ALLUVIAL BASIN OF THE MISSISSIPPI 



■49 



basin. It, too, lias a portion of 

 its drainage area protected from 

 the prevailing winds and approxi- 

 mates to the rainfall condition of 

 the Mississippi. The Ganges and 

 the Hoang Ho are in regions of 

 copious seasonal rainfalls. 



The portion of the Mississippi 

 Basin subject to inundation in- 

 cludes land on both sides of the 

 river south of Cape Girardeau, 

 Mo., and aggregates 29,700 square 

 miles, or an area equivalent to the 

 state of South Carolina. Through 

 this alluvial basin (Fig. 2) the 

 river winds in a rather tortuous 

 path, the distance by river (1,700 

 miles) being nearly three times 

 as long as a straight line drawn 

 from Cape Girardeau to the Gulf 

 of Mexico (GOO miles). 



It is characteristic of many 

 flood plains that the land imme- 

 diately adjacent to the river is 

 higher than the more distant 

 parts of the plain (Fig. 3). 

 These higher parts are called nat- 

 ural levees by some writers. If, 

 therefore, the river rises beyond 

 the limit of its banks, there is 

 likely to be a general inundation 

 of the alluvial basin. This fea- 

 ture of flood-plain form is further 

 illustrated by the tributaries of 

 the lower river. From an ordi 

 nary valley slope a river flows 



fairly direct into the trunk stream. If a tributary crosses a flood plain 

 of some size, the stream must force an entrance against the rise of the 

 back slope of this plain and finally must breach the banks of the river. 



Fig. 2 The Lower Mississippi, with Tp.ib- 

 utaries, alluvial basin, and hlghep. lands 

 (dotted). CG, Cape Girardeau; C, Cairo: C", 

 Columbus; M, Memphis; H, Helena; A, Arkan- 

 sas City: G, Greenville; V, Vicksburg; N, Nat- 

 chez ; BR, Baton Rouge; ISO, New Orleans. 



flood plain deposit- - 1 



-OL3 V ft L 1_ £ V S OS b 



-_-J 



--I 



Fig. 3. Cross Section of a River Valley. 



