Sediment 



Sediment has many effects on aquatic ecosystems in the Mississippi 

 and Illinois River valleys. Sediment can occur in an aquatic ecosystem 

 in several ways. Suspended sediment is that particulate matter that 

 is carried in the water column. Deposited sediment is that particulate 

 matter that has dropped from the water column. Resuspended sediment is 

 stirred up from the bottom by water currents, wave action, boat traffic, 

 or by the rooting activities of fish, such as carp. Sedimentation is 

 defined as the deposition of the solid particulate material by water. 



When sediment is suspended or resuspended in the water column it 

 contributes to turbidity. In the case of the lower Illinois River val- 

 ley, the sediments have formed a soft or false bottom that is readily 

 resuspended. 



From Keokuk, Iowa south to Alton, Illinois, the bottom of the Mis- 

 sissippi River is dominated by sand which is mixed with silt in some 

 locations (Platner, 1946: Table 5). This type of bottom is less easily 

 resuspended than the Illinois River bottom. 



The Mississippi has been historically described as carrying a large 

 silt load (Saxon, 1927: 78; Galtsoff, 1923: 371). The Illinois River, 

 on the contrary, was relatively clear. Barrows (1910: 4) described the 

 original discharge of the Illinois River as relatively small, being less 

 than the Rock River and a small fraction of the Mississippi. The Illinois 

 flows in an unusually wide floodplain and drops sediment on its slug- 

 gish edges resulting in the formation of natural lateral levees and bottomland 

 lakes (Mills, Starrett, and Bellrose, 1966: 3). The Mississippi flows 

 at a higher velocity but also forms natural levees, although these 

 levees are somewhat smaller than those on the Illinois River (Rubey, 

 1952: 123). 



Two major changes have occurred in the lower Illinois and Mississippi 

 valleys. The first major change took place mainly in the first quarter 

 of this century, when large tracts of bottomland were drained and leveed 

 from the rivers. In the Illinois River valley, approximately half of 



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