98 WJ MCGEE — SHEETFLOOD EROSION. 



ranges that the ground-water reaches or approaches the surface so that 

 settlements can be maintained. 



The chief source of stream water is the sporadic storm, especially the 

 thunder-gust or cloud-burst, which fills old channels and gouges out new 

 ones, though the flow may last but a few minutes, and seldom continues 

 more than a few hours. In the sierras the slower drizzles produce stream 

 floods which sometimes find their way out on the valley-plains, though 

 the drizzle on the plain commonly does no more than wet the surface or 

 produce feeble sheetfloods ; and on the broader plains only a relatively 

 small part of even the heaviest rainfall ever collects in streams. 



In brief, the streams of the district are strikingly short and small in 

 proportion to the area, and only less strinkingly few and feeble in pro- 

 portion to the scant precipitation. 



Streamways and stream-work. — In the sierras the permanent streams are 

 slender threads of water slipping over ledges, now gathering in tinajas,* 

 and again disappearing in fissures or gravel pockets at the bottoms of 

 rugged barrancas; and the barrancas dividing narrow aretes are excep- 

 tionally parallel and close laid, while it is the combes or amphitheaters 

 in which the barrancas head, in conjunction with the peaks in which the 

 aretes join the crests, that produce the characteristic sierra profile. Most 

 of the multitudinous barrancas are supplied only by storm torrents, and 

 these usually end about the base of the sierra, the margin of the valley- 

 plain ; it is only the deeper and longer barrancas that send arroyas or 

 permanent channels far enough over the plain to unite with other water- 

 ways in dendritic systems ; and it is partly for the reason that most bar- 

 rancas end at the plain that their remarkable parallelism is maintained. 

 Outside the sierras the typical channel is at first a rugged or flat-bot- 

 tomed barranca cut in the country rock ; it soon diminishes in depth 

 and increases in width and becomes lined with boulder beds ; still fur- 

 ther down stream it changes into a broad, steep-banked arroya cut in 

 alluvium and burdened with gravel beds or sand sheets ; and it finally 

 ends in an alluvial fan, usually of imperceptible slopes miles in length 

 and furlongs in width. If the stream is permanent it is, in its low-water 

 stage, but a thin ibbon of water rippling over the rocks of the upper 

 .course or the sands of the middle course. The streamways are notable, 

 first, for high grades, and, second, for the width of their channels, which 

 may exceed that of the Ohio, or even that of the Mississippi, for a stream 

 less than 50 miles long; but during most of the year nine-tenths of the 

 few streamways are broad wastes of barren sand, the most forbidding 

 lines of the desert, often littered with skeletons of famished stock. 



* Natural bowls, or water-pockets; defined in Science, new series, vol. iii, 1896, p. 494. 



