IRRIGATION 



353 



they become chilled and 

 lose moisture by condensa- 

 tion. The moisture which 

 condenses falls on the moun- 

 tain slopes as snow or rain. 

 The winds continuing their 

 eastward journey soon en- 

 counter the great peaks of 

 the Sierra Nevada, where 

 they lose more moisture by 

 condensation. The winds 

 which leave the Sierra 

 Nevada are poor in mois- 

 ture and give little to 

 the thirsty plains beyond. 

 Finally, the nearly dry 

 winds encounter the Rocky 

 Mountains and in rising 



over these towering peaks lose practically the last remnant of 

 their moisture. Such winds blow onward with almost nothing 

 for the lands beyond, which therefore lie parched and barren. 



FIG. 219. The ocotillo. This tree is in full 

 bloom although its leaves can hardly be seen. 



D 



Pacific Coast Coast Range 

 Plain 



Sierra Nevada Rockies Oesen 



FIG. 220. High mountains rob winds of their moisture. 



The " lay of the land " is responsible for meager rainfall in 

 our desert lands. 



Irrigation. Many western plains and valleys which lack 

 rainfall have large rivers fed by rains falling on distant moun- 

 tains. By using the waters of these rivers for irrigation much 

 of the barrenness due to scant rainfall has been overcome, 



