260 FLOODS OF THE ARDECHE. 
The Rhone, therefore, is naturally subject to great and sudden 
inundations, and the same remark may be applied to most of 
the principal rivers of France, because the geographical char- 
acter of all of them is approximately the same. 
The volume of water in the floods of most great rivers is de- 
termined by the degree in which the inundations of the differ- 
ent tributaries are coincident in time. Were all the afiluents 
of the Lower Rhone to pour their highest annual floods into its 
channel at once—as the smaller tributaries of the Upper Rhone 
sometimes do—were a dozen Niles to empty themselves into its 
bed at the same moment, its water would rise to a height and 
rush with an impetus that would sweep into the Mediterranean 
the entire population of its banks, and all the works that 
man has erected upon the plains which border it. But such a 
coincidence can never happen. The tributaries of this river 
run in very different directions, and some of them are swollen 
principally by the melting of the snows about their sources, 
others almost exclusively by heavy rains. When a damp south- 
east wind blows up the valley of the Ardeéche, its moisture is con- 
densed, and precipitated in a deluge upon the mountains which 
embosom the headwaters of that stream, thus producing a flood, 
while a neighboring basin, the axis of which ties transversely 
or obliquely to that of the Ardeéche, is not at all affected.* 
* “There is no example of a coincidence between great floods of the 
Ardéche and of the Rhone, all the known inundations of the former having 
taken place when the latter was very low.” —Marpieny, Mémoire sur les Inon- 
dations des Riviéres de V Ardéche, 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 MrEssEDAGLIA, Analisi del? opera di Champion, ete., p. 103. 
The floods of the affluents of the Tiber form an exception to this law, being 
generally coincident, and this is one of the explanations of the frequency of 
destructive inundations in that river.—LOMBARDINI, Guida allo Studio del? 
Idrologia, ff. 68; same author, Hsame degli studi sul Tevere. 
I take this occasion to acknowledge myself indebted to Mardigny’s interest- 
ing memoir just quoted for all the statements I make respecting the floods of 
the Ardéche, except the comparison of the volume of its waters with that of 
the Nile. 
