140 A NATIONAL PLAN FOR AMERICAN FORESTRY 



available water supplies in satisfactory amount and condition are 

 limited, and our needs have been met thus far only with enormous 

 effort and cost. Forested lands favorably influence the " water crop " 

 in so many ways and to such an extent that the protective values 

 inherent in these forest influences rival those of any other use of 

 forest land. 



Floods ordinarily cost us something like 40 million dollars annually, 

 to say nothing of the expenditures for engineering-control works. 

 Single floods sometimes amount to calamities. The Mississippi flood 

 in 1927 is estimated to have cost some 300 million dollars. Forests 

 will not prevent floods but adequate areas of forested land strategi- 

 cally located exercise such a favorable influence that no effective plan 

 for flood control can fail to include forest lands as one essential 

 element. 



Forest cover delays the melting of the snows, the litter retards 

 run-off, and the soil is very porous; these together induce soil absorp- 

 tion of water in the forest at a rate many times greater than for field or 

 cultivated lands. The effect is measurably to retard and reduce flood 

 peaks and, therefore, the destructiveness of floods. 



The reduced and retarded run-off on forested land, the absorption 

 of water by the soil, and the soil-holding effect of the tree roots com- 

 bine to prevent the washing away or erosion of forest soils. Investiga- 

 tions have revealed that but a fraction of 1 percent as much soil per 

 unit of area is eroded from forest land as from open-crop land. 

 Surface erosion depletes the fertility of the land. In the aggregate, 

 soil and fertility losses by erosion on cultivated and open land are 

 tremendous. It has been estimated that 7 inches of the top soil of 

 such lands in Illinois has been lost through erosion. The erosive 

 effect of silt-laden streams is much greater than where the water is 

 clear; the silt is deposited in reservoirs, clogs up hydroelectric plants 

 and engineering works, forms sand bars and otherwise interferes with 

 navigation, and prevents the use of the water for domestic and certain 

 industrial purposes. The beneficial effects of forested lands strategi- 

 cally located on river watersheds in reducing erosion and its resultant 

 ills are therefore diverse and highly important. 



The benefits of forest cover in keeping water in springs, streams, 

 and reservoirs clear and pure for domestic use are universally recog- 

 nized. An adequate supply of domestic water for our urban centers 

 has become a vast and expensive problem. San Francisco is securing 

 water from the Sierras 200 miles away; Los Angeles across 200 miles 

 of desert and mountain from Owens River; New York City is drawing 

 its supply from the Catskills by costly conduit and is reported to be 

 looking for additional sources at much greater distance. The im- 

 portance to navigation of clear water and stabilized flow and to the 

 enormous hydroelectric plants, are too obvious to require description. 

 The importance of clear and permanent streams to fish life and to 

 recreation generally are perhaps not so widely appreciated but are 

 none the less important. 



The shelter against strong drying winds afforded by forests to 

 homes, crops, and livestock is very important in some localities and 

 constitutes a definite protective function of forest land. This 

 function also includes protection against shifting sands and the 

 formation and movement of sand dunes. 



The protective value of forested lands is largely independent of the 

 exact type of cover though not of its condition. The dense spruce 



