THE MISSISSIPPI VALLEY. 57 



has provided that the fluid shall be thoroughly exposed to sun, and hght, 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 arti- 

 ficial reservoirs in all respects analogous to those of Nature, liable to be drained 

 off, to some extent, 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 not intended to be wholly exhausted ; 

 and need only to be reduced at the surface, so as to lay bare a portion of their 

 rocky borders. 



But the salubrity of rivers, when no longer subject to become dry, and have 

 their sands and vegetable deposits exposed to the summer's sun, must necessa- 

 rily be increased ; for the same expei'ience which teaches that large masses of 

 fresh water, existing as lakes, are salubrious, also teaches that shallow, stagnant 

 pools, such as are found in the place of an exhausted river, are deleterious to 

 health. 



It is difficult to imagine a serious objection to the improvement of this great 

 natural system of inland navigation, by a method which accomplishes so much 

 for an outlay so small — by a plan which places no incidental impediment in the 

 way of trade, and the application of which is limited to no State or section of 

 the Union. 



From the base of the Rocky Mountains to the base of the Alleghany, there is 

 not a great river or navigable tributai'y that may not be benefitted by this pro- 

 cess ; while on the eastern slope of the dividing range there are numerous rivers 

 flowing into the Atlantic which have been improved by other means, and which 

 must ultimately be subjected to this treatment, and relieved of the dams by 

 which they are now obstructed. 



The North Branch of the Susquehanna may be easily made navigable, from 

 its mouth into the State of New York, for a convenient class of steamboats, 

 by this simple expedient ; and there are several rivers in Virginia, which, for an 

 insignificant cost, may probably thus be supplied with abundant water for a 

 permanent navigation. 



The personal observation of the writer does not extend to the great rivers of 

 the southern States. But the elevations of their surfaces above tide, from point 

 to point, seem to indicate that they are even more susceptible of the application 

 of this method of improvement than those further north ; while the Cumberland 

 and Tennessee, and the rivers of Kentucky, possess all the essential character- 

 istics of the Ohio, and will always aflford a good navigation for steamboats 

 whenever they are adequately supplied with water. 



Further west the field of valuable improvement is immense. Probably two 

 thousand miles of precarious navigation on the Missouri alone, may be rendered 

 permanent and safe by a few dams constructed upon the great tributaries above 

 the mouth of the Yellow Stone ; and, as civilization is carried by steam into 

 those distant regions, it is reasonable to suppose that the same incidental 



