The Spokane Flood beyond the Channeled Scablands

Abstract

The channeled scablands of the Columbia Plateau in Washington are extensive elongate denuded tracts of basalt, deeply scored by huge high-gradient glacier-born rivers. As physiographic features they are unique. All the rivers which produced them converged to Snake and Columbia valleys, and the record of the tremendous flood thus engendered has been found all the way from the pjateau scablands to Portland. Columbia River Valley below the confluence of Snake River carried a flood which at Wallula Gateway was 2 miles wide and reached 750 feet above present river level, at Arlington was 7 miles wide and 450 feet above present river level, at Lyle was 3 miles wide and 430 feet above present river level, and at Portland was nearly 20 miles wide and 350 feet above present river level. The field evidence for the flood consists of (1) noteworthy, even spectacular, denudation and erosion of slopes below the upper surface of the flood, (2) spreading of the river in favorable places back among preglacial hills, (3) deposition of great gravel bars, some of them nearly as thick as the flood was deep, (4) damming of Snake River above the entrance of the glacial rivers by a delta built upstream against the course of the Snake, and (5) deposition of the Portland Delta, 350 feet thick, 200 square miles in area, and with bars 100 feet high on its surface.