on the Bonneville Flood path, not the Missoula Flood path, this is an important distinction. The Bonneville Flood occurred around 17.4 ka when Lake Bonneville (in present-day Utah) overtopped its northern outlet at Red Rock Pass, sending a single catastrophic flood north down the Snake River through Hells Canyon to the Columbia. Floodwater scoured Hells Canyon and deposited large gravel bars at the canyon mouth; the modern park lies on one of those flood-deposited terraces. The Missoula Floods did not reach this point, they were a Columbia-system event upstream of the Snake-Columbia confluence at present-day Pasco. The park's inclusion in the Ice Age Floods Trail reflects its Bonneville-Flood relevance, which is part of the broader "Ice Age Megafloods" story the trail interprets. The Pomona Basalt columns at the south end of the park (~14 Ma) predate both floods.
No site-specific updates since O'Connor (1993, USGS Prof Paper 1369) on Bonneville Flood hydraulics. The 17.4 ka Bonneville Flood timing is still standard.
No dedicated IAFI chapter in Lewiston; the Palouse Falls Chapter (Pullman/Spokane area) covers the closest interpretation. The Lewis & Clark Discovery Center exhibits do mention Ice Age flood geology alongside expedition history.
Best March through October; summer temperatures match Lewiston's "banana belt" reputation. The Discovery Center is the main interpretive draw; the park's flood-terrace geology is best read from the river bluffs.
Flood-extent overlay shows the maximum reach of the Missoula Floods.
The site in its place along the flood path, with the maximum flood extent drawn over the modern map.
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