EEPOKT FOR 1912 AND 1913 55 



cover, prevent erosion, and check surface runoff in general, thus in- 

 creasing the underground seepage and so tend to maintain a steady 

 flow of water in streams. 



Forests in hilly and mountainous country, even on the steeper 

 slopes, create conditions with regard to surface runoff such as obtain 

 in a level country. The steeper the slope the less permeable the soil, 

 and the heavier the precipitation the greater is the effect of forests 

 upon streamflow. 



FORESTS AND EROSION. 



Forests are the most effective agency for protecting the soil from 

 erosion, 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, precipita- 

 tion, 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 summer 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 regu- 

 larity 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 extent 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. 



