CHANNELED SCABLANDS OF THE COLUMBIA PLATEAU 639 



cutting by these glacial streams has 

 been overestimated, the depth of the 

 stream to flood across the divide must 

 be correspondingly increased. This 

 view promptly runs into an absurdity, 

 for the less the canyon-cutting is held 

 to have been, the deeper and there- 

 fore more competent to erode the 

 stream must have been. 



One of the best cases for measure- 

 ment is Washtucna Coulee at the head 

 of Devils Canyon. Though there are 

 but two small rock knobs out in the 

 coulee floor, the summit of neither 

 indicating the original valley bottom, 

 there are good rock terraces to record 

 it (Figs. 12 and 13). Near Kahlotus 

 they lie 1,000 to 1,200 feet above tide. 

 The glacial stream here, at the begin- 

 ning of its history, overflowed through 

 the loessial hills to Snake River, at 

 an altitude of at least 1,350 feet. It 

 was, therefore, from 150 to 350 feet 

 deep. Its width averaged at least a 

 mile. 



And this was no ponded condition, 

 for Washtucna Coulee opened widely 

 into Esquatzel Coulee, an even more 

 capacious valley, and Esquatzel in 

 turn into Columbia and Snake valleys, 

 and the glacial waters cut deeply into 

 the bottom of both coulees. Figuring 

 the preglacial floor as 1,000 feet A.T. 

 at Kahlotus and as 675 feet A.T. at 

 Eltopia, 34 miles farther down the 

 valley, the great glacial stream had a 

 gradient of about 10 feet to the mile. 



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