1512 



AMERICAN FORESTRY 



ged the topography, the more marked is this contrast. 



In hilly country some erosion is, of course, inevitable 

 under any conditions. When the soil cover of trees, 

 underb rush, 

 and litter is 

 kept intact, 

 however, this 

 is more often 

 beneficial than 

 otherwise, since 

 only the light- 

 er soil particles 

 are washed 

 away, to be 

 later deposited 

 in the more 

 level lands be- 

 low, adding to 

 their fertility. 

 But when this 

 protective cov- 

 er is interfered 

 with, whether 

 by fire, destruc- 

 tive lumbering, 

 overgrazing, or 

 i n j u d i c ious 

 clearing of 

 land for agri- 

 culture, the 

 proportion of 

 coarser, infer- 

 tile materials 

 washed away 

 increases great- 

 ly and trans- 

 forms erosion 

 from a con- 

 structive into 

 a dangerously 

 d e s t r u c tive 

 force, difficult 

 of control and 

 capable of do- 

 ing untold 

 damage. 



From the 

 standpoint of 

 the water user, 

 the tendency of 

 the mountain 

 forests to pre- 

 vent erosion is 

 of the utmost 

 import ance. 

 Wherever stor- 

 age reservoirs must be used, whether for municipal sup- 

 plies, irrigation, or water power, they are exposed to the 

 ever-present danger of silting up. Every bit of soil 



WATER FOR IRRIGATION AND POWER 



Upper.— Roosevelt Dam and power plant (in right center foreground), 

 feet of water and, together with the Verde 



This reservoir stores 1,140,000 acrr- 

 ._ River, furnishes the water supply for the Salt River 

 Reclamation Project in southern Arizona. The bulk of the water for the project originates on 

 the Tonto National Forest and the White River Indian Reservation. 

 Lower.— Minidoka Dam and power plant. This dam supplies water for the irrigation of 120,300 acres on the 

 Minidoka Reclamation Project in southern Idaho. The electricity 

 is used on many farms for lighting, heating, and cooking. 



brought down by the streams and deposited in them 

 reduces their capacity and consequently their effective- 

 ness by just so much. This sedimentation is serious 



under any con- 

 d i t i o n , but 

 doubly so 

 when, as not 

 i n f r e quently 

 h a p p e ns, no 

 other satisfac- 

 tory dam sites 

 are available 

 and the reser- 

 voir can not be 

 replaced at a 

 r e a s ona ble 

 cost. 



Water heavi- 

 ly laden with 

 eroded ma- 

 terial often de- 

 creases the ef- 

 ficiency and in- 

 creases the cost 

 of maintaining 

 diversion dams, 

 pipe lines, 

 flumes, canals, 

 and other irri- 

 gation works. 

 Sometimes 

 such water 

 damages the 

 ctops to which 

 it is applied, 

 and not infre- 

 quently it ser- 

 iously injures 

 or even ruins 

 the land by 

 burying it un- 

 der a mass of 

 sand, gravel, 

 bowlders, and 

 other infertile 

 debris. Exces- 

 s i v e erosion 

 may interfere 

 seriously with 

 navigation by 

 filling the 

 streams with 

 material which 

 is deposited in 

 their lower 

 reaches and in 

 the harbors into which they empty. The action of the 

 forest in reducing surface run-off tends also to regulate 

 the flow of streams. Instead of rushing away in uncon- 



developed at the power plant 



