DAMAGE BY EROSION 



173 



generally recognized by the stockmen. This is not surprising 

 when it is recalled that not less than 85 per cent of the water 

 used for irrigating the 3,200,000 acres of western farm lands has 

 its origin in the mountains of the National Forests. Of about 

 156,000,000 acres within the boundaries of the National Forests 

 in 192 1, practically all of which contributes in one way or an- 

 other to agriculture, approximately 8,000,000 acres lie above the 

 altitude of forest growth. Of this acreage about 4,200,000 

 acres are barren; about 1,500,000 acres support a cover of brush; 

 and the remaining 2,300,000 acres are grassland. The value of 

 the lands at these dizzy heights lies chiefly in their efficiency as 

 watersheds, that is, in the water which they contribute for 

 irrigation. Because of the ruggedness of their surface and 

 their economic relation to the adjoining communities and to 

 agriculture generally, no better opportunity is offered for con- 

 sidering the erosion problem than that afforded on the National 

 Forests. Furthermore, the principles developed in the con- 

 trol of erosion on these lands are applicable to farm and pasture 

 lands generally. 



On the National Forests, as elsewhere, the greatest damage 

 from erosion occurs on the lands that have been badly over- 

 grazed and whose ground cover has been destroyed or seriously 

 impaired. After the breaking up of the vegetative cover caused 

 by overstocking during the happy-go-lucky grazing period, 

 many streams that had originally a steady, yearlong flow and 

 teemed with trout became treacherous channels with inter- 

 mittent flow through which plunged the water from rainstorms. 

 Most of the streams swelled suddenly and subsided as quickly 

 according to the size and frequency of the storms; and many 

 such flows carried so much sediment that fish and similar life 

 could not exist in the water. 



The damage is not confined to the decrease in the forage 

 yield of the lands eroded or to the silting over of adjoining 

 agricultural lands to which the torrential floods carried the 

 debris; the efiiciency of the watershed in maintaining a per- 

 manent flow of irrigation water is also greatly decreased. 



Many typical examples of the damage of erosion that have 



