Ch. 23] SAN SIMON VALLEY 409 



and features associated with channels of this type, the San Simon gully 

 of southwestern Arizona, might be considered typical. From its con- 

 fluence with the Gila River near the town of Solomonsville, Arizona, 

 this gully cuts southeasterly for nearly 70 miles through the heart of 

 the lower San Simon Valley. The gully is not continuous, as one short 

 reach of the valley floor, approximately 2 miles in length situated some 

 40 miles above the mouth, is uncut. It is not known if this reach has 

 previously been dissected, but it is evident that unless remedial meas- 

 ures are taken the two discontinuous segments will shortly be inte- 

 grated into one continuous channel. In depth, the gully, in both 

 branches, varies from 6 to 60 feet; in width from 75 to more than 

 1,000 feet. Surveys made by the U. S. Soil Conservation Service * 

 in the middle 1930's show that silt removed from the main trench is of 

 the order of 20,000 acre-feet. Additional excavations from gullied 

 tributaries and from sheet and badland erosion on interfluviatile belts 

 adjacent to the bank, both traceable in the main to rejuvenation re- 

 sulting from development of the master gully, probably equal or ex- 

 ceed this amount. Prior to the 1929 completion of Coolidge Dam, 

 located on the Gila River some 70 miles downstream, silt disposal 

 from the eroding area offered no particularly critical problem. Since 

 that date it has become a very real threat to the life of the San Carlos 

 Reservoir. 



The San Simon Valley, like many others of its kind located through- 

 out the West, contains a deep alluvial fill which originated as outwash 

 from the surrounding mountains. Older portions of the fill, repre- 

 sented by both lacustrine and continental deposits, form a part of 

 the Gila conglomerate of late Pliocene and Pleistocene age, described 

 in the literature by Knechtel (1937). These beds, which are usually 

 indurated to some extent, are exposed around the margins of the valley, 

 and although extensively dissected they generally exhibit no serious 

 recent erosion. 



Distributed along the central axis of the valley and extending moun- 

 tainward along the tributaries, are deposits consisting of clay, silt, and 

 fine sand intermixed with occasional lenses and stringers of coarse 

 sand and gravel. These are typical flood-plain deposits obviously 

 laid down by gently flowing streams capable of carrying fine-textured 

 sediments only, except during periodic floods, when the coarse sand 

 and gravel were brought in. Archaeologic evidence f indicates that 



* Unpublished data obtained through personal communication with earlier 

 employees of U. S. Soil Conservation Service. 



t Personal communications from Dr. Emil Haury, Department of Anthropology, 

 University of Arizona, furnished probable dating of artifacts found in the valley. 



