Eastern Washington includes two dissimilar regions—a northern highland area and a southern plateau area. The northern highland consists of high, north-south mountain ranges, separated by broad, and rather straight north-south valleys, sculptured dominantly in crystalline rocks. The range tops rise from about 3000 feet, at the southern end, to more than 7000 feet in places at the northern end, whereas the profiles of the present major streams descend, from 1500 to 2000 feet, in the north and east, to 800 to 1000 feet, in the south and west.
The highland is limited abruptly southward by the Columbia Plateau, whose basalt flows abut against, and lap over on, the highland. No drainage now passes from the highland across the plateau surface, for the contact of basalt and crystalline rock is followed closely by the west-flowing Spokane River and its continuation in the west-flowing Columbia, to which all drainage . . .