Semi-arid Portion of Southwestern Neio Mexico. 241 



deposition within a very few years at most, clearly indicates 

 that, in recent times, the maximum volume of water coming 

 down the valley in times of flood has been greater than while 

 the alluvium was accumulating. 



On the basis of such evidence of the increased volume of 

 flood waters, we are justified in suspecting that the waters of 

 these floods may have been responsible for the cutting of the 

 stream trenches in question. In an endeavor to explain the 

 trenching it is, therefore, necessary to look for the cause of the 

 increased volume of the flood waters. 



Such an increase may be the result of one or the other of 

 two conditions or a combination of both. These conditions 

 are : either an increase in the amount of water by increased 

 precipitation, or a more rapid rim-off with no increase in pre- 

 cipitation. An analysis of the results of each of these two 

 conditions taken separately may serve to indicate which is 

 most probably the responsible one in the case at hand. 



Taking first the condition of increased precipitation, we will 

 start with a semi-arid climate, such that the streams are not 

 permanent and are silting up their valleys — in other words, 

 with conditions as we conceive them to have been at the time 

 of the formation of the valley till of Cane Spring Canyon and 

 other valleys of the region. There would undoubtedly be 

 some vegetation which might serve as a fairly efficient cover 

 and constitute an important factor in the absence of heavy 

 floods. With an increase in precipitation we should expect 

 the vegetation to become more luxuriant and more effective as 

 a protective covering ; in this way perhaps equaling or over- 

 balancing the tendency of the stream toward cutting, resulting 

 from the increased volume of water. Even though the storms 

 became heavier, the run-off would be slower, so that it is likely 

 that, until the time that the precipitation became so great that 

 the streams became permanent, there would be little, if any, 

 increased tendency toward cutting. 



Considering next the second factor : 

 climate favorable to the formation of 



tion cover, and the balance between erosion and deposition is 

 so adjusted that the streams are silting up with tine material ; 

 with considerable rainfall at certain times of the year, pre- 

 vented, however, by the vegetation cover from forming heavy 

 floods, and if by some means we remove the vegetation cover 

 without changing the amount or distribution of the rainfall, 

 marked results of a different nature will follow. The rain, 

 still as heavy as before, will fall on a bare surface unprotected 

 by vegetation and with little capacity to hold in reserve the 

 excess precipitation. A rapid run-off in the form of floods 

 must result. These floods, rushing down the valleys, will have 



if we have a semi-arid 

 a fairly efficient vegeta- 



