186 REPORT OF THg FORESTRY COMMITTSE 



FOEESTS AND EROSION 



FORESTS are the most effective agency for protecting the soil from ero- 

 sion because: (1) The resistance of the soil to erosive action is increased 

 by the roots of the trees, which hold the soil firmly in place, and (2) at 

 the same time the erosive force of the runoff is itself reduced because the rate of 

 its flow is checked and its distribution over the surface equalized. 



FORESTS AND FLOODS 



THE total discharge of large rivers depends upon climate, precipitation, 

 and evaporation. The observed fluctuation in the total amount of water 

 carried by rivers during a long period of years depends upon climatic 

 cycles of wet and dry years. 



The regularity of flow of rivers and streams throughout the year depends 

 upon the storage capacity of the watershed, which feeds the stored water to the 

 streams during the stunmer through underground seepage and by springs. In 

 winter the rivers are fed directly by precipitation, which reaches them chiefly 

 as surface runoff. 



Among the factors, such as climate and character of the soil, which affect 

 the storage capacity of a watershed, and therefore the regularity of streamflow, 

 the forest plays an important part, especially on impermeable soils. The mean 

 low stages as well as the moderately high stages in the rivers depend upon the ex- 

 tent of forest cover on the watersheds. The forest tends to equalize the flow 

 throughout the year by making the low stages higher and the high stages lower. 



Floods which are produced by exceptional meteorological conditions can not 

 be prevented by forests, but without their mitigating influence the floods are more 

 severe and destructive. 



CONCLUSIONS 



THE extent of forest land necessary for the regulation of streamflow and 

 the protection of the soil against erosion must be not less than from one- 

 fifth to one-third of the total area of the country. 



Forests must be protected, not so much in localities which already suffer 

 from lack of moisture, as in regions which lie in the path of prevailing winds 

 and are still abundantly supplied both with ground water and precipitation. 



In the dry regions large bodies of forest may have an unfavorable effect up- 

 on the available water supply. There rows of trees or windbreaks surrounding 

 fields and orchards, by preventing the drifting of the snow and increasing the 

 activity of the wind, will act more as conservers of moisture in the soil than solid 

 bodies of timber. 



The care with which forests should be protected in the eastern half of the 

 United States must increase from north to south and from west to east. 



In the Atlantic plain and southern Appalachians, which are the gateway for 

 the prevailing winds from the Gulf of Mexico and the Atlantic Ocean, forests 

 must be especially conserved if the humidity of the Central States and the prairie 

 region is to be maintained. 



