218 turnbull et al. RIVER ALLUVIUM PROBLEMS [Ch. 12 



The deposits are mainly very thinly laminated clays and silts which 

 may contain organic matter, especially in southern portions of the 

 valley. In addition, deltaic backswamp deposits contain much fresh 

 organic material such as wood and roots of trees (Russell, 1933). 

 The sediments are nearly always calcareous, as soluble materials tend 

 to be concentrated in these basins. They vary in thickness from less 

 than 20 feet in the latitude of Memphis to over 70 feet southward in 

 Louisiana, where they merge into the deltaic deposits along an in- 

 distinct line. Adjacent to the natural levees of Fig. 3, backswamps 

 can be recognized from their low-lying, wooded appearance and their 

 lack of directional drainage. 



Influences of Flood-Plain Deposits 



The flood plain in the Lower Mississippi Valley covers approximately 

 35,000 square miles, a value slightly greater than the area of Louisi- 

 ana. In addition there is a non-flood-plain portion of the Recent al- 

 luvium which consists of levels between the lowest terrace and the 

 present flood plain. These surfaces are low fans which extend from 

 valley entrances of tributary streams and are being dissected under 

 present conditions. Areal extents of these fans are pictured in Fig. 1. 



Within its flood plain, the Mississippi River has distinct regional 

 variations of behavior which are related to the sediments through which 

 it flows. Southward from Cairo, Illinois, the river flows in a region 

 of coarse deposits where its channel is chiefly in sands and silts. Un- 

 der these circumstances the river tends to develop wide, shallow chan- 

 nels and meanders extensively. Below Helena, Arkansas, sands and 

 silts continue to predominate but with the influence of varied meander- 

 belt deposits. Farther southward in the area of deep, fine-grained 

 deltaic sediments, which are resistant to scouring action, channels tend 

 to be narrower, deeper, and more stable. 



In general, as a result of regional sediment changes, the Mississippi 

 River may be designated according to the following behavior units: 



Commerce, Missouri, to Helena, Arkansas. The river is broad and 

 shallow and has many towheads and a high percentage of reaches. 

 Channels shift rapidly. 



Helena, Arkansas, to Angola, Louisiana. Here there are deeper, 

 narrower channels with fewer reaches and fewer towheads, although 

 meandering still takes place rapidly. In this region the channels are 

 complicated through influences of excessively variegated meander-belt 

 sediments. 



Angola, Louisiana, to the gulf. The channel is deep and narrow but 

 with much less meandering. False River, Louisiana, is the southern- 



