EROSION AND ITS SIGNIFICANCE. 121 



covers, but is apt to be swept away from time to time by 

 violent floods." — (Geikie, Geology, p. 385.) 



44 More work may thus be done by a stream in a day than 

 oould be accomplished by it during years of its ordinary 

 condition."— (Ibid., p. 381.) 



The case however for ordinary streams was first stated 

 scientifically by Gilbert in 1883 (The Topographic Features 

 of Lake Shores, U.S. Geol. Survey,Fifth Ann. Report, 1883-4 

 p. 89). ". . . It gives to the exceptional flood a power 

 greatly in excess of the normal or annual flood. Not only 

 is it true that the work accomplished in a few days during 

 the height of the chief flood of the year is greater than 

 sM that is accomplished duriug the remainder of the year, 

 but it may even be true that the effect of the maximum 

 flood of the decade or generation or century surpasses the 

 combined effects of all minor floods. It follows that the 

 dimensions of the channel are established by the great 

 flood and adjusted to its needs." 



Floods. — This leads us to a definition of the term flood. 

 It is common to hear of "floods" in rocky mountain gorges, 

 of the "sheet flood erosion " of broad valley bottoms and 

 the "floods" in areas of deep alluvium. It is evident that 

 two distinct processes are here implied, the one implying 

 corrasion, the other bringing about an actual deposition of 

 material. We are here mainly concerned with the mean- 

 ing of the term flood as it is related to corrasion, never- 

 theless both cases are considered. 



In physiographical studies the term corrasion implies 

 the mechanical abrasion, quarrying and sapping of the 

 sides and the bases of stream channels; that is, the 

 corrasion of channel structures implies "work" done on 

 them by external forces. For channel structures, such 

 external forces arise as the result of stream action. It is 

 thus evident that a stream which moves the upper layer 



