because of the dams. This decrease in the Mississippi and Illinois 

 Rivers' current velocity has reduced their silt-carrying capacity, 

 increasing the siltation problem. 



Increased water levels created by the construction of the dams 

 also increased sedimentation rates. This statement is supported 

 by the fact that the sedimetation rate in Lake Meredosia increased 

 as its depth became greater. This has been further illustrated for 

 eight lakes in the middle Illinois valley (Bellrose et al., 1977: 

 C-ll, C-15). The construction of the dams increased water levels at 

 Clarksville, Missouri, Cap Au Gris, Missouri, and Grafton, Illinois ap- 

 proximately 9, 10, and 9 feet, respectively. These water depths were 

 greatest just above the dams and diminished as the distance upstream 

 from the dams increased (Simons, 1975: 94). The increased water levels 

 in the channel and backwater areas created by the construction of the 

 dams, combined with a reduction in current velocity, increased the 

 rate of sedimentation. 



Barge traffic, associated with the impoundment of the navigation 

 channel, increases suspended sediments in two ways: (1) Movement of the 

 barges and the associated towboat causes a strong current on the 

 silt bottom, which resuspends the silt particles, thereby increasing 

 turbidity. (2) Bank erosion and resuspension of silt in shallow areas 

 result from the wake produced by a passing barge. Starrett (1971: 273) 

 observed an increased in suspended sediments on the Illinois River r 



On November 18, 1964 in the Alton Pool at river mile 65.1, 

 the turbidity just prior to the passing of two towboats 

 was 108 units and within 6 minutes after the tows had passed, 

 the turbidity was 320 units. Sixteen minutes later the 

 turbidity had dropped to 240 units. 



Pools 24, 25, and 26 have complex networks of channels, pools, 

 and backwater areas. Water is generally supplied to the backwater 

 areas through runoff, and during high river stages, spillage occurs 

 from the main channel to the backwater areas (Karaki and Van Hoften, 1974: 19) 



146 



