THE SOIL CONSERVATION SERVICE 259 



contributions to the work as well as to agree to maintain all structures, 

 dams, plantings, and other improvements for a period of five years. 



The principal reliance is upon vegetation as a means of control. 

 There are two general types of erosion by water: (1) sheet erosion, 

 or the general washing away of the soil; (2) gully erosion, which often 

 includes the advanced stages after sheet erosion has washed away the 

 top soil. Wind erosion is still another problem requiring slightly modi- 

 fied control methods. 



Two classes of control measures are used in curbing erosion: (1) 



FIG. 129. An erosion control project along the Angeles Crest Highway in South- 

 ern California. Boys of the CCC camps are planting shrubs and other plants 

 to prevent the soil from washing away during and after heavy rains. 



vegetative methods, such as the planting of grasses, trees, and shrubs; 

 (2) mechanical methods, such as dams of various types or terracing. 



Under the Service an attempt is made to control arroyos in the 

 West by means of diversion dikes and dams to spread the available 

 water over the upland plains in order to increase the growth of grass, 

 trees, and other types of vegetation. 



The principal species of trees used in the vegetative control work 

 are black locust, willows, cottonwood, various species of pine, includ- 

 ing white, red, shortleaf, and slash and Russian olive and caragana. 



