54 
The Illinois River from 2 miles below Havana to 1.5 miles be- 
low the Head of Grand Island 
(Mile 209.0—215.0 below Lake Michigan) 
Neither in July—October, -19183—1915 nor at the same season in 
1920 were foul odors or abnormal bubbling noted at the muddier collect- 
ing stations in this somewhat swifter and harder-bottomed section of 
river than the two previously described. The conspicuous change in the 
physical appearance and odor of the bottom sediments that occurs in less 
than three miles below the wagon bridge at Havana is no doubt influenced 
by several factors, including as the principal ones the effect of the high 
Spoon River bar in favoring sedimentation in the widened river immedi- 
ately north of Havana, and the lesser rate of settling and greater mobility 
of bottom sediments in the stretch of faster channel that begins just be- 
low it. 
The improvement in the condition of the stream bottom as we pro- 
ceed southward of Havana is only a little less strikingly shown in the 
composition of the small bottom-fauna—if we except the Mollusca—than 
in the physical condition of the bottom deposits. Here in 1920, for the 
first time below Chillicothe, we found thriving the nymphs of the common 
willow-fly of the middle and lower Illinois, Hexagenia bilineata; some- 
thing like normal numbers of the common sand-caddis, Hydropsyche sp., 
as well as immature Gomphidae and other Odonata; and among non-in- 
sect bottom-animals, about the usual variety of Bryozoa, leeches, and 
planarians. Even on the molluscan side we did not find serious decrease 
in total-weight valuations over 1915, either in the channel or in the 1—3- 
ft. zone. But there was a decline of 95 per cent. in snail poundage com- 
pared with five years before, indicated for the 4—7-ft. zone, and the full 
list of snails for the section showed the disappearance since 1915 of not 
less than eight species, representing five families. The only species sur- 
viving were, in fact, the same three—one each of Viviparidae, Pleuro- 
ceridae, and Sphaeriidae—which have pulled through in the Chillicothe— 
Peoria and Liverpool—Havana sections. The disappearance of these 
snails from the river below Havana is not, however, to be compared in 
seriousness with the visitations of mortality above Havana, both for the 
reason that neither in the channel or the shallower zones were average 
snail-yields here anything like as high some years ago as above Havana, 
and because the remaining species have apparently quite or more than 
held their own in at least a part of the range since the disappearance of 
the more sensitive forms. 
