EROSION BY UNCONCENTRATED WASH 747 



channeling, even where steamlets are sufficiently concentrated to 

 carry their loads and have some power to spare. Even here, however, 

 the failure to cut channels may be due less to the actual withstanding 

 of wear than to the retardation of currents and their continued sub- 

 division, that is, the power which would result from concentration of 

 the water is prevented rather than withstood. This case and all 

 others which are complicated by the nature of the material or by 

 special initial slopes are omitted from this discussion, which is con- 

 cerned only with the simplest and most typical case of gradual con- 

 centration of run-off and its effects. If there is any categorical dif- 

 ference between the wash above the gully-head and that below, it 

 must appear best where there is perfect freedom for either mode of 

 activity to occur. 



Loss of power due to subdivision of stream. — It is a well-recognized 

 principle that the carrying power of a given flow of water is greater 

 when concentrated into a single stream than when subdivided into 

 several streams. Its application to deltas is familiar, in which case 

 it might be shown that although the united stream were able to carry 

 its entire load, a sufficient degree of subdivision into distributaries 

 would bring about a condition in which no one of these could trans- 

 port its sediment; in other words, while the volume of water and 

 sediment are divided arithmetically, the power of the water decreases 

 in a greater ratio. 



The same principle applies equally well to the opposite case, 

 that is, to the union of several streams into one. Applied to this 

 case it may be stated thus: Given a stream whose power is more 

 than necessary to carry its load; suppose this to be formed by the 

 union of smaller streams, each of which in turn was similarly formed, 

 and so on back to the origin of all in unconcentrated wash; previous 

 to a certain degree of concentration, when all streams were below a 

 certain size, all were overloaded and hence unable to cut definite 

 channels. It is a commonplace observation that definite and con- 

 tinuous channels cannot be cut until a certain degree of concen- 

 tration is reached; but the point here emphasized is that this con- 

 dition may be and often is due to actual overloading of the primary 

 streamlets with sediment. 



Sudden change from overloaded to cutting condition. — There is 



