Reservoirs to the Improvement of Rivers. 119 



Such are essentially the characteristics of all the great 

 rivers of the Mississippi valley. 



Many of these streams rise in the mountain ridges, and 

 flow great distances through depressions parallel with the 

 range in which they originate. Those which descend from 

 the Alleghany break through the subordinate ranges of 

 Laurel Hill, Greenbriar, Big Sewell, and other parallel and 

 analogous formations, where many gorges are presented, 

 easy to dam up, and where the lakes to be formed will lie 

 inclosed within a rim of rock, which will insure a purity equal 

 to that of the waters of Erie or Ontario. Tens, and per- 

 haps hundreds, of such sites exist in the valleys of the Alle- 

 ghany, Monongahela, Great Kanawha, and their tributaries ; 

 and, indeed, along all the rivers that flow from the mountains 

 on either slope of the great dividing ridge. 



It is not to be maintained that the water will become less 

 salubrious because it is confined. The lakes which it is pro- 

 posed to form are in all respects analogous to the great 

 fresh-water lakes of the globe, which are provided with out- 

 lets to the Ocean, through which the water is slowly dis- 

 charged, but, nevertheless, so adjusted as to retain the same 

 water for a long series of years. 



The salubrity of the fluid is not impaired by this exposure. 

 The Falls of Niagara probably do not vent the volume of water 

 which is contained in Lake Erie more than once in six or eight 

 years ; and it is certain that the contents of all the upper 

 lakes would not pass over the cataract in half a century. 



Nature relies for effecting the change which is for ever 

 taking place in great bodies of fresh water, almost exclu- 

 sively on the process of evaporation ; and has provided that 

 the fluid shall be thoroughly exposed to sun, and light, and 

 air, by the agitation of its surface when in volume, and by its 

 suspension in the clouds after its evaporation. 



The healthfulness of the country cannot be impaired by 

 the formation of artificial reservoirs in all respects analo- 

 gous to those of nature, liable to be drained off, to some ex- 

 tent, more than once in every year. They need not cover 

 vegetable matter in sufficient quantity to cause apprehension 

 from the effect of its decomposition. These reservoirs are 



