56 PHYSICAL GEOGRAPHY OF 



OF THE PROPER APPLICATION OF RESERVOIRS TO THE 

 IMPROVEMENT OF RIVERS. 



It is not intended to recommend the application of this mode of improvement 

 to all streams. It is only those rivers, or parts of rivers, on vihich the imperfec- 

 tions of the channel are caused essentially by a deficiency of water, in seasons 

 of drought, and not by the rapidity of their fall, or obstructions in their beds, 

 that are susceptible of this mode of improvement. Rivers which, like the Ohio, 

 Alleghany, Cumberland, and Tennessee, are always navigable when there is 

 sufficient water in their channels to float the boats freely, but of which the navi- 

 gation fails because the supply of water fails, and on which lakes may be formed 

 at small expense, without injury to valuable property or to the salubrity of the 

 country — such rivers as these can be best, most cheaply, permanently, and 

 effectually improved, by collecting a portion of the waters which are wasted in 

 producing floods, and holding them in store for the season when the sources of 

 supply fail to render their customary tribute to the channel. 



Such are essentially the characteristics of all the great rivers of the Missis- 

 sippi 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 forma- 

 tions, where many gorges are presented easy to dam up, and where the lakes 

 to be formed will he enclosed within a rim of rock, which will insure a purity 

 equal to that of the waters of Erie or Ontario. Tens, and perhaps hundreds, 

 of such sites exist in the valleys of the Alleghany, Monongahela, Great Kana- 

 wha, 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 proposed to form are in all respects analo- 

 gous to the great fresh water lakes of the globe, which are provided with outlets 

 to the Ocean, through which the water is slowly discharged, 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 cateract in half a century. 



Nature relies for effecting the change which is forever taking place in great 

 bodies of fresh water, almost exclusively on the process of evaporation ; and 



