632 / HARLEN BRETZ 



gradient and in the development of canyons.^ Though a much 

 greater volume of water passed through this tract, the gradients were 

 steep enough to draw off the flood and prevent a complete spreading 

 over the area. Much water came to it from the ice margin to the 

 east in Idaho and no estimate can now be made of the length of ice 

 front which contributed to it. There were five or six places of 

 distributary discharge where this flood crossed preglacial divides and 

 one of them^ eventually obtained most of the discharge. Along this 

 distributary route, the glacial flood swept away the loessial hills for 

 a width of 10 to 15 miles and eroded 500 feet into the basalt. 



The glacial drainage route which possessed the highest crossing 

 of the preglacial surface of the plateau is Grand Coulee. It also 

 found the steepest gradient^ and its volume was sufficient with this 

 gradient to cause the deepest erosion in the basalt. Upper Grand 

 Coulee, across the preglacial divide, is 1,000 feet deep. But its 

 floor, after the epoch had closed, was lower than that of any other 

 glacial drainage route at the head. In its early history it drew 

 water from about 40 miles of ice front, but never spread widely. 

 The gradient, determined here by exceptional warping of the 

 basalt, prevented that. There was no noteworthy preglacial stream 

 along its course. Grand Coulee is, therefore, the simplest but 

 grandest case of canyon-cutting by glacial streams on the plateau. 



DEPTH OF GLACIAL STREAM EROSION IN THE SCABLANDS 



Criteria. — The courses of the larger valleys in the mature drain- 

 age system were determined by the warped surface of the basalt. 

 The dominant feature of this warping is the southwestward dip 

 from Spokane River and Columbia River on the north to Snake 

 River on the south and Columbia River on the west. Many 

 minor folds are superposed on this dip slope, so recent geologically 

 that anticlines determine divides and synclines contain stream 

 valleys.4 Commonly, only the major valleys are intrenched in the 



' Cow Creek, Rock Lake, and Creek and lower Palouse River now occupy the most 

 striking of these canyons. 



^ Palouse River Canyon below Hooper. 



3 In one place, 1,000 feet in about a mile. 



■» Examples are Moses Coulee east of Palisades, WUson Creek above Almira, Crab 

 Creek below Corfu, Washtucna Coulee, Lind Coulee below Lind, Snake River near 

 Lewiston and Clarkston, Union Flat Creek, Rebel Flat Creek, and Palouse River 

 above Winona. 



