98 LITERATURE ON TORRENTS. 



" I make no mention of the influence of forests in regard to evaporation 

 — in regard to the direct absorption of water — and in regard to the 

 humidity of the atmosphere, &c. I take up one point of view alone of the 

 torrential management of waters in the high mountains, and these relate to 

 this only indirectly. 



" If a storm of rain beat upon a forest the whole of the water which falls 

 is temporarily retained, all penetrates more or less deeply the soil without 

 flowing on the surface; and, it may be objected, if the subsoil is imperme- 

 able the result will be the same. But the objection is without foundation. 

 I shall suppose, what is frequently the case, that there is impermeable 

 rock underlying the hwrrms : all the water should arrive at this bed of rock 

 and flow down, but the hydroscopicity and capillarity of this humus — of the 

 ground — of the foliage — of the branches of the trees — in a word, of the 

 material of which the forest is composed — will arrest the water to such a 

 degree and measure as to regulate temporarily the delivery. 



" In support of what has just been said," says he, " I shall cite an obser- 

 vation made quite recently in the canton of Appenzel, in Switzerland. The 

 torrent of Weissenbach formerly appeared in a swollen state at Weissenbach 

 about three hours subsequent to the bursting of the storm on the mountain ; 

 but since the woods have been destroyed — and this has only been done to a 

 partial extent, and those destroyed have been replaced with a fine gazonne- 

 ment — the floods appear at Weissenbach within an hour after the storm. 

 In this we have a very striking illustration of the influence of forests, 

 and of the gazonnermnt which has taken their place. And whoever has 

 Kesided in the mountains will understand that a delay of two hours in the 

 appearance of the flood, and in its protraction (which augments by four 

 hours the period of flow), may suffice to prevent the most serious disasters ; 

 for there everybody knows that the great danger from Alpine torrents arises 

 from the suddenness, amounting almost to instantaneousness, of their flood. 



" I have glanced rapidly at the action of forests, in view simply of their 

 efiect on the water which falls on their surface ; but their function is by no 

 means limited to this, for they serve also to arrest the waters which come 

 from the pastures above them. They constitute in some measure a kind of 

 immense and powerful barrage, or barrier, placed between the summit and 

 the bed of the valleys. 



" In support of this allegation, I shall cite personal observations which 

 seem to me conclusive. Never have I seen, during the most violent storms 

 of rain, superficial flowings of water in the forests situated under pastures, 

 though such flowings may have existed in the meadows at a greater eleva- 

 tion than the forests ; all the waters which these supplied were literally 

 absorbed and retained by the forest soil. I except, intentionally, well- 

 marked ravines, which coming from above traverse forests, for the question 

 here is only of slopes somewhat uniform, or hut slightly undulated; it is 

 evident that the soil of the forest will not absorb the water of a stream which 

 traverses it encased in a bed. 



" I take, for example, a valley which rises to a summit line somewhat 

 elevated. The end situated at a great height is formed entirely of pastm-e 

 lands, which stretch out equally on the summits of the brows of the moun- 

 tains ; at a lower level beneath these are the forests. The waters which 

 fall into the cistern formed by the head of the valley rapidly accumulate, 

 and give birth to a torrent which traverses the forest. On the contraiy, 

 that which falls on the pasture lands above the brows do not commonly reach 



