CHANNELED SCABLANDS OF THE COLUMBIA PLATEAU 635 



In addition to the deepening of valleys already existing, the 

 glacial flood actually made, de novo, drainage ways of greater width 

 and depth than any previously developed on the plateau. This 

 happened where divides were crossed and unusually high gradients 

 down the farther slopes were found. Five such places are especially 

 noteworthy: Devils Canyon, Palouse Canyon below Hooper, 

 Drumheller Channels, Othello Channels, and Grand Coulee. 



Preglacial Palouse River joined Snake River at Pasco, its sub- 

 parallelism with the larger stream for 150 miles being structurally 

 determined. The glacial flood from the north entered it in mid- 

 length at several places between Winona and Washtucna. The 

 volume of this flood was more than the valley could carry away. 



miillTTi 



Fig. 10. — Devils Canyon, cross-section a mile and a half south of Kahlotus, show- 

 ing (i) steepened loessial bluffs (33°), (2) narrow scabland above brink of basalt cliffs 

 (3) canyon 450 feet deep and less than one-fourth mile wide, and (4) post-Spokane 

 talus, three-fourths the height of the cliffs. Horizontal ana vertical scale the same. 



Two leaks across the divide to the Snake developed, one near 

 Kahlotus, and one near Hooper, and in both very great gradients 

 were encountered. 



The Devils Canyon distributary, south of Kahlotus, cut 50 

 feet or so through the loess and then by recession of waterfalls over 

 ledges of basalt in the north slope of Snake River Valley, it eroded a 

 canyon 5 miles long, a quarter of a mile wide and 500 feet deep 

 (Fig. 10). Every fall but the lowest retreated completely through 

 the divide. The remaining ledge, like a dam separating the two 

 canyons, is less than 100 feet above the floor of Washtucna Coulee 

 and not half a mile wide.^ An abandoned half of a double cascade 

 stands in mid-length of Devils Canyon (Fig. 4). 



^ The Spokane, Portland and Seattle Railroad tunnels through this ledge. In 

 1916, Washtucna Lake was so high for a time that it overflowed through this tunnel 

 into Devils Canyon. 



