robbinT°^] climate and EVIDENCE OF CLIMATIC CHANGES 63 



of a much heavier rainfall than we now have. River valleys in many cases show 

 only dry gravelly or sandy beds which evidently were formerly occupied by con- 

 tinuous streams. The floods that once carved their way across the slopes or over 

 the plains are no longer seen, at least not in the same volume as in former time. 

 Even existing streams do not reach in times of great flood their former volume and 

 carrying capacity. All tell of diminished volume, whether in the desert regions or 

 in the regions of abundant plant-growth. 



We may not judge of the accuracy of this conckision without 

 knowing the evidence on wliich it is based; it is certainly contrary 

 to the opmion of many geologists. Throughout the West and the 

 Southwest are many dry channels which at first give the impression 

 of having once been the beds of perennial streams but which, on 

 careful investigation, seem to have been formed by the intermittent 

 flow of water during wet seasons or after storms. The absence of 

 great floods can not be inferred without very long-continued obser- 

 vation, for extraordinary floods in any region occur scarcely more 

 than once in a generation. There are numerous cases in which 

 destructive floods have visited regions that have been free from 

 them for many years, in a few hours cutting deep channels where 

 none were known before. In many instances the failure of floods 

 to reach former levels results from the deepening of the channels. 

 On this subject Gilbert's remarks^ are timely: 



As in other desert regions, precipitation here results only from cyclonic disturbance, 

 either broad or local, is extremely irregular, and is often violeijt. Sooner or later the 

 "cloud-burst" Adsits every tract, and when it comes, the local drainage-way discharges 

 in a few hours more water than is yielded to it by the ordinary precipitation of many 

 years. The deluge scours out a channel which is far too deep and broad for ordinary 

 needs and which centuries may not suffice to efface. The abundance of these trenches 

 in various stages of obliteration, but all manifestly unsuited to the every-day condi- 

 tions of the country, has naturally led many to believe that an age of excessive rain- 

 fall has but just ceased — an opinion not rarely advanced by travelers in other arid 

 regions. So far as may be judged from the size of the channels draining small catch- 

 ment basins, the rare, brief, paroxysmal precipitation of the desert is at least equal 

 while it lasts to the rainfall of the fertile plain. 



Though for other reasons it seems probable that the flow of streams 

 in this region was at one time much greater than now, the canyons 

 themselves do not necessarily indicate it. It is hkely that all the 

 erosion could be accomplished in course of time with no greater 

 volume of water than now flows, and even such canyons as those 

 now without perennial streams could be cut by storm waters, though 

 the work would require more time. However, more rapid cutting 

 by a greater volume of water during the last few thousand years, 

 by shortening the requisite time, would make it easier to under- 

 stand how the high vertical cliff in the yielding tufa of the Frijoles 

 canyon has been able to stand during the down-cutting period. 



I Gilbert, G. K., Lake Bonneville, Monogr. U. S. Geol. Surv., i, 9, 1890. 



