THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XXVII. — Recent Stream Trenching in the Semi-arid 

 Portion of Southwestern New Mexico, a Result of Removal 

 of Vegetation Cover y* by John Lyon Rich. 



A conspicuous development of recent stream trendies in the 

 valleys of many of the temporary streams of the western states 

 is a feature of such widespread and common occurrence that it 

 cannot be assigned to accidental causes, but calls for an explana- 

 tion which shall have more than a local application. This 

 trenching is particularly well developed in portions of south- 

 western New Mexico. Here it is apparently an effect of the 

 removal of vegetation cover by excessive grazing. A brief 

 description of the nature of the trenching and a statement of 

 the evidence on which the above explanation of the phenom- 

 enon is based is the purpose of the following discussion. 



The portion of New Mexico on which our study is particu- 

 larly based is included within the Silver City quadrangle in 

 the southwestern part of the state. The quadrangle may be 

 roughly divided into two physiographic units, a mountainous 

 portion and a desert plain formed by the accumulation of 

 gravels derived from the mountains. Of the whole area of the 

 quadrangle, about one-third is covered by these desert accumu- 

 lations. 



The climate is semi-arid, with an average rainfall in the 

 neighborhood of 18 inches in the mountainous parts, and con- 

 siderably less in the lower, desert portions. The precipitation 

 comes mostly in the form of showers during the hot summer 

 months, from about the middle of June until the latter part of 

 September. General rains are not of common occurrence. As 

 a result of these climatic conditions there are no permanent 

 streams in the area outside the higher mountains, and even in 



- * Published by permission of the Director of the U. S. Geological Survey. 



Am. Jour. Scr.— Fourth Series, Vol. XXXII, No. 190.- 



18 



-October, 1911. 



