SUMMARY 



1. Several factors are responsible for increased sedimentation 

 and turbidity in the Illinois and Mississippi Rivers. Agricultural 

 practices, such as clearing of fencerows and bottomland timber, plus 

 increased acreages of row crops have created serious erosion problems 

 contributing to the sediment load. In addition, municipal sewage 

 effluents, construction activities, and certain industrial discharges 

 add to the sediment loads. 



2. A major long-term effect of the 9-foot channel project was to 

 increase turbidity and the rate of sedimentation. In general, 



in backwater areas the sedimentation rate is 

 highest in the deeper waters. The construction of the navigation 

 dams increased the water levels at the dam sites in Pools 24, 25, and 26 

 by 9, 10, and 9 feet respectively. The dams also slowed the velocity 

 of the Illinois and Mississippi Rivers, reducing their silt-carrying 

 capacity and resulting in a greater sedimentation rate. Increased 

 barge traffic associated with the 9-foot channel increased turbidity 

 in two ways: (a) movement of the barges and the associated towboat 

 resuspends sediment, and (b) the wake from barges contributes to bank 

 erosion and resuspends sediment in shallow areas. During rising river 

 stages this resuspended sediment is carried into the backwaters where 

 it settles, eventually destroying fish and aquatic wildlife habitats- 



3. Backwater habitat for some species of fish, such as carp, 

 bass, and sunfishes, increased following impoundment of the Mississippi 

 River by navigation dams. Within the study area of Pools 24, 25, and 

 26, there is a total of approximately 73 square miles of aquatic habi- 

 tat at normal pool elevations. Although this certainly represents a 

 gain in backwater or slackwater habitat, pre-construction acreage was 

 unavailable for comparison. Physical loss or physical alteration of 

 backwater habitat as a result of sedimentation has been, and continues 

 to be a problem. 



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