478 BASINS OF KECEPTIOIT. 



be seriously proposed is already approximately known by mete- 

 orological records, and the quantity of water delivered by the 

 greatest floods which have occurred within the memory of man, 

 may be roughly estimated from their visible traces. From these 

 elements, or from meteorological tables, the capacity of the neces- 

 sary reservoirs can be calculated. Let us take the case of the 

 Ard^che. In the inundation of 1857, that river poured into the 

 Rhone 1,305,000,000 cubic yards of water in three days. If we 

 suppose that haK this quantity might have been suffered to flow 

 down its channel without inconvenience, we shall have about 

 650,000,000 cubic yards to provide for by reservoirs. The 

 Ard5che and its principal afiluent, the Chassezac, have, together, 

 about twelve considerable tributaries rising near the crest of the 

 mountains which bound the basin. If reservoirs of equal capac- 

 ity 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 inundation. 



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 Hmits 

 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 physi- 

 cally, 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 be- 

 comes stiU more glaringly 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 capa/« 



