The remarkable system of abandoned valleys and channels trenched into the basalt of the Columbia Plateau in eastern Washington has long attracted attention. I have had opportunity to examine many parts of this system during three seasons’ study of the drift border and glacial features at the northern margin of the plateau. During that time it gradually became apparent that the published explanations of the genesis of these drainage ways did not meet the requirements of the field data as I saw them, and, accordingly, a fourth season (summer, 1936) was devoted to an intensive examination of one of the principal drainage tracts of the complex. The results of the study are offered herewith, in the hope that the additional data presented may lead toward a more general agreement than was possible before, on the cause and character of the streams that formerly flowed across the plateau. . . .