WORK OF WATER IN ARID REGIONS 577 



mountain waste. The alluvial fans which are formed appear to become 

 confluent, especially when the river's channel rapidly meanders. On 

 either side of the river broad plains are thus built up, and these are in- 

 clined strongly toward the channel of the master stream. Were the river 

 free from lateral swinging these plains would doubtless become contin- 

 uous and even, like the intermont plains beyond the great valley. 



The valleys of the through-flowing rivers afford a means of measure- 

 ment between general desert-leveling and lowering of the country by the 

 winds alone and general erosion by the winds assisted by stream work. 

 The lowering of the bolsons through which these rivers flow, as compared 

 with those without streams, seems to be about 50 per cent greater. On 

 the whole, these trunk streams appear to transport from the region 

 through which they pass about as much rock waste as do the Missouri, 

 the Platte, the Arkansas, and other long rivers rising in the Eocky Moun- 

 tains. 



There is no doubt but that vast deflation goes on in the valleys of the 

 through-flowing rivers. I am inclined to agree with Walther,^^ that in 

 the walls of the Grand Canyon of the Colorado Eiver deflative effects 

 predominate over those of water corrasion. On the other hand, between 

 the mouth of the Grand Canyon and the mouth of the river the cross 

 profiles of the valley appear to be controlled more by the lateral arroyo 

 grades. In the Eio Grande Valley this phenomenon is still more clearly 

 displayed. The cross profile of this valley is peculiar as river trenches 

 in general go, in that for so wide a valley it is not broadly U-shaped, but 

 very broadly V-shaped. The controlling factor of the valley contour is 

 the arroyo, which is a uniformly graded line from the mountains to the 

 river's channel. In this great valley, often a score of miles and more in 

 width to the mountain bases, water might at first thought appear to be 

 the sole erosive agent, but wind scour operates with equal facility upon 

 any form of surface, and its leveling influences are the same on an in- 

 cline as on a horizontal plain. The profile of the valley of the Eio 

 Grande at Socorro, for instance, is essentially as represented below 

 (figure 2). 



The Eio Grande is probably the most remarkably terraced river in the 

 world. Bordering the stream is an extensive succession of high-level 

 mesas that constitute the most striking feature of the great valley's sur- 

 face relief. These escarpmented plains, or mesas, inclining strongly 

 toward the river, are abruptly cut off as they near the banks of the stream. 

 These so-called aggraded terraces I have recently shown^^ to be directly 



25 Verhandl. d. Gesellschaft f. Erdkunde zu Berlin, XIX Bd., 1892, p. 52. 

 8» American Journal of Science (4), vol. xxiv, 1907, p. 467. 



