BASINS OF RECEPTION. 
399 
this quantity might have been suffered to flow down its chan¬ 
nel without inconvenience, w^e shall have about 650,000,000 
cubic yards to provide for by reservoirs. The Ardeche and 
its principal affluent, the Chassezac, have, together, about 
twelve considerable tributaries rising near the crest of the 
mountains which bound the basin. If reservoirs of equal 
capacity were constructed upon all of them, each reservoir 
must be able to contain 54,000,000 cubic yards, or, in other 
words, must be equal to a lake 3,000 yards long, 1,000 yards 
wide, and 18 yards deep, and besides, in order to render any 
effectual service, the reservoirs must all have been empty at 
the commencement of the rains which produced the inun¬ 
dation. 
Thus far, I have supposed the swelling of the waters to be 
uniform throughout the whole basin; but such was by no 
means the fact in the inundation of 1857, for the rise of the 
Chassezac, which is as large as the Ardeche proper, did not 
exceed the limits of ordinary floods, and the dangerous excess 
came solely from the headwaters of the latter stream. Hence 
reservoirs of double the capacity I have supposed would have 
been necessary upon the tributaries of that river, to prevent 
the injurious effects of the inundation. It is evident that the 
construction of reservoirs of such magnitude for such a purpose 
is financially, if not physically, impracticable, and when we 
take into account a point I have just suggested, namely, that 
the reservoirs must be empty at all times of apprehended flood, 
and, of course, their utility limited almost solely to the single 
object of preventing inundations, the total inapplicability of 
such a measure in this particular case becomes still more glar¬ 
ingly manifest. 
Another not less conclusive fact is that the valleys of all 
the upland tributaries of the Ardeche descend so rapidly, and 
have so little lateral expansion, as to render the construction 
of capacious reservoirs in them quite impracticable. Indeed, 
engineers have found but two points in the whole basin suit¬ 
able for that purpose, and the reservoirs admissible at these 
would have only a joint capacity of about 70,000,000 cubic 
