Anderson 1959:338-339). Much of the Illinois River 

 (as well as its adjoining bottomland lakes) is now char- 

 acterized by populations of pollution worms of the 

 family Tubificidae. However, some of the original di- 

 versity of benliiic organisms, such as immature insects, 

 clams, snails, leeches, moss animals, and the like, does 

 exist in some parts of the river and its lakes. 



Reduction in the abundance of the clean-water ani- 

 mals would be expected to have an adverse effect on 

 animals which rely on them for food, and, as we shall 

 see later, this undoubtedly has happened. 



Changes which have occurred since 1913 in the zones 

 of pollution based on the bottom fauna are shown (Fig. 

 5). The chart shows that approximately half of the 

 river in the La Salle-to-Beardstown section in the 1913- 

 1915 period contained principally clean- water benthic 

 forms, whereas in the 1964-1965 period most of the or- 

 ganisms in the same section of the river were pollution 

 worms, which are poor food for fish and ducks. 



Fingernail clams (Sphaeriidae) occurred in large 

 numbers in the Illinois River and some of its bottomland 

 lakes up to 1954 (Paloumpis & Starrett 1960:423-425, 

 and unpublished). The cause for the virtual dis- 

 appearance of these important food items is not 

 known, but there are strong indications that it was 

 a pollution complex of some kind. These tiny clams 

 (Fig. 6) still occurred in ihe river below the mouth of 



the Sangamon at Beardstown in 1964 (Starrett & 

 Paloumpis, unpublished). 



Snails of the genera Campeloma and Pleurocera also 

 occur at the present time in greater abundance in the 

 lower river than elsewhere, but we collected a few living 

 specimens (Campeloma) in 1964 from the river channel 

 below Henry by means of an otter trawl. In Quiver 

 Lake, above Havana. Paloumpis & Starrett (1960:425) 

 found that a small snail (Cincinttali tmarginata) disap- 

 peared simultaneously witii the fingernail clam. In the 

 lower part of this lake, which is properly a part of the 

 river, all species of snails decreased from 10.76 grams 

 per square foot (exclusive of shells) in 1952 to 6.07 

 grams per scjuare foot in 1954. and none in 1964 (Star- 

 rett & Paloumpis, unpublished) . 



Starrett & Paloumpis did not take midge larvae 

 abundantly anywhere in the river in 1964 and 1965. At 

 times they were more abundant in fish stomachs than 

 in the benthic collections, especially in the Peoria Pool. 

 It is possible that seasonal variations and local concen- 

 trations of larvae may have accounted to some extent 

 for this disparity. 



Burrowing mayflies (Hcxagciiia) were considered 

 by Richardson ( 1928) to be clean-water organisms in the 

 Illinois River. According to Hunt (1953:55) nymphs 

 of Hcxancnia limhata were unable to withstand stagnant 

 conditions when tlie dissolved oxygen dropped below 1 



Fig. 5. — Historical rhango in 

 pollution of the Illinois River as 

 indicated by bottom fauna sam- 

 ples. (From Riehardson, 191iH, 

 and Starrett & Paloumpis, un- 

 published.) 



MILES BELOW 

 LAKE 



LOCATION MICHIGAN 

 LA SALLE 101.5 



SPRING VALLEY 108.6 



CHILLICOTHE 146.5 



SPRING BAY 154.0 



NARROWS 



PEORIA NARROWS 161.0 



HAVANA 



BEARDSTOWN 



207.0 



238.0 



1913 -1915* 



EARLY 



POLLUTIONAL 



EARLY POLLUTION- 

 AL TO EARLY 

 SUB-POLLUTIONAL 

 IN I9II-I9I2 



LATE 

 SUB-POLLUTIONAL 



EARLY CLEAN-WATER 

 WHEN NOT AFFECTED 

 BY LOCAL SEWAGE 



PRINCIPALLY 

 CLEAN-WATER 



CLEAN-WATER 



I 



RATING OF TERMS FROM MOST TO 



EARLY POLLUTIONAL 

 LATE POLLUTIONAL 

 EARLY SUB-POLLUTIONAL 

 LATE SUB-POLLUTIONAL 

 EARLY CLEAN-WATER 

 CLEAN-WATER 



1920-1925* 



PRINCIPALLY EARLY 

 SUB-POLLUTIONAL 



EARLY 



POLLUTIONAL 



EARLY TO LATE 

 POLLUTIONAL 



POLLUTIONAL TO 

 EARLY SUB-POLLU- 

 TIONAL ABOVE 

 COPPERAS CREEK 

 0AM, LARGELY EAR- 

 LY SIJB- POLLUTION- 

 AL BELOW 



PRINCIPALLY LATE 

 SUB-POLLUTIONAL, 

 1920-1923, SHIFT- 

 ING TO EARLY 

 CLEAN-WATER AFT- 

 ER 1923 



I 



I I 



LEAST POLLUTED. 



1964-1965 



»* 



POLLUTIONAL 

 MAINLY TUBIFICID 

 WORMS AND 

 MIDGE LARVAE 



HEXAGENIA NYMPHSi 

 AND SPHAERID 

 CLAMS APPEAR 

 HERE SAMPLES 

 ALSO WITH TUBI- 

 FICID WORMS AND 

 MIDGE LARVAE 



* FROM RICHARDSON (1928:402) 

 **FROM STARRETT AND PALOUMPIS (UNPUBLISHED) 



12 



