CAUSES OF TORRENTS IN MOUNTAIN FORESTS 151 



and, moreover, it is ordinarily little protected by forest vegetation, for 

 in France much land of this type is in fields or is devoted to agriculture. 

 The formation of a torrent gorge involves three distinct areas: (1) 

 An area on which rain water falls before it runs into the "thalweg" or 

 collecting basin; (2) an eroded area, the torrential gorge or bed; and 

 (3) an area of deposit, the delta or torrential cone. Surell designates, 

 under the name of the basin of reception — 



". . . The region in which the water collects and floods the ground, but, when 

 concerned with work of restoration, it is preferable to consider the entire collecting basin 

 under the general definition of basin; . . . the space at the bottom of which runs a 

 course of water and into which all the slopes are drained." 



Torrent gorges that form on steep mountain slopes are short and 

 usually run at right angles to the slopes. Where the slope is gentle 

 the length of the gorge increases and the curves or bends in the gorge 

 probably increase. These curves seem to be due to the unequal resist- 

 ance of different parts of the hills. A torrential cone does not always 

 exist at the base of the torrent gorge. Sometimes the gorge or the 

 drainage way extends to the bottom of the valley and the material borne 

 by the torrent is deposited in a river which carries it away. 



Causes of Torrents in Mountain Forests. — Huffel says that torrential 

 rains, easily eroded surfaces, and steep slopes promote torrents. Tor- 

 rent gorges — the products of erosion — may be due immediately or 

 directly to natural conditions or operations, but are usually directly 

 or indirectly due to the destruction of the soil cover by the residents of 

 the region in which they occur. It is conceivable that an exceptional 

 storm might start erosion that would form a torrent gorge in a virgin 

 forest. An accumulation of overmature timber or natural windfall 

 coupled with a heavy snowfall and rapid melting may produce avalanches 

 and denuded slopes. The area at the upper limit of tree growth is 

 always liable to damage from the normal snowslides which are so preva- 

 lent in high mountains. Numerous other causes of "normal" damage 

 by nature to forested areas could be cited, but under the usual condi- 

 tions this change in topography — a change which is continually going 

 on all over the world — is slow and locahzed. Where valley after 

 valley that was once forested is being eroded and where there are numer- 

 ous torrents nature is not alone to blame. The destruction must be 

 due (1) to deforestation and (2) to the breaking of the soil surface. 

 In France the once forested mountain areas that are now being repaired 

 at so great expense were overcut, burned, and overgrazed. This might 

 be termed collectively "abusive use." Even to-day typical examples 

 abound (see also p. 153). 



At Mont Dore (Puy-de-D6me) there are Roman mineral springs, and 

 around the watering places there has been partial deforestation caused 



