areas where there is a moderate amount of organic pollution and where 

 soft mud bottoms are available. 



Between 1964 and the 1970' s, the fingernail clam populations de- 

 clined slightly (the declines are very slight, and perhaps insignificant), 

 Asiatic clams invaded the river, and the snails either disappeared 

 entirely or were reduced to such low numbers that they did not show up 

 in the recent collections. Asiatic clams were first found in the Illi- 

 nois River in 1974 by Thompson and Sparks (1977: 34-36) and Colbert et 

 al. (1975: G15-G19). The oldest Asiatic clams Thompson and Sparks found 

 were 2- and 3-year-old clams, indicating that Corbicula first entered 

 the Illinois River in 1970-1971. They may have been introduced in 

 gravel delivered to a ferry landing at Kampsville (river mile 32) . We 

 do not know whether the Asiatic clam will displace or compete with 

 the native sphaerid fauna (Thompson and Sparks, 1978: 391). Asiatic 

 clams do not appear to be as nutritious a food for fish and waterfowl 

 as the native fingernail clam, Musculium transversum , but the Asiatic 

 clam may furnish a food source in portions of the Illinois River where 

 fingernail clams have been eliminated by pollution (Thompson and Sparks, 

 1978: 394-395). 



Table 4 shows that the same trends which occurred in the river 

 border and side channel habitats between 1915 and the 1970 's also oc- 

 curred in the main channel: the midges, oligochaete worms, and may- 

 flies increased, Asiatic clams appeared, and the snails disappeared. 

 The increase in the numbers of organisms which burrow in the mud, such 

 as the worms , midges , and burrowing mayflies ( Hexagenia and Pentagenia ) , 

 indicates that the construction of the dam at Alton may have reduced 

 the current velocity and caused some deposition of mud in the main channel. 

 Other studies have confirmed the absence of snails in the lower Illinois 

 River. No snails were found in an intensive 2-year study at a power 

 plant site at Meredosia (WAPORA, 1974: Appendix C15-C26) . Butts (un- 

 published report, 1975: 4-5) found no snails in a benthic survey of 

 Meredosia Lake. Some toxic agent present in the lower Illinois River 

 may be eliminating the snails. Sparks and Walter (unpublished data) 



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