2G7 



moisture, and the balance, between the consumption and the saving of evaporation 

 by forest growth, is largely in favor of this kind of vegetation as compared with 

 any other vegetable cover or with naked ground, provided the forest floor of 

 decayed leaves, twigs, etc., is not destroyed. Furthermore, the melting of snows 

 is retarded under the forest cover, and finally the mechanical retardation of the 

 surface water flow promotes subterranean drainage, insuring to springs a greater 

 supply for a longer time. 



This observation, very generally made, used to be explained by popular 

 writers as due to the sponge-like condition and action of the forest floor, being 

 able to take up water and then gradually giving it up to the soil below. For- 

 tunately, the forest floor is rarely like a sponge, for a sponge never gives up water 

 below, but always by evaporation above after the supply has ceased. The simile 

 was an unfortunate one. 



The open runs, i. e., brooks, rivulets and rivers, receive their supply mainly 

 from springs, but also from the surface waters which flow without definite channels 

 down the slopes. The more the supply is derived from springs the more even is 

 the water flow of the river ; the greater the suppjy of the surface drainage the 

 more dependent is the water flow on the changeful rains and on the melting of 

 the snows, and the more changeful is the water flow. While, then, in the first 

 place, the water flow in rivers is dependent upon the amount and frequency of 

 rainfall and snow, the manner and time in which the water reaches the channels 

 determines the greater or smaller extremes of water stages. 



The retardation of the melting of the snow, which in a well-covered moun- 

 tain district may be prolonged for two or three weeks under a forest cover, is of 

 great significance in reducing the spring floods. The main influence, however, lies 

 in the mechanical impediment which the forest floor Opposes to the rapid surface 

 drainage, promoting filtration to the soil and preventing the rapid filling of sur- 

 face runs and lengthening the time during which the water is to run off. Obser- 

 vations in one of the reforested parts of the French Alps showed this retardation 

 to be in the ratio of 5 to 3. 



Thus, while in extreme cases, with excessive rainfalls or sudden rises of tem- 

 perature in early spring, with steep declivities and impermeable rock formation, 

 even a forest cover may have no practical effect in preventing a flood, it may be 

 accepted as a generally true proposition that a forest cover has a tendency to 

 lengthen the time to run off, and hence to reduce in -amount and frequency flood 

 conditions and to maintain the water flow more even with fewer excessively low 

 and high stages. 



Lastly, but of greater importance than has often been conceded to this influ- 

 ence, the forest cover prevents erosion of the soil and formation of the so-called 

 detritus of rocks, gravels, and sands which, carried into the rivers, increase the 

 danger from floods, impede navigation, and if deposited on fertile lands may. as in 

 France, destroy the soil value of whole districts. Along the coast and in the 

 sandy plains the protection of the loose soil and dunes against the disturbing 

 action of the winds, and in the mountains which are liable to avalanches and 

 snow-slides, as in Switzerland, the protective value of a forest is also well estab- 

 lished. If there were any doubts regarding the influence of forest cover upon 

 water and soil conditions before they have been entirely dispelled by the extenT 

 sive reforestation work undertaken by the forest department of France. 



There seventeen departments or counties had been impoverished and depopu- 

 lated by the washing of the soil, torrential action of the rivers, and repeated 

 floods, due to deforestation of the mountains, when the government adopted the 

 policy of reclothing the denuded slopes with tree growth and sod. The popula- 



