topographic features oio 



Topography axd Draixage 



The Columbia Ijasalt plateau is separated from several mountain 

 ranges on the north and from the Cascade Eange on the west by the 

 trench in which flows Columbia Eiver. On the east, the plateau abuts 

 ao-ainst and interflnoers with the Coeur d'Alene Mountains of Idaho. 

 When the Miocene basalt flows of this region ceased, the lava plain 

 abutted in a similar fashion against the mountains on the north and 

 west. The cutting of the Columbia Valley between mountains and plain 

 in the great arc known as the "Big Bend" has partially isolated this 

 portion of tl;ie former lava plain and thus given it the character of a 

 plateau. The Columbia Valley north of the plateau is a great canyon. 

 1,500 to 2,000 feet deep, its bottom less than 1,000 feet above the sea. 

 It does not, however, serve for the complete northern boundary of the 

 plateau. Spokane Eiver, its tributary from the Coeur d'Alene Mountains 

 on the east, may be considered as marking the northern limit of the 

 pleateau east of the Columbia in Washington. The canyon of Snake 

 Eiver in Washington marks the southern margin of that portion of the 

 plateau with which this study is concerned. 



AYidespreacl, gentle warping of the Columbia basalt flows in Washing- 

 ton has made a great shallow, structural basin, the rim of which is 

 roughly the margin of the plateau on the west, north, and east. The 

 flows, originally horizontal, are 2,500 feet above tide about the eastern 

 and northern margins and 4,000 to 6,000 feet above tide in the Cascade 

 Eange, along the western margin. The lowest tract in the basin is the 

 general vicinity of the junction of Columbia and Snake rivers. Here 

 the basalt flows dip below the Columbia, perhaps below sealevel. Drain- 

 age escapes to the Pacific through the Columbia Gorge, cut in the Cascade 

 Eange, the greatest water gap on the North American continent. 



The expression "the plateau" will be understood in this paper to denote 

 that portion of the Columbia Plateau bounded by Spokane, Columl^ia, 

 and Snake rivers and the mountains of Idaho. 



There are many lesser basins of structural origin in the great feature 

 above outlined and many sharp flexures, particularly in the western part, 

 near the youthful Cascade Eange. Most of the drainage lines of the 

 plateau are consequent on its warped surface. The Columbia is the great 

 exception. It approaches the rim from the north, but, instead of cross- 

 ing and entering the ])asin^ it swings westward and then southward in 

 the Big Bend already described, before entering the basin. As a conse- 

 quence of this, no waterway of importance enters the Columbia from the 

 concave side of the Big Bend. Drainage of the plateau is centripetal. 



