Chapter 23 



THE PROBLEM OF GULLYING IN WESTERN VALLEYS * 



H. V. Peterson 



Geologist, U. S. Geological Survey 

 Salt Lake City, Utah 



To distinguish between gullying and other types of erosion, the term 

 gully is generally applied to any erosion channel so deep that it cannot 

 be crossed by a wheeled vehicle or eliminated by plowing. When ap- 

 plied to the arid and semi-arid valleys of the West, this limitation re- 

 garding minimum size seems somewhat inconsistent, for with many 

 gullies having dimensions measured in tens of feet in depth, hundreds 

 of feet in width, and scores of miles in length, a definition aimed at 

 distinguishing them from canyons or small valleys might have been 

 more appropriate. 



Generally the term arroyo has been used synonymously with gully, 

 but by some (Thornthwaite et al., 1942, p. 72) arroyos are considered 

 the result of natural conditions of erosion in contrast to gullies, which 

 have developed under "culturally accelerated erosion" or, in other 

 words, land misuse. Because it is difficult or impossible to learn from 

 the appearance of the channel the conditions under which it developed, 

 the distinction has little significance, and in this discussion the less 

 euphonius but more descriptive term "gully" will be used. 



Gullies appear to have been a common feature of stream erosion in 

 the recent geologic history of western valleys and possibly have always 

 formed a part of the landscape in localities favorable to their develop- 

 ment. Evidence of former gullies, similar in cross section and dimen- 

 sions to existing ones, can usually be found wherever any extensive 

 section of fine-textured alluvial fill is exposed. Just as at present, 

 gullies have developed in localities underlain by soft, easily eroded 

 materials, which offered minimum resistance to downcutting. It is dif- 

 ficult to determine the extent of this earlier gullying or to ascertain 

 whether the channels were integrated into a continuous system wherein 

 the gradients of all branches were accordant with the parent stream, 



* Published by permission of the Director, U. S. Geological Survey. 



407 



