by increasing the turbidity. The increase in turbid- 

 ity is more noticeable in the lower three pools, par- 

 ticularly in the Alton Pool, than it is upstream be- 

 cause of differences in bottom types.... The outrush 

 of water from shore toward the channel caused by a 

 towboat also temporarily exposes the shallow areas. 

 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 (Jackson turbidity 

 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." 



Figure 5 shows that the turbidity in mid-channel at mile 25.9 was in- 

 creased by approximately 100 Jackson turbidity units (JTU) as towboats 

 passed on three occasions. It took approximately 2% hours for the tur- 

 bidity to return to background levels following passage of towboats. 



In addition to increasing turbidity, the resuspended sediment ex- 

 erts an oxygen demand. Butts (1974) reported that the oxygen demand 

 of sediment in the Upper Illinois River increased several-fold when the 

 sediment was disturbed. The oxygen measurements plotted in Figure 5 

 suggest that resuspension of sediment by barge traffic significantly 

 depresses oxygen levels. The oxygen levels at the bottom and surface 

 declined following passage of a towboat. The oxygen levels at mid-depth 

 increased slightly. The decline of 0.4 mg/1 oxygen at the bottom is 

 significant, because the standard deviation of the method used to meas- 

 ure dissolved oxygen (azide modification of the Winkler method) is 

 0.1 mg/1. The present impact of boat traffic and the potential impact 

 of increased traffic on dissolved oxygen levels and turbidity in the 

 river should be investigated further. 



Future Impacts 



Improvements in waste treatment 



The Illinois State Water Survey has developed a model to predict 

 dissolved oxygen levels in the middle and upper sections of the Illinois 

 River under various waste loads during low-flow conditions (Butts, 

 et al. 1975). Under extreme low-flow conditions likely to occur once 



63 



