8 MISC. PUBLICATION 10 9, U. S. DEPT. OF AGRICULTURE 



farther into the decayed leaf mold, and so on down into the mineral 

 soil and the fissures of the rocks. There it becomes tributary to the 

 great reservoir of underground water that feeds springs, creeks, and 

 rivers with a steady flow of water. 



The advantage of increasing the proportion of the rainfall which 

 is absorbed into the earth and decreasing that which runs off on the 

 surface is that the surface run-off, unless it is caught in reservoirs, 

 ultimately goes to the ocean and is permanently lost. Also, it is 

 difficult, often impossible, to impound or control the surface run-off 

 when suddenly increased by heavy storms, and its power of erosion 

 is increased out of all proportion to the increase in the volume of 

 water. 



The forest-controlled water supply, of course, does not all go into 

 springs. Some of it is absorbed by roots and carries up to the 

 leaves of plants and trees the dissolved mineral substances which they 

 need for growth. A part of the water is then used in producing the 

 increasing substance of the tree, but much of it is given off by the 

 tree again into the atmosphere. 



Some people believe we could save these water losses by burning 

 or otherwise destroying the vegetation, especially chaparral. They 

 forget the tremendous damage and losses which would certainly 

 follow that destruction, due to increased erosion and silting and 

 other disturbance of stream flow. This could well make a large part 

 of the land useless and uninhabitable in the course of time. The 

 service which the forest or brush renders to man in these directions 

 is so much greater than the loss due to the water which they use that 

 such a remedy would be disastrous. 



3. How do forests regulate stream flow? 



The forest acts in two ways to regulate stream flow. First the 

 canopy of leaves and branches breaks the impact of rain falling 

 upon the earth. Even during a heavy rain one has the sense of the 

 dripping of water from the tree tops rather than a deluge from the 

 sky. The second is in the effect of the layer of ground litter which 

 acts as a sponge to absorb the water and reduce surface run-off. 

 Much more significant than its spongelike absorption of water, how- 

 ever, is the action of the litter in keeping the soil mellow, porous, and 

 more permeable, and j^reventing the sealing up of the seepage chan- 

 nels into the substratum where the great supply of earth water is 

 stored, to appear again at the surface through springs. 



This action is effective and continues, regardless of the intensity 

 or duration of the precipitation received, although it can not entirely 

 prevent floods when the rate of rainfall exceeds greatly the rate 

 at which the soil can absorb the water. The sponge action ceases as 

 soon as the litter reaches its saturation point, and would not there- 

 fore continue to operate far beyond a certain period in long-continued 

 storms such as are responsible for floods. 



IF YOU DON'T KNOW, ASK A UNITED STATES FOREST 



RANGER 



