44 PHYSICAL GEOGRAPHY OF 



OF THE FLOODS OF THE OHIO. 



The records exhibited in the preceding pages, and the volumes of water dis- 

 charged in periods of twenty-four hours, when the river is at given heights, 

 shown in Table IV, are useful in another application. They teach us the 

 important fact, that the great floods of this river may be so controlled as to be 

 deprived of their destructive power, and forced, by the hand of man, to glide 

 harmlessly to their place in the Ocean. 



A flood in a great river, like the Ohio, is but a wave, spreading along a limited 

 portion of its coui'se. The banks of the stream are only overflowed by water 

 from the same tributary, for a very short distance at the same time, and the 

 channel of the river is never filled simultaneously from its source to its mouth. 

 When there is a great rise at Pittsburg, there is no perceptible rise, produced 

 by water from the Alleghany and Monongahela, at Louisville; and when the 

 flood originating at Pittsburg has reached Louisville, the water has always sub- 

 sided again at Pittsburg. 



A great flood in the upper Ohio rarely continues at its extreme height more 

 than ten or twelve hours ; and but a single instance can be found in the records 

 of the last eleven years, of a rise which continued at a greater height than 25 

 feet at Wheeling, longer than four days. When the river is at its highest mark 

 at any point, as it usually remains at that level but the fraction of a day, it is 

 rising 100 miles below, and falling 100 miles above; it has quite a moderate rise 

 UOO miles below ; and has receded again, within moderate bounds, 200 miles 

 above. From these extreuies it rises towards the point which, for the moment, 

 is the top of the swell, forming a wave, represented in — 



FIG. 3. 



This figure is not designed to show the absolute form of the wave, the height 

 of which is due to the diflerence between the inclination of the surface during 

 the flood, and in the ordinary regimen of the stream ; and varies from hour to 

 hour, as the supply becomes greater or less than, or just equal to, the discharge. 

 The slope of the river is not represented in the cut, and the height of the wave 

 is greatly exaggerated. In fact, the surface descends from A to C, although C 

 is, compared with the summer level of the stream, higher than A. 



At the point A the river may have fallen to 25 feet, and at the point B it may 

 have risen to 25 feet above low water ; while at C, the centre of the swell, it 

 might stand — as during the great flood of 1832, at Wheeling — 44| feet above 



