474 
Changes in the Quantity of the Bottom-Fauna Stocks 
between 1913 and 1915 
Various conditions or agencies besides any heretofore mentioned 
are doubtless capable of effecting local or temporary changes in the com- 
position and weight of the bottom and shore animals either in the river 
or the lakes. Among those peculiar to the river may be mentioned the 
occasional scouring effect of floods in the regions of steepest slope, re- 
sultant natural filling at points farther down stream, and the cutting away 
of the river floor by artificial dredging for channel improvement. In the 
lakes in which the heaviest commercial fishing is carried on, injury may 
be done to the bottom animals in the fall of the year by the heavy tackle 
used. Either in the river or in the backwaters variations of importance 
in the size of the bottom-fauna stocks may doubtless result from changes 
in the size of the population of bottom-feeding fishes, these variations 
being in the direction either of decrease or accumulation. Mortality 
from other unknown causes no doubt occurs at times, as seemed to be 
the case with the larger snails in Quiver Lake between July 1914 and 
July 1915. Of water pollution I note that this was not anywhere an im- 
portant cause of mortality in the river bottom-fauna below Chillicothe up 
to and including 1915. 
Comparison of such data as we have for the season of 1913 with 
the more complete results obtained in 1915 indicates generally a quite 
satisfactory correspondence both in average composition and size of the 
bottom-fauna stocks in the longer reaches of river channel between 
Chillicothe and the Kampsville dam, both series of collections bringing 
out clearly the contrast between the more productive reaches of channel 
above Havana and the decidedly poorer stretches below. The most 
important single point of disagreement between the 1913 and 1915 river- 
figures concerns the finding in 1913 in the lower 30 miles, where in 1915 
the average channel stocks of bottom animals were no larger than any- 
where else in the lower 75 to 100 miles, of a rich local fauna of Sphaerii- 
dae which apparently compared very well with the best found in the 
rich Havana district. The presence, only locally, of so rich a bottom 
population seems to imply the existence in that part of the river of 
an adequate food supply, and is doubtless sufficiently explained other; 
wise by its occurrence in a region of less than average slope and velocity 
and more favorable conditions for sedimentation than are found in most 
of the lower 100 or more miles of channel. For its disappearance be- 
tween 1913 and 1915 no certain explanation offers. I note, however, that 
it is in this wider, and on the average swifter portion of the channel that 
bar formation is most frequent in the Illinois and that dredging opera- 
tions for channel maintenance are oftenest carried on. - It is also in this 
part of the river, where for more than 70 miles the far greater portion 
of the bottom-land lakes had been leveed and drained before 1915, that 
sudden depletion of sporadic bottom-animal populations might most 
