EANGE MANAGEMENT IN" NEW MEXICO. 6 



Socorro County and the Estancia Valley in Torrance County are 

 typical examples. These plains are everywhere dissected by dry 

 watercourses, or arroyos, which serve to collect the flood waters that 

 are sometimes temporarily very abundant, and sheet erosion of unpro- 

 tected soil surfaces is everywhere very rapid, largely because of the 

 steep gradient of even that part of the surface, which, by -comparison 

 with the bolder relief features, seems to be level. Many of the striking 

 topographic features, both of sculpture and deposition, are due to 

 wind action. On the eastern side of the State the mountains and 

 mesas gradually subside into the wide expanse of the Staked Plains, 

 while to the west Arizona is but a repetition of the alternation of 

 plains and mountains so characteristic of New Mexico but at a slightly 

 lower general level. 



The river valleys are narrow and not infrequently constricted to the 

 river channels alone or " boxed in" where they cut through moun- 

 tains or mesas. The two largest rivers, the Rio Grande and Rio 

 Pecos, run entirely or almost across the State from north to south. 

 The San Juan and the Gila flow out of the State westwardly in San 

 Juan and Grant Counties, respectively, while the northeastern part 

 of the State contains some of the tributaries of the Arkansas River 

 and the headwaters of the Canadian, all of which flow eastwardly. 

 The most conspicuous feature of all these streams is the small number 

 of permanent tributaries possessed by each when the size of its drain- 

 age area is considered. The valley bottoms throughout the State, 

 wherever there is a permanent flow of water, are always turned into 

 cultivated fields, and many acres of such lands produce alfalfa. 

 (PL II; fig. 2.) 



CLIMATE. 



Precipitation. — Precipitation is everywhere relatively small in 

 amount in New Mexico. On the plains of the southern part of the 

 State and in all of the river valleys outside of the mountains it is 

 always scanty. In the mountains at altitudes of 5,500 feet or more 

 it is more abundant, but even in the more moist. regions the amount 

 of water that falls during the year is rarely equal to that which is com- 

 mon in the humid regions farther east. 



To say that there is a summer rainy season does not mean what the 

 same expression tells in regard to a tropical country. It merely says 

 that most of the rain of the year comes during July, August, and Sep- 

 tember, and that this is the growing season on the plains and almost 

 everywhere in the mountains. In some of the higher mountains, 

 where there is considerable snow during the winter, the ground is left 

 wet enough by the melting of these snows to cause a certain spring 

 and early-summer growth; and some of the perennials of even the 

 drier plains put out thnir blossoms and grow some new leaves in May 



