Small particles of clay and organic matter have both a greater affinity 

 for toxicants than larger particles such as sand, and also a much 

 greater surface area per unit weight. The finer particles settle out 

 where the current velocity is reduced: above the dams and in areas 

 lateral to the main channel, hence one would expect the concentration 

 of toxicants in sediment to follow the same distribution. 



Illinois River 



We were able to find very little water quality data collected 

 before 1938 on the lower 80 miles of the Illinois River. Most of the 

 early water quality studies by the Illinois Natural History Survey 

 and Illinois State Water Survey were directed toward measuring the 

 degree and extent of pollution in the middle and upper reaches of 

 the Illinois River. Richardson (1921b) states that by 1920, the 

 bottom fauna in the river and bottomland lakes as far downstream 

 as Browning (mile 97,0) had been affected by organic waste and low 

 dissolved oxygen levels. His statement indicates that the lower 

 river either was not affected, or affected to a lesser degree than the 

 middle and upper reaches of the river. 



The section on benthos in the Illinois River described how the 

 tubificid worms, fingernail clams, and chironomid larvae increased 

 between 1915 and 1964. These results indicate that the organic loading 

 of the lower river may have increased, while the dissolved oxygen 

 levels had declined slightly. Figure 8 shows that a pronounced oxygen 

 sag occurred in the LaGrange Pool, immediately upstream from the study 

 reach. The water was reaerated as it flowed over the LaGrange Dam, 

 and then a slight sag occurred in the lower 80 miles of the Illinois 

 River. The minimum dissolved oxygen levels in the study reach in 1965-66 

 were 2-3 mg/1 (Figure 8 ) — lower than the current Illinois standard of 

 4.0 mg/1, and probably low enough to stress some types of aquatic organisms. 

 Recent data indicate that dissolved oxygen levels in the backwaters and 

 lakes have declined, probably due to the high oxygen demand exerted by 

 the sediments, while the oxygen levels in the river have improved. Studies 

 conducted in the summer of 1977 by the Illinois Natural History Survey 



138 



