East of the Cascade Mountains the Columbia and Spokane rivers mark the boundary between the Okanogan Highlands to the northward and the Columbia plateau to the southward; The Okanogan Highlands, an outlier of the Rocky Mountains extending westward from the Coeur d'Alene Mountains in Idaho, reach heights of 5000 to 6000 ft.
The Okanogan Highlands, the Columbia plain, the E.
Boundary) are its principal tributaries from the E.; the Yakima (a little above the mouth of the Snake) from the W.; and the Okanogan (in the north central part of the state), from the N.
Slope of the Cascades and most of the Okanogan Highlands are clothed with light forests consisting chiefly of yellow pine, but containing also Douglas fir, cedar, larch, tamarack and a very small amount of oak.
In the eastern part of the Okanogan Highlands there is some western white pine, and here, too, larch is most abundant.