Ch, 23] CAUSES OF GULLYING 



TABLE 1 

 Measueed Progress of Gully Advancement 



417 



CAUSES OF GULLYING 



As gully cutting represents but one aspect of erosion, consideration 

 of its cause involves an inquiry into the broad phases of the erosion 

 problem. Specifically, erosion may be assumed to occur in a given 

 locality when the resistance of the surface to erosion is less than the 

 erosive power of the eroding agent. Thus, in gully formation, which is 

 clearly the result of water action, the erosion may logically be at- 

 tributed to a condition that either lowered the resistance of the sur- 

 face in a given locality or increased the size or the velocity of the 

 stream. A combination of the two would make the action doubly ef- 

 fective. 



As the fine-textured alluvium that floors practically all the western 

 gullied valleys has little if any inherent cohesion, the greatest re- 

 sistance to erosion is afforded by the protective vegetative cover. De- 

 pletion or destruction of this cover in any manner sets the stage for 

 incipient erosion under normal conditions of flow, and, likewise, flows 

 of extraordinary magnitude might also be expected to accomplish the 

 same action under normal condition of cover. 



Recognition of the critical nature of the erosion problem, particularly 

 since the early 1930's, has stimulated much study of both its cause 

 and methods for curing or ameliorating it. As could be expected, the 

 studies have evoked controversies on both points which, unfortunately, 

 are no nearer settlement today than when first advanced. Doubtless 



