393 



types of organisms. This vit-w receives support also from the fact that 

 in this portion of the river such cleaner preference bottom species as are 

 represented at all occur on the average in less variety than do similar 

 forms in the lower part of Peoria Lake. 



In respect to the number of species of the old 11)13-1!)] 5 bottom 

 fauna that have apparently been exterminated by the sewage since some- 

 what less than 10 years ago, the various reaches below the head of Peoria 

 Lake examined in 1!)23 compare as follows : Peoria Lake, twelve 

 species exterminated ; Wesley to Copperas Creek Dam, thirteen species ; 

 Spring Lake Canal to Havana, 68-G9 species ; Matanzas to Beardstown, 

 24-25 species. 



It is particularly noteworthy that the greatest injury since 1913-1915 

 seems to have been done in the short but formerly exceedingly rich sec- 

 tion just above Havana ; where current is slackest and sedimentation most 

 abundant after floods ; or where, in other words, there was at the same 

 time most danger and the most that was capable of being destroyed. 



The similarity of its hydrographical conditions to those of the one- 

 time rich Liverpool-Havana section, and the fact that the list of species 

 known to be missing from Peoria Lake since 1913-1915 is so small in 

 comparison strongly suggests that at least the portion of Peoria Lake 

 above the lower narrows had been to some extent injured by Chicago 

 sewage previous to 1915. 



Evidence that the lower section of Peoria Lake, below the lower 

 narrows and opposite the city, has for several years been receiving serious 

 injury from wind and wave-borne local pollution is seen in the fact that 

 the improvement noted both in 19'20, 1922, and 1923 as we proceed south- 

 ward through the "upper" and "middle" lakes is wholly discontinued in 

 the "lower lake", when we properly discount the occurrences of a few 

 current-loving cleaner preference forms restricted to areas of unusual 

 current. 



The bottom animals covered above and in the following pages in- 

 cludjE only the smaller kinds, and are wholly exclusive of adult commer- 

 cial mussels, which have been largely exterminated recently in Peoria 

 Lake and in the river as far south as Havana. 



Illinois River above Peoria Lake, 

 La Salle— Chillicothe, 44 Miles 



With the exception of the single occurrence of the little shrimp 

 Hyalella kuickerhockcri in swift water near the shore below the dam at 

 Henry — an insignificant circumstance — all of the small bottom species 

 taken in 1923 in the river between La Salle and the head of Peoria Lake 

 belonged to the more strictly pollutional or tolerant groui)s, including 

 only: ui>wards of half a dozen varieties of Tubiticidae, small worms 

 some of which are characteristic of .septic sludge; two or three varieties 

 of leeches ; a similar number of kinds of chironomid larvae, of which one, 



