390 
DAMAGE DONE BY FLOODS. 
which lies transversely or obliquely to that of the Ardeche, 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 
mechanical force of the superficial currents. In treating of 
the devastations of torrents in a former chapter, I confined 
myself principally to the erosion of surface and the transporta¬ 
tion of mineral matter to lower grounds by them. The gen¬ 
eral action of torrents, as there 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 chan¬ 
nels—and sometimes when extraordinary floods give the tor¬ 
rents 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 rolling 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 structures.f 
* “ There is no example of a coincidence between great floods of the 
Ard&che and of the Rhone, all the known inundations of the former hav¬ 
ing taken place when the latter was very low.” —Haedignt, Memoire sur 
les Inondations des Rivieres de VArdeche, p. 26. 
I take this occasion to acknowledge myself indebted to the interesting 
memoir just quoted for all the statements I make respecting the floods of 
the Ard&che, except the comparison of the volume of its waters with that 
of the Nile, and the computation with respect to the capacity required for 
reservoirs to be constructed in its basin. 
t 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—and especially where they are fed by glaciers not overhung by 
crumbling cliffs, the channel may remain almost unchanged for centuries. 
This is observable in many of the tributaries of the Dora Baltea, which 
drains the valley of the Aosta. Several of these small rivers are spanned 
by more or less perfect Roman bridges—one of which, that over the Lys at 
Pont St. Martin, is still in good repair and in constant use, An examination 
