at river mile 99 (which is probably representative of conditions in 

 the study area, although it is located upstream from the LaGrange Lock 

 and Dam) showed that the minimum dissolved oxygen level in water 

 coming out of a backwater through Panther Slough was few tenths of a 

 ppm, while dissolved oxygen levels at the same time in the main 

 channel were on the order of 5-6 ppm. 



The benthos section of this report showed that the numbers of 

 oligochaete worms, midges, and perhaps fingernail clams declined be- 

 tween 1964 and 1974-1975, possibly indicating a reduction in the 

 organic load of the river and improvement in the dissolved oxygen 

 levels. The increase in number of mayflies from 4 per square meter 

 in 1964 to 172.2 per square meter in 1975 also indicates an improvement 

 in dissolved oxygen levels. 



The disappearance of snails in the lower Illinois River between 1964 

 and 1974 may indicate a toxicity problem, and pesticides were implicated 

 in studies where caged snails rapidly accumulated dieldrin when placed 

 in the Illinois River (see the benthos section) . The demise of vascular 

 aquatic vegetation in the lower Illinois River in the late 1950' s 

 may be attributable to increasing turbidity which reduces the penetra- 

 tion of light needed for photosynthesis, or to a toxicity problem. 

 Colbert et al. (1975: 42-43, Table D-II) found that iron concentrations 

 in the water at all stations except one exceeded the 1.0 mg/1 Illinois 

 standard, and mercury concentrations at most stations exceeded the 

 0.0005 mg/1 Illinois standard. The only factor which differed between 

 the Illinois and Mississippi Rivers within the study area was total 

 phosphorus, which was highest in the Illinois River. Colbert et al. 

 (1975: 46-47) reported that the sediments of the lower Illinois River 

 contained higher concentrations of ammonia, phosphorus, and iron than 

 sediment in the Mississippi River. Mean PCB and pesticide concentra- 

 tions in the sediments of the side channels were comparable for both 

 rivers. Sediment in the main channel of the Illinois River contained 

 detectable pesticide concentrations, whereas the Mississippi River did 

 not. Slightly higher PCB concentrations also occurred in the sediments 

 of the main channel of the Illinois River. 



