418 pkterson. PROBLEM OF GULLYING [Ch. 23 



these differences reflect the extreme complexity of the problem and 

 the tenuous and uncertain nature of the evidence available for its 

 interpretation. 



In general two theories have been advanced as the cause of the 

 recent excessive erosion of the West. Advocates of the first ascribe 

 gullying exclusively to land use, or misuse, the chief form of which is 

 overgrazing, but establishment of roads and trails and other activities 

 which locally destroy the protective vegetative cover are also included. 

 From this conception has developed the term "accelerated erosion," 

 now so widely used in the literature on conservation. Advocates of 

 the second theory consider the present cycle merely another in the 

 sequence of similar events that happened previously, each ascribable 

 to slow changes in climate wherein aggradation occurred in wet and 

 degradation in dry periods. Adherents of this theory hold generally 

 that overgrazing merely acted as a trip to set off in advance events 

 which were ultimately bound to happen. It is to be noted that there 

 is no disagreement among advocates of either school regarding the 

 importance of vegetation in controlling erosion, nor does either side 

 deny that vegetation has deteriorated. Opinions are split only on the 

 cause of the deterioration. 



A third suggestion, that increase in stream gradient resulting from 

 regional or differential uplift has been the cause of recent increased 

 erosion, has been considered by some as completely untenable (Bryan 

 and Post, 1937, p. 78) and, by others, has been accorded only minor 

 consideration chiefly because of the difficulty of obtaining substantiat- 

 ing evidence. Another possibility, that irrigation diversion from the 

 many western streams has had an influence on initiating the gully cycle, 

 has not, in the writer's opinion, been given the study it merits. It 

 appears entirely logical to conclude that interference with the regimen 

 of a stream, particularly where diversions have affected the natural 

 protective bank vegetation, might well lead to changes in the parent 

 channel that would be reflected in the tributaries. 



The arguments for and against the two principal theories are vol- 

 uminous, and there is no need for their review except to point out a 

 few of the salient features of each. 



Land Misuse 



It would appear that the occurrence of widespread gullying shortly 

 after the white man deployed his herds across the West was no coin- 

 cidence. According to Bryan (1925), Gregory (1917, p. 130), Thorn- 

 thwaite, et al. (1942, pp. 102-104), and the testimony of many living 



