ALLUVIAL BASIN OF THE MISSISSIPPI 



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The sequence of floods from the tributaries is first, the Ohio, then 

 the upper Mississippi, followed by the Missouri and the western streams. 

 Great floods are not annual occurrences. Eecently the years marked 

 by excessive floods have been the years of 1893, 1897 and 1903. The 

 floods of the years 1898 and 1901 did not fall much short of the 

 records of the seasons previous. 



The hydrographs of floods (Fig. 4) and the profiles of rivers at 

 different seasons (Fig. 5) show 

 that the floods proceed down 

 stream somewhat as a wave. The 

 highest point or crest marks the 

 extreme danger limit due to 

 height of flood. The hydrograph 

 also indicates that the crest flat- 

 tens somewhat in its down-river 

 progress. The reason for this 

 may be seen in the river profiles 

 which are drawn from three 

 stages of the river's annual fluc- 

 tuations. The down-stream slope 

 (AC) of the flood wave has in- 

 creased over the normal slope of 

 the river (GE). This results in 

 an increase of speed of the waters 

 on this slope. On the other hand, 

 the up-stream slope (BA) is a 

 less one than the normal slooe 

 (EG), and a decrease of speed 

 over normal flow results. At cer- 

 tain times the slope (BA) may be 

 against gravity, and a further 

 retardation will be experienced. 

 This would occur when a tribu- 

 tary added a large volume of 

 water to the Mississippi, as in the 

 case of the Ohio during the 

 spring freshets. The tendency 

 of this increase of speed on the 

 down-river side of a flood wave 

 and the decrease of velocity on 

 the up-river side is to reduce the 

 size of the flood wave by draining 

 off the excess of water faster than 

 it can accumulate. If the river 

 is long enough to allow this proc- 



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Fig. 5. Profiles of the Mississippi River 

 at Flood Stage (BC), at low water stage (DE), 

 and at an intermediate stage. 



