opinion that some additional factors were responsible for the disap- 

 pearance of plants from the river proper. Coontail, longleaf and sago 

 pondweeds, and wild celery have disappeared from the Starved Rock Pool 

 since the 1940's, even though the transparency of the water has been 

 adequate for their growth in many years since then (Mills, et al. 1966). 

 Summary 



In the early part of the 20th century, the bottomland lakes and 

 backwaters along the Illinois River produced most of the fish because 

 they contained most of the food, which in turn was associated with the 

 aquatic weeds that were abundant in the shallow, clear waters. Approxi- 

 mately half the bottomland acreage subject to overflow was drained or 

 leveed. The river became increasingly polluted, with low dissolved oxy- 

 gen levels occurring farther downstream, while some of the lakes, which 

 were fed by springs or tributaries, continued to offer havens for fish 

 and food organisms. More recently, some of the lakes have developed 

 periodic low dissolved oxygen levels during the summer, while the oxygen 

 levels in the middle and upper sections of the river have improved some- 

 what due to more and better waste treatment by industries and munici- 

 palities in the drainage basin. 



The number of fish and duck food organisms in the lakes have de- 

 clined in recent years, probably because of low oxygen levels, absence 

 of aquatic plants, and presence of flocculent sediments. The once- 

 abundant rooted aquatic plants may have also served to lock up nutrients 

 that are now available to bacteria and plankton. Bacteria and plankton, 

 and the biochemical oxygen demand exerted by the sediments, are probably 

 responsible for low oxygen levels in the lakes during overcast days or 

 at night when photosynthesis does not occur. 



Present Impacts 



Many of the factors which have had a detrimental impact on the 

 biota of the Illinois River in the past continue to have an impact at 

 the present time. Two major factors are wastes from urban areas and 



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