THE MISSISSIPPI VALLEY. 53 



The valley of the Upper Alleghany is, froiri hill to hill, about one-third of a 

 mile in width. A reservoir, 25 miles in length, would, therefore, cover an area 

 of some 8 or 9 square miles along the principal valley, and much more than that 

 in its lateral branches. Such a dam as is here spoken of — from 55 to 60 feet in 

 height — would probably form a lake covering from 16 to 18 square miles, with 

 an average depth of near 30 feet, and containing more than 12,000,000,000 

 cubic feet of water. This is nearly double the quantity which was ascertained 

 above to have been requisite to supply a depth of 2^ feet during the whole of 

 the summer of 1838, the season of greatest drought ever known west of the 

 Alleghany range. 



Now, we have already seen that the whole volume of water constituting the 

 injurious portion of the flood of 1841 could have been retained in reservoirs 

 capable of holding 44,000,000,000 cubic feet. It follows, then, that we should 

 need but four dams, such as are here described, less than 60 feet high, to secure 

 the valley of the upper Ohio against all destructive floods ; and these same 

 dams, as already shown, would be capable of supplying the navigation with suf- 

 ficient water for boats of 5 feet draught during a period of 60 days, if the natural 

 discharge should fail entirely. 



But, though the Ohio never ceases entirely to send water to the Ocean, it will 

 be seen, by reference to Note D, in the Appendix, that, during the memorable 

 drought of 1838, there was a period of 120 consecutive days when the depth on 

 the bar at Wheehng was less than 5 feet. The actual average daily discharge 

 during these 120 days, computed by the formula given in this paper, was about 

 220,000,000 cubic feet. The daily discharge, when there is 5 feet on the bar, 

 has been found to be 864,000,000 cubic feet. The difference between these 

 quantities is 644,000,000, which is the volume that should have been furnished 

 daily for 120 days, in 1838, to have secured a depth of 5 feet throughout that 

 year. 



Reservoirs capable of furnishing this volume must possess a capacity of 

 ■77,000,000,000 cubic feet. But this was a period of unexampled suffering from 

 drought ; and an application of the facts furnished by the tables and records 

 in these pages shows that a supply which would have been sufficient to secure 

 a depth of five feet in 1838, would have secured full six feet in any ordinary 

 season. 



We shall then require three or four high dams upon the Alleghany and Mo- 

 nongahela, or an equivalent number upon other tributaries of the Ohio, to 

 insure, with this single known exception, a perpetual navigation for boats of 5 

 feet draught. 



It is not appropriate in this place to enter into any detailed estimate of the 

 cost, or description of the mode of constructing, such dams. It may be said, 

 however, that they should be formed of massive masonry, set in hydraulic 

 cement, and built more with reference to the part they arc to perform in 

 advancing the commercial prosperity of the country, than with a view to stinted 

 economy. Yet, formed as monuments of the art and enterprise of the age, it is 

 Art. 4—8 



