254 FLOODS OF THE ARDECfHE. 



deluge upon the mountains wliicli embosom the headwaters of 

 that stream, thus producing a flood, while a neighboring basin, 

 the axis of which lies transversely or obliquely to that of the 

 Ard^che, is not at all affected.* 



It is easy to see that the damage occasioned by such floods as 

 I have described must be almost incalculable, and it is by no 

 means confined to the effects produced by overflow and the me- 

 chanical force of the superficial currents. In treating of the 

 devastations of torrents, I have hitherto confined myself prin- 

 cipally to the erosion of surface and the transportation of mineral 

 matter to lower grounds by them. The general action of tor- 

 rents, as thus far shown, tends to the ultimate elevation of their 

 beds by the deposit of the earth, gravel and stone conveyed by 

 them ; but until they have thus raised their outlets so as sensibly 

 to diminish the inclination of their channels — and sometimes 

 when extraordinary floods give the torrents momentum enough 

 to sweep away the accumulations which they have themselves 

 heaped up — the swift flow of their currents, aided by the abrasion 

 of the roUing rocks and gravel, scoops their beds constantly 

 deeper, and they consequently not only undermine their banks, 

 but frequently sap the most solid foundations which the art of 

 man can build for the support of bridges and hydraulic struc- 

 tures.f 



*" There is no example of a coincidence between great floods of the Ar- 

 d^che and of the Rhone, all the known inundations of the former having 

 taken place when the latter was very low." — Maedignt, Memoire sur les 

 Inondations des Rwi^res de VArdeche, p. 26. 



The same observation may be applied to the tributaries of the Po, their 

 floods being generally successive, not contemporaneous. The swelling of the 

 affluents of the Amazon, and indeed of most large rivers, is regulated by a 

 similar law. See Messedaglia, Analm delV opera di Champion, etc., p. 103. 



The floods of the affluents of the Tiber and the Seine form an exception to 

 this law, being generally coincident, and this is one of the explanations of the 

 frequency of destructive inundations in the former river. — Lombardesti, 

 Ouida alio Studio delV Idrologia, ff. 68 ; same author, Esa/me degli studi sul 

 Tevere ; Belgrand, La Seine, p. 388. 



I take this occasion to acknowledge myself indebted to Mardigny's interest- 

 ing memoir just quoted for all the statements 1 make respecting the floods of 

 the Ardeche, except the comparison of the volume of its waters with that of 

 the Nile. 



f In some cases, where the bed of rapid Alpine streams is composed of very 

 hard rock — as is the case in many of the valleys once filled by ancient glaciers 



