From Washington To You
Flood-scoured basalt walls of Wallula Gap on the Columbia River.
Photograph by Umptanum, released to the public domain.

From Washington to you

Wallula Gap

Wallula Gap · Walla Walla County · Ice Age floods

Ice Age floodwaters hit a two-mile gap that could not pass them all — and backed up into a temporary lake 900 feet deep.

All of the Ice Age floodwaters that cut the Channeled Scabland were channeled through Wallula Gap in the Horse Heaven Hills. NPS says more than 833 cubic kilometers (200 cubic miles) of water per day arrived at a gap that could discharge less than 166 cubic kilometers (40 cubic miles) per day. The rest ponded in the Pasco Basin and the Yakima and Touchet valleys as Lake Lewis.

The Ice Age Floods brochure calls the gap two miles (3.2 km) wide. Like the neck of an hourglass, the constriction backed water, ice, and debris into a temporary pond 900 feet (274 m) deep. The National Natural Landmark page says the gap is more than 1.6 kilometers (1 mile) from rim to rim and calls it the largest and most significant water gap through a basalt anticline in the Columbia Basin.

At the height of the floods, NPS estimates flow through the gap at 166 cubic kilometers (40 cubic miles) per day — nearly twice the combined flow of all the present rivers on Earth. The sources below are National Park Service Ice Age Floods pages.

  • Geography & Geology

Sources (2)

Last reviewed: September 21, 2026

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