Chief Joseph Reservoir is located along the Columbia River in southern Okanogan County,Washington (Fig. 108). The general area is a rugged highland, underlain chiefly by pre-Tertiary resistant rocks, and marked by welldefined north-south mountain ranges separated by broad troughs, now well dissected, into which the present main tributary streams, such as the Okanogan and the Sanpoil, have cut deep narrow trenches to the Columbia. The highland, whose summits reach 7000 and even 8000 feet at the international boundary, slopes rather uniformly south and passes beneath the broad expanse of the Columbia Plateau, where flat-lying mid-Tertiary basalt flows conceal the older rocks (Flint, 1935, p. 171). Along the river the country is rather barren of flora and fauna, except for sagebrush and an occasional clump of pine trees (Fig. 109, a). Several miles north of the river the pine forests begin and extend north into Canada.