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POPULAR SCIENCE MONTHLY 



Fig. 4. Hydrograph of the Mississippi River from June, 1902, to May, 1005. 



It is apparently easier for the tributaries of the lower Mississippi not 

 to overcome this rise, but to flow down stream in the back swamp lands 

 or bottoms until some more formidable barrier forces them to empty 

 their waters into the main channel. In this manner the St. Francis 

 (Fig. 2) flows for 100 miles and is forced into the Mississippi just 

 above Helena, where the main river, after crossing the alluvial basin, 

 touches the higher land on the west. The Yazoo Eiver is forced in at 

 Vicksburg, after flowing along the flood plain for 200 miles. Other 

 tributaries show this same characteristic. The Atchafalava pursues 

 its course to the Gulf as an independent stream. The height of the 

 river bank over the back swamp districts varies from 10 to 25 feet. 



The large area of the drainage basin of the Mississippi would yield 

 an unmanageable amount of water to the lower river during the stage 

 of flood if the excess of discharge at that time resulted from a uniform 

 rainfall condition. The very size of the basin with the tributaries of 

 the main stream reaching far into rainfall areas of different types and 

 seasons is beneficial to the control of floods. Reference to the map 

 (Fig. 1) and the appended explanation may aid one in understanding 

 the condition of rainfall over the basin. In addition, it is well to 

 bear in mind that the condition of the ground affecting the amount 

 of run-off of water is an important factor in the amount of discharge. 

 The Ohio basin has its heaviest rains in January, February and March. 

 Its largest tributaries, the Cumberland and the Tennessee, rise in 

 regions of copious winter rainfall and add enormous volumes of water 

 to the Ohio. The basin of the Ohio is less than one half that of the 

 Missouri, yet it furnishes over twice as much water to the Mississippi. 

 The melting of the snow and the frozen condition of the ground which 

 increases the percentage of run-off at the time of the early spring rains 

 swell the volume of the Ohio, while the late spring rains over the 

 Missouri basin fall on an absorptive soil. Only 15 per cent, of the 

 rainfall is drained from the latter basin and 24 per cent, from the 

 former. The percentage over the Ohio during the flood months be- 

 cause of the conditions stated above is probably much higher. 



