NATURAL BASINS OF EECEPTION. 479 



cious reservoirs in them quite impracticable. Indeed, engineers 

 have found but two points in the whole basin suitable for that 

 purpose, and the reservoirs admissible at these would have only a 

 joiat capacity of about Y0,000,000 cubic yards, or less than one- 

 ninth part of what I suppose to be required. The case of the 

 Ard(^che is no doubt an extreme one, both in the topographical 

 character of its basin and in its exposure to excessive rains ; but 

 all destructive inundations are, in a certain sense, extreme cases 

 also, and this of the Ardeche serves to show that the construction 

 of reservoirs is not by any means to be regarded as a universal 

 panacea agairist floods. 



Nor, on the other hand, is this measure to be summarily re- 

 jected. ]S'ature has adopted it on a great scale, on both flanks 

 of the Alps, and on a smaller, on those of the Adirondacks and 

 of many lower chains. The quantity of water which, in great 

 rains or sudden thaws, rushes down the steep declivities of the 

 Alps, is so vast that the channels of the Swiss and Itahan rivers 

 would be totally incompetent to carry it off as rapidly as it would 

 pour into them, were it not absorbed by the capacious basics 

 which nature has scooped out for its reception, freed from the 

 transported material which adds immensely both to the volume 

 and to the force of its current, and then, after some reduction 

 by evaporation and infiltration, gradually discharged into the 

 beds of the rivers. In the inundation of 1829, the water dis- 

 charged into Lake Como from the 15th to the 20th of September 

 amounted to 2,600 cubic yards the second, while the outflow from 

 the lake during the same period was only at the rate of about 

 1,050 cubic yards to the second. In those five days, then, the 

 lake accumulated 670,000,000 cubic yards of superfluous water, 

 and of course diminished by so much the quantity to be disposed 

 of by the Po.* In the flood of October, 1868, the surface of 

 Lago Maggiore was raised twenty-five feet above low-water mark 

 in the course of a few hours.f There can be no doubt that with- 

 out such detention of water by the Lakes Como, Maggiore, Garda, 

 and other subalpine basins, almost the whole of Lombardy would 

 have been irrecoverably desolated, or rather, its great plain would 



*BArRD Smith, Italian Irrigation, i., p. 176. 



f Bollettino delta Societd Oeog. Italiana, iii., p. 466. 



