EROSION BY UN CONCENTRATED WASH 749 



for one in that condition, since the making of a gully is its characteristic 

 function. 



The whole area over which rills alone are formed may well be 

 spoken of as subject to " unconcentrated wash," This term is 

 necessarily relative, for even a drop of water represents a degree of 

 concentration and there is no categorical distinction until the cutting 

 stage is reached. The terms "sheet flood" and "sheet wash" are 

 appropriate both in a descriptive and a technical sense for certain 

 phenomena, generally in arid regions, where the run-off from torrential 

 rains descends a slope in visible sheets. The same terms are mis- 

 leading when applied to the ordinary phenomena of unconcentrated 

 wash. The "sheet" in this latter case is rather a net work of con- 

 stantly changing pattern. 



Down-cutting by unconcentrated wash and ^'overloaded streams.'' — 

 It will probably not be questioned that unconcentrated wash may 

 and commonly (perhaps universally) does degrade that part of a 

 slope which lies above gully-heads. If the assumption made above 

 be correct, we then have the case of degradation being performed 

 by currents which, according to our accepted terminology, are over- 

 loaded. It is difficult to deny that this is the case. If it seems to 

 involve a contradiction of terms, it may be necessary to define an 

 overloaded stream (if the term be retained) not as one which deposits 

 at a certain place more than it removes but as one which behaves in a 

 certain way with reference to its load. The main features of such 

 behavior are the building of bars, the shifting of channels, sub- 

 division and braiding, and above all, the inability of the streams 

 to incise any one channel beneath the level upon which it wanders. 



It would be a mistake to assume that all streams which build 

 bars, anastomose, shift their channels and "braid" are of necessity 

 aggrading their valleys or even that they are not degrading them. 

 The relations between these phenomena of "overloaded streams" on 

 the one hand and aggradation on the other is not so simple as that. 

 It may safely be assumed however that some of the conditions which 

 favor anastomosing, etc., are also favorable to aggradation. The 

 classical example of an "overloaded stream" (the Platte) is quite 

 probably aggrading its valley at the present time in that part where 

 the phenomena listed above are most pronounced, and it has surely 



