WORK OF WATER IN ARID REGIONS 575 



sheet in a single night compelled the Mexican Central Eailway to move 

 its track for many miles a distance of 7 kilometers from the original line 

 of location.^* 



ARROYO-BUNNINa 



The nearest approach to normal stream work in the desert country is 

 found in the loftier mountain ranges. As the storm waters of the moun- 

 tain drainageways enter the plains they quickly sink from sight. This 

 disappearance occurs even when there are exceptionally heavy local 

 showers. Under these conditions streams extend but little farther from 

 the bases of the mountains than at other times. With the exception of a 

 few days out of each year, the lines occupied by storm waters, including 

 the liwer reaches of the mountain streams, are without water. Arroyos, 

 or dry creeks, the Spanish-speaking people of the region call them. 



To the dwellers of the arid region the running of the arroyo is always 

 an event of more than passing notice. It has the constant solicitude of 

 the traveler. At times of heavy rainfall in the neighborhood the dry 

 drainagcway, which in the sparsely settled districts is also often a road- 

 way as well, becomes a raging torrent. It sweeps everything before it. 

 Trains of gravel and boulders mark its course when it ceases flowing. 

 The fan that it builds up at the mouth of its mountain canyon is often 

 a mile or more across. From the point where the streamway debouches 

 from its canyon the flood waters spread out in a broad sheet over the 

 surface of the fan, for the latter has no deep channelways. It is arroyo- 

 running after debouching upon the alluvial fan that McGee so well de- 

 scribes under the title of sheetfiood erosion, rather than the action of the 

 true floodsheet of the desert plains. 



INFLUENCE OF THROUGH-FLOWING STREAMS 



In the American arid region there are three large perennial rivers orig- 

 inating without the area that, after traversing the desert, flow through 

 to the sea. They are the Eio Colorado, the Rio Grande, and the Rio 

 Pecos. With the exception, perhaps, of the Colorado River, none of 

 these streams receives tributaries in its passage through the dry region. 

 After leaving the State of Colorado the Rio Grande, for instance, flows 

 for 1,000 miles without notable lateral augmentation to its waters. 



While these streams flowing through to the sea receive practically no 

 additions to their volume from lateral waters within the arid country 

 through which they flow, they have a very interesting geologic history. 

 The huge valleys which they occupy appear to be out of all proportion to 



2* Keyes : American Journal of Science (4), vol. xvi, 1903, p. 377. 



