The Clearwater embayment of western Idaho represents the easternmost encroachment of Columbia River Basalt against the western side of the Rocky Mountains. Several new units of the Columbia River Basalt Group have been identified in the embayment, and the distribution of these, and other previously recognized flows, reflects (1) the location of vents, feeder dikes; and source areas from which the flows were erupted; (2) tectonic events during the eruptive episode, and (3) the development of erosional channels that were filled subsequently by successive flows.
The Lewiston and Stites basins were filled by Wanapum and Saddle Mountains flows erupted in the North Fork of the Clearwater River area. Flows poured into the Stites basin from surrounding plateau surfaces, but they spread into the Lewiston basin after advancing down ancestral Clearwater River canyons. As an ancestral Clearwater River would lose its canyon to an invading flow, water would spill onto the low-lying areas of the Lewiston basin and deposit sediment. This event was repeated several times and resulted in a sequence of alternating flows and intercalated sediment in the Lewiston basin.