THE MISSISSIPPI VALLEY. 45 



low water. On that remarkable occasion the flood rose 13 feet in the 24 hours 

 preceding the attainment of its greatest height ; and, as we shall presently see 

 that the apex of the curve did not move more than 61 miles per diem, this could 

 only have been a common spring flood at any point 100 miles distant from the 

 top of the wave. 



Now, it is not the height at A and B which the river occupies, 100 miles 

 above or below the crest of the curve, that causes the destruction of life and 

 property. It is the top of the wave only, which is sweeping through the valley, 

 buoying up and bearing ofi' every floating object above the line of harmless rise, 

 A B, that is injurious to the works of men. 



To control a flood, and render it harmless, we must retain in reservoirs the 

 volume of water which produces that portion of this wave above the Hne A B, 

 and fills two-thirds of the rectangle, a A, 6 B. A rise of twenty-five feet is a 

 flood which, at Wheeling, is shown by experience to cause but little damage. 

 The difference, therefore, between the discharge of the river when at 25 feet, 

 and the actual discharge for whatever period the height rules above 25 feet, is 

 the volume of water to be retained to render the flood entirely harmless. 



To make a practical application of the facts, we will take the flood of March, 

 1841, which, because of its long continuance, would have been the most difficult 

 to control of all the freshets which have occurred within the last eleven years, 

 although there have been instances when the height of the water has been 

 greater for a brief period. 



The records, (see Note D,) and Table IV, furnish the following data : 



March 24 • = 



• - height. 



, 31 feet • ■ 



■ = discharge, 



17,972,000,000 cubic feet. 



" 25 - . 



« 



32 «« " ■ 





18,760,000,000 





" 26 - - 



, . ff 



30 " . ' 





17,088,000,000 





- " 27 - « 



(( 



28 " - . 





15,322,000,000 





« 28 - . 





80 " - = 





17,088,000,000 





« 29 - . 



u 



34 '« - - 





20,624,000,000 





" 80 = - 



(t 



33 '^ . . 





19,700,000,000 





« 31 - . 



a 



30 '( = - 





17,088,000,000 





April 1 - - 



(( 



28 " - - 





15,322,000,000 





Total discharge during these nine days of high water, 158,964,000,000 " « 



This is the entire volume of water which passed down the river while the 

 height of its surface was more than 25 feet upon the Wheeling bar. But to 

 have kept this flood down to 25 feet all that time, we need only to have 

 retained the excess of the discharge over and above the quantity which would 

 have been vented during the same period, at a 25 feet stage. 



The diurnal discharge of the Ohio, when there is 25 feet in the channel, is, 

 by Table IV, 12,752,000,000 cubic feet; and the total discharge for these nine 

 days would then have been, at that depth, 114,768,000,000 cubic feet. The dif- 

 ference between this quantity and the actual discharge is 44,196,000,000 cubic 

 feet 5 and this is the volume which the reservoirs must be capable of holding, to 

 Art. 4.-7 



