88 WJ MCGEE — SHEETFLOOD EROSION. 



known ; it is undoubtedly by reason of this tendency that running- 

 waters commonly flow in streams which cut channels and eventually 

 fashion most of the lands of the earth. The tendency of flowing water 

 to break up into transverse waves is not so well known, though beauti- 

 fully exemplified in sluices for irrigation or for bringing logs and lumber 

 down from mountains (where it is the succession of waves which prevents 

 the sluice from clogging and equalizes the movement of the floating load) ; 

 it is of subordinate importance geologically. 



Under certain conditions, sand-laden water flowing over an erodable 

 plain tends at first to divide into parallel streams like those of pure water 

 on an indestructible surface, yet, since the streams formed in this way at 

 once begin to scour and overload themselves and thus check their own 

 flow, this tendency is soon counteracted and the water is distributed 

 again ; so that the ultimate tendency is toward movement in a more or 

 less uniform film or sheet. This tendency is well known to laymen in 

 those regions in which it prevails, but it seems not to be generally recog- 

 nized among geologists. Colloquially a moving water-body of this type 

 is sometimes known as a " wash ; " but since the term is commonly ap- 

 plied primarily to the product and only secondarily to the agency, and 

 since it is usually restricted to limited, though broad, channels (e. g., 

 in San Francisco wash), it seems desirable to use some other designation 

 for the water-body ; and the term sheetflood has come into use in notes 

 and in conversation. 



Thus there are in nature two strongly contrasted types of moving water 

 bodies, namely, (1) streams, and (2) sheetfloods. The first type is char- 

 acterized by a tendency toward concentration in narrow and relatively 

 deep bands which quickly cut channels for themselves ; the second is 

 characterized by a tendency to spread widely in relatively shallow sheets. 

 The second is logically coordinate with the first as a geologic agent. In a 

 general way streams prevail in humid regions, sheetfloods in arid regions, 

 though streams occur locally in arid lands, while it seems probable that 

 sheetfloods occur under certain conditions in nearly all lands, howsoever 

 humid. 



Sheetflood Work in the Sonoran District. 

 features of the district. 



Location. — Sheetflooding is characteristic of the broad expanse of plain 

 and mountain in southwestern Arizona and western Sonora (Mexico), 

 stretching from the Sierra Madre to the gulf of California, and lying be- 

 tween Gila and Yaki rivers. In physical characteristics and geologic 

 history this territory forms part of Powell's great province of " Basin 



