CAUSES AND PREVENTION OF INUNDATIONS. 39T 
dent that if such results are produced by this method, its intro¬ 
duction upon an extensive scale must also have the same 
climatic effects as other systems of irrigation. 
Whatever may he the ultimate advantages of reclothing a 
large extent of the territory of France with wood, or of so 
shaping its surface as to prevent the too rapid flow of water 
over it, the results to he obtained by such processes can he 
realized in an adequate measure only after a long succession 
of years. Other steps must he taken, both for the immediate 
security of the lives and property of the present generation, 
and for the prevention of yet greater and remoter evils which 
are inevitable unless means to obviate them are found before 
€ 
it is forever too late. The frequent recurrence of inundations 
like those of 1856, for a single score of years, in the basins of 
the Rhone and the Loire, with only the present securities 
against them, would almost depopulate the valleys of those 
rivers, and produce physical revolutions in them, which, like 
revolutions in the political world, could never he made to “ go 
backward.” 
Destructive inundations are seldom, if ever, produced by 
precipitation within the limits of the principal valley, hut 
almost uniformly by sudden thaws or excessive rains on the 
mountain ranges where the tributaries take their rise. It is 
therefore plain that any measures which shall check the flow 
of surface waters into the channels of the affluents, or which 
shall retard the delivery of such waters into the principal 
stream by its tributaries, will diminish in the same proportion 
the dangers and the evils of inundation by great rivers. The 
retention of the surface waters upon or in the soil can hardly 
be accomplished except by the methods already mentioned, 
replanting of forests, and furrowing or terracing. The current 
af mountain streams can be checked by various methods, 
among which the most familiar and obvious is the erection of 
barriers or dams across their channels, at points convenient for 
forming reservoirs large enough to retain the superfluous 
waters of great rains and thaws. Besides the utility of such 
basins in preventing floods, the construction of them is recom- 
