MAMMALS AND SOIL TURNOVER III 



the grazing permits, for the restriction of the number of animals al- 

 lowed in each area, and for the more or less continual movement of 

 sheep, so that they may not be grazed and bedded too long in one lo- 

 cality. 



In the semi-arid southwestern states many streams which flowed 

 in very shallow channels when settlers first entered the region, have 

 since become deeply entrenched in the alluvium, with vertical walls. 

 Large cottonwoods and willows which lined some of the streams have 

 died because they were left on the high, dry banks where their shallow 

 roots no longer reach the water line. Swamps and ponds which occu- 

 pied some of the valleys and aided in retarding run-off have been de- 

 stroyed by the establishment of more complete drainage through the 

 deep, narrow channels. These changes have by some writers been at- 

 tributed to a very recent change in climate, which is not consistent with 

 the facts. The evidence seems to point to more rapid run-off of storm 

 waters, resulting from the removal of the vegetative cover by over- 

 grazing, as the probable cause in many or most cases. 8 Run-off has been 

 accelerated by trails worn by cattle and sheep in going to and from 

 water and feeding grounds, the trails providing small channels of least 

 resistance and thus concentrating the sheet flow during and after storms 

 into numerous tiny rivulets. However, the trails of themselves could 

 not have produced such general and pronounced trenching as is found 

 in the Southwest. There were numerous bison trails on the Great Plains 

 when white settlers arrived, which had been traveled by those animals 

 for centuries, and they had produced only very local erosion, but the 

 bison herds moved about freely and made long migrations, not grazing 

 in one locality long enough to seriously reduce the vegetative cover. 



"Rich, Recent stream trenching in the desert of southwestern New Mexico a 

 result of removal of vegetative cover, Amer. Journ. Sci., 4th series, xxxu, 237- 

 245, 1911; abstract, Ann. Assn. Amer. Geographers, i, 135, 1911. Duce, The effect 

 of cattle on the erosion of canyon bottoms, Science, XL, 450-452, 1918. Reagan, 

 Recent changes in the plateau region, Science, LX, 283-285, 1924. Bryan, Date of 

 channel trenching (arroya cutting) in the arid Southwest, Science, LXII, 338-344, 

 1925 ; with a great deal of evidence as to dates of cutting and citation of numerous 

 publications. 



