160 BUREAU OF AMERICAN ETHNOLOGY [BuU. 179 



PHYSIOGRAPHY OF THE PLATEAU 



The Plateau is a large basin with mountain ranges on all sides 

 (see map 1). It includes the eastern slopes of the Cascades from the 

 Eraser River south to Crooked River. Eastv/ard, it extends to the 

 edge of the Plains Area which would be the Flathead River in the 

 north and the continental divide in the south. The exact boundaries 

 on the north have never been set. On the south, the Plateau merges 

 into the Northern Great Basin. 



Most of the Plateau is characterized by horizontal sheets of basaltic 

 lava piled one on top of the other, the result of Miocene eruptions. 

 Pleistocene and recent eruptions have been numerous, but have not 

 materially changed the landforms. Erosion and deposition have gone 

 on, cutting and filling, carving deep canyons and creating flat alluvial 

 valleys. Still the thick layers of lava dominate the landscape. 



The Columbia River, with its major tributary the Snake, is the most 

 important feature in altering the landscape. Throughout the Plateau 

 these two rivers have carved deep canyons along the main courses and 

 side canyons where each tributary joins the main stream. The Co- 

 lumbia is an exotic stream in that it rises in a climate zone different 

 from the Plateau, and carries a tremendous volume of water through 

 a semidesert whose rainfall could never support more than intermit- 

 tent streams. The Columbia and its tributaries made it possible for 

 man to utilize an area which otherwise might have been comparable 

 to the Great Basin in its aridity and barrenness. 



The course of the Columbia River through the Cascades is in the 

 form of a deep gorge with high mountains on both sides. At The 

 Dalles, as one leaves the coastal strip and enters the Plateau, the 

 country changes suddenly, and east of there the trees disappear. The 

 lush vegetation of the gorge gives way to semidesert, and almost true 

 desert conditions. As one proceeds upstream, he can see little change 

 in several hundred miles. The river is entrenched in a basalt-cliff- 

 lined canyon one hundred to one thousand feet below the rolling and 

 broken uplands. Here and there, some miles away from the river, 

 the hills rise to a height capable of intercepting moisture from the 

 Pacific Ocean winds, and thus supporting some vegetation. Other- 

 wise, the canyon and its flanks are barren except for those trees and 

 gardens recently planted by white men. Life is possible in this interior 

 basin only where water is available, and that is limited to those major 

 streams that rise in the mountains that surround the basin. 



Not until the river traveler reached the Okanogan highlands on his 

 way upstream, would he notice much change. Here the river leaves 

 the basalt cliffs and winds through rounded hills that feature bunch- 

 grass and pine in a typical park landscape. Farther to the north 

 the vegetation increases gradually to become more forestlike. 



