FLOODS OF THE AEOilCHE. 253 



therefore, is naturallj subject to great and sudden inundations, 

 and the same remark may be applied to most of the principal 

 rivers of France, because the geographical character 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 different 

 tributaries are coincident in time. Were all the affluents 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 melt- 

 ing of the snows about their sources, others almost exclusively by 

 heavy rains. When a damp southeast wind blows up the valley 

 of the Ardeche, its moisture is condensed, and precipitated in a 



Durance, above its junction with the IsSre, an equal quantity, per second. — 

 MoNTLUiSANT, NoU sur les Dessechements, etc., Annales des Fonts et Ghausees, 

 1833, 2me seinestre, p. 288. 



The Upper Rhone, which drains a basin of about 1,900 square miles, includ- 

 ing seventy-one glaciers, receives many torrential affluents, and rain-storms 

 ind thaws are sometimes extensive enough to affect the whole tributary 

 system of its narrow valley. In such cases its current swells to a great 

 volume, but previously to the floods of the autumn of 1868 it was never 

 known to reach a discharge of 2,600 cubic yards to the second. On the 28th 

 of September in that year, however, its delivery amounted to 8,700 cubic 

 yards to the second, which is about equal to the mean discharge of the Nile. 

 — BericTite der Experteti- Commission uber die Ueherschwemmungen im Jahr 

 1868, pp. 174, 175. 



The floods of some other French rivers, which have a more or less torren- 

 tial character, scarcely fall behind those of the Rhone, The Loire, above 

 Roanne, has a basin of 2,471 square miles, or about twice and a half the area 

 of that of the Ardeche. In some of its inundations it has delivered above 

 9,500 cubic yards per second, or 400 times its low-water discharge. — Bel- 

 GRAi^D, De I' Influence des Forets, etc., Annales des Fonts et Ghausees, 1854, ler 

 semestre, p. 15, note. 



The ordinary low-water discharge of the Seine at Paris is nearly 100 cubic 

 yards per second. Belgrand gives a list of eight floods of that river within 

 the last two centuries, in which it has delivered thirty times that quantity. 



