THE EFFECT OF THE FOREST UPON WATERS 



i6i 



like those that devastated Languedoc 

 in the autumn of 1907 and very re- 

 cently. Man is necessarily helpless in 

 the face of meteorological phenomena, 

 hut he can lessen their consequences by 

 protecting the soil against erosion, and 

 by diminishing the proportion of detri- 

 tus carried bv the run-off. It is not 

 necessary for us to offer proof upon 

 this point, the truth of which is defi- 

 nitely admitted by Messrs. Lauda and 

 Charles Rabot. 



vSince everyone is willing to recog- 

 nize the efficacy of the "geological" role 

 of the forest (to make use of the term 

 employed by Mr. Rabot), it is a point 

 calculated to make one concede also its 

 action upon floods. The fact that the 

 volume of detritic matter carried by the 

 streams in periods of flood is notably 

 diminished by the presence of the forest 

 must have as a corollary a decrease in 

 the violence of the floods, since it is 

 recognized that the presence of the for- 

 eign matter borne along in the current 

 greatly roughens the action of the 

 water. In fact, this detritic matter — 

 sand, gravel, pebbles, and rocks — torn 

 from the soil by the stream, raises the 

 level of the stream beds, increases to a 

 large degree the density of the waters, 

 and, as a consequence, their power of 

 undermining, and aids in the erosion 

 of the steep banks. 



There is no lack of facts to estab- 

 lish this moderating action of the forest. 

 Mr. Marchand gives an example of a 

 torrent at Weissenbach, in the canton 

 of Appenzell, Switzerland, which for- 

 merly became swollen at Weissenbach 

 about three hours after the storms had 

 burst upon the mountain ; following a 

 partial deforestation of the mountain, 

 the floods became manifest at Weissen- 

 bach only one hour after the appear- 

 ance of storms. The presence of the 

 forest, then, had the effect of delaying 

 by two hours the manifestation of flood, 

 and of increasing bv four hours the 

 duration of the run-ofif. 



Mr. Marchand has noticed a fact of 

 a nature to prove that there is a diminu- 

 tion in detritus carried by the water 

 because of the forest growth. In the 



great forest of Ofen, canton of Grisons. 

 he saw numerous mud-burdened chan- 

 nels deposit the sediment with which 

 they were charged among the fir trees, 

 and yield only pure, clear water where 

 they issued from the forest. 



The diminution of run-ofl^ resulting 

 from the presence of forests must be 

 attributed on the one hand to the re- 

 tention by the forest foliage of a part 

 of the rainfall, a fact verified by the 

 experiments of Marshal Vaillant in 

 1866 in the forest of Fontainebleau, 

 by those of Mr. Fautrat in the forests 

 of Halatte, and of Ermenonville (1874- 

 1877). and by those of Mr. Mathieu in 

 the forest of Haye ; on the other hand, 

 to the innumerable obstacles, the boles 

 of the trees and shrubs, the twigs, the 

 dead leaves, and the inextricable net- 

 work of roots, all of which break up the 

 rainfall and delay the ofif-flow. But it 

 is above all to the great absorptive 

 power of the layer of dead leaves, of 

 plant debris and humus, which covers 

 the surface of the forest soil ; of moss, 

 herbs, and bushy plants, which grow 

 under the leafv arches of the trees, 

 which, altogether, form what we call 

 the forest floor, that we must attribute 

 the retention of the greater part of the 

 rainfall and of the water formed by 

 the melting of snow. This water, held 

 at the surface, penetrates the soil slowly 

 and unites with the subterranean waters 

 which give birth to springs. A great 

 part is absorbed, besides, by the roots 

 of the plants in the ascensional move- 

 ment of the sap, and forms a part of 

 their nourishment. 



THE RETENTION OF WATER BY THE 

 FOREST FLOOR 



The retentional capacity of the forest 

 floor has been established by experi- 

 ments made in Germany and in France, 

 with great scientific accuracy, the re- 

 sults of which we will state briefly. 



Gerwig admits that, upon a surface 

 of one square meter, moss retains on 

 an average 4,466 kilograms of water. 

 Consequently, it is able to retain ten to 

 thirtv millimeters of water. 



