Bottom Fauna

The bottom fauna of a river consist of benthic organisms, such as insect larvae, fingernail clams, and snails which spend all or part of their life in the bottom sediments. Much of the Illinois River now contains primarily populations of pollution worms of the Tubificidae family (Mills 1966), although there are areas where a diversity of benthic organisms remains.

Approximately half of the river from La Salle to Beardstown contains clean-water benthic organisms in the period from 1913-1915, but by 1964-1965, mostly pollution-adapted organisms are found (Mills 1966). These species are poor food sources for fish and ducks. Fingernail clams disappear in 1954 due to some type of pollution complex. All snail species also decrease from 10.76 grams per square foot in 1952 to 6.07 grams per square foot in 1954, and none in 1964 (Mills 1966).

Other clean water species also disappear or decrease in numbers through 1965, among them the midgefly and mayflies. Mills reports that

...mayfly emergencies can be spectacular when millions of these insects are drawn to lights and may concentrate in such numbers as to be hazardous to traffic...The last time we observed a large emergence at Havana was in 1949 (Mills 1966).

The predominant benthic organisms we find in 1964-65 are pollution worms, even downstream of Beardstown. Such dramatic changes in benthic populations can only be explained by the accumulative effects of pollution in the bottom sediments (Mills 1966).