244 •/. /.. L'lch -Recent Stream Trenching in tin 



torica] evidence that such a reduction has actually taken place, 

 with an accompaniment of increased floods, we may further 

 test ilie theory by inquiring whether the formation of the 

 trenches corresponds in time with the removal of the vegeta- 

 tion. On this point the testimony of the settlers is unanimous 

 that, in the early days, there were few, if any, trenches in the 

 valley bottoms, ami that trenching has followed the appear- 

 ance of tin' heavy Hoods. The deep trenches in the Mangas, 

 one of which is shown in the photograph (fig. 3), have practi- 

 cally all been formed within the past '20 or 30 years, and are 

 still deepening. The town of Silver City is built partly on an 

 alluvial flat, such as those we have described. Running through 

 the town is a trench or gulley cut through the alluvial valley 

 tilling to a depth of at least 20 feet and with a width of about 

 Ion feet. A little over 15 vears agro the site of this trench 

 was one of the main streets of the town. During a severe 

 storm the street was partly washed out and the trench begun. 

 This trench has been cut deeper every year until the present 

 condition lias been reached. 



From physiographic evidence of this sort, and from the tes- 

 timony of the early settlers, there is good proof that the trench- 

 ing is of recent date, corresponding in time with the removal 

 of the grass by too close pasturing. 



Our own observations, in so far as we had opportunity to 

 make them in the short space of one summer, bear out the 

 statements of Mr. MacMillan and others as to the normal vege- 

 tation conditions. In certain portions of the Silver City quad- 

 rangle, which were so far removed from water that the cattle 

 seldom visited them, we found a fairly good cover of grass. 

 These portions were on rocky hills of lava, never, at the best, 

 fitted to maintain a great amount of vegetation, so that in the 

 more fertile valley bottoms and on the gravel plains one would 

 expect a good growth of grass. Within the Fort Bayard mili- 

 tary reservation there is a tract of land which has been kept 

 fenced for several years, and has been subjected to very slight 

 grazing. Within this area the grass grows thick and often 

 knee high. There can be no doubt that this grass as a cover- 

 ing is efficient, both in preventing a rapid run-off, and, on 

 account of the sod which is formed, directly hindering stream 

 cutting. Mr. MacMillan reports that in a watershed which he 

 has kept fenced for several years and has not allowed to be 

 closely pastured, the stream trenches which had begun to form 

 before the fencing are gradually filling up. 



The conspicuous nature of the physiographic results of over- 

 stocking of a region, and the rapidity with which they are 

 brought about are surprising, and afford a striking illustration 

 of the influence of man on geologic processes. If the grazing 



