314 



OUR FORESTS 



ground. Devastation of immense areas in China and considerable 

 damage by floods in parts of Switzerland, France, and the United 

 States have resulted where the forest covering has been removed. 

 The annual spring " freshets " in the Ohio and Mississippi valleys, 

 with all too frequent loss of both life and property, are due to the 

 lack of forest protection at the sources of these rivers. It has been 

 estimated that the carrying power of water is increased 64 times 



if its rate of flow is 

 doubled; that is, the 

 transporting power of 

 water varies as the sixth 

 power of its velocity. 

 This accounts for the 

 tremendous destruction 

 produced by a mountain 

 stream in flood. 



Prevention of Erosion 

 by Covering of Organic 

 Soil. — It is hard to 

 realize the vast amount 

 of soil that is dug out 

 by ungoverned streams 

 and carried far from its 

 original source. Exam- 

 ples of what streams 

 have done may be seen in the great canyon of the Colorado River 

 and the filling up of the Gulf of California with debris, or in 

 the huge deltas of the Nile and of the Mississippi River. The 

 forests prevent erosion in other places by holding back the water 

 supply and letting it out gradually. This they do by covering 

 the inorganic soil with humus or decayed organic material which, 

 like a sponge, holds water through long periods of drought. It is 

 estimated that the forest floor can absorb and hold for some time 

 a rainfall of between four and five inches. Thus it regulates the 

 flow off. The roots of the trees, too, help hold the soil in place 

 and prevent erosion. The gradual evaporation of water through 

 the stomata of the leaves cools the atmosphere, and this tends to 

 precipitate the moisture in the air. Eventually the dead bodies 



Result of deforestation in China. This land 

 has been ruined by erosion. (Carnegie Institu- 

 tion Research in China.) 



