220 



E. C. ANDREWS. 



live rock structures over which it is dragged. So long 

 then as any stream has power to move the total load over- 

 lying any area, large or small, of the channel structures, 

 so long will that stream have power to cut vertically or 

 laterally into the rock structures forming the channel base 

 or side at that particular point, for by moving the total 

 load at that point it drags it over the live rock structures. 

 If the stream flows over the channel debris however with- 

 out urging it along as a whole at any one place, it is not 

 corrading at that spot. Thus when the stream volume is 

 much reduced the corrasion is of very limited extent, while 

 in times of great stream volume, when the load of earth 

 material is rolled along in its entirety at many places, great 

 corrasion of the channel bed and sides ensues. 



Fig. 2 (b). 



Fig. 2 (a). 



Let us consider this a 

 little more in detail inas- 

 much as its proper under- 

 standing is necessary in 

 stream studies so as to 

 prevent any confusion of 

 the terms " corrasion" and 

 "aggradation." 1 



In Fig. 2 (b) let A BOD 

 represent a unit area of a 

 channel bottom, NMOP 

 the upper limit of a load of 

 stream debris, and SS'S"S'" 

 the stream surface. We 

 assume the channel base 

 to be unaffected by outside 

 influences (such as those 



1 As an example we sometimes hear of the " sheet-flood erosion " of 

 wide valley bottoms, when an examination of the evidence shows that 

 "sheet-flood" aggradation (not erosion) of the valley floor is apparently 

 indicated. 





