EVANESCENCE OF THE STREAMS. 99 



The character of the channels expresses well the characteristics of 

 stream erosion : In the sierras the storm torrents gather loosened rock 

 masses (there is little disintegrated detritus and still less decomposed 

 rock matter on the steep slopes), hurl them down the cliffs and hurry 

 them through the barrancas, bursting them asunder and knocking 

 loose other masses on the way ; toward the base of the sierras the larger 

 boulders lodge, to be removed and reduced during later storms, while 

 the pebbles and finer debris are swept further. Then general or local 

 conditions either spread the torrent into a sheetflood, or else maintain 

 the stream character; and in the latter case cobbles and pebbles are laid 

 down after much trituration, the sand is carried far, scouring the channels 

 as it goes, and finally nothing coarser than silt is borne by the diminish- 

 ing flood, which is constantly robbed by dry earth below and drier air 

 above ; the silt burdens the flood without giving much aid in corrasion, 

 and it gradually expands either into a labyrinth of interlacing channels 

 or into a sheetflood, when evaporation and absorption rapidly sap the 

 strength of the torrent until it ceases to be. It is significant that despite 

 the high declivity of the barrancas the freshet torrents are often surpris- 

 ingly clear, evidently by reason of the dearth of comminuted and lixivi- 

 ated detritus, so that the streams are often underloaded and thereby 

 enfeebled as erosive agents. 



The Streamwork in the district is notable partly in that it is excep- 

 tional in occurrence, partly in that it is reduced and rendered subordi- 

 nate by the tendency of the streams to pass into sheetfloods with 

 diminishing declivity; for most of the barranca torrents are transformed 

 at once on reaching the valley-plain, while the sand-lined channel of the 

 typical arroya is but a sheetflood of limited width. So strong indeed is 

 the tendency toward transformation that it is only in the few streams of 

 permanent supply and in the valley-plains oversupplied by exception- 

 ally extensive drainage basins that definite channels are maintained. 



SHEETFLOODING IN THE DISTRICT. 



Character of sheetfloods. — In distribution the streams are mainly con- 

 fined to the sierras, including the Sierra Madre and the higher foot-ranges 

 as well as the lower outlying ranges and masses and the isolated mesas 

 and buttes. and are local and exceptional in the valley -plains, while half 

 the vast valley-plain area is the area of sheetflooding. Although there is a 

 general increase in precipitation with altitude throughout the district, 

 the cloudburst and drizzle usually affect both sierra and valley-plain ; 

 and in such cases the plain is flooded by the direct rainfall as well as by 

 drainage from the sierra. The character of the flooding is known from 



