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8. Owing to the much greater volume of the main stream since 1899, 
the effect of tributary waters is now much less than before, whether 
this be the diluent effect of a cleaner stream than the Illinois, such as the 
Fox at Ottawa, or the polluting effect of a local inflow of sewage from 
one of the river towns, as at Pekin or Peoria. For the same reason these 
diluent effects are much less obvious and important at a high stage of 
water than at a low stage. Peoria sewage, for example, can not now 
pollute the stream at any time nearly as much as it formerly did when 
the river was low. 
9. The present greater average depth of the stream and the conse- 
quent lower average temperature must retard somewhat the rate of 
spontaneous chemical change by which the proteid compounds of the 
raw sewage, quite unfit in that condition to serve as food for chlorophyl- 
bearing plants, become converted by successive steps into forms fit for 
assimilation by green plants—into free ammonia, nitrites, and nitrates, in 
sequence—and must carry these stages of change farther down the stream 
than formerly; and this effect is to be added to that due to a virtual 
doubling of the rate of the down-stream movement. 
10. As a consequence of several of the foregoing considerations it 
follows that the uppermost part of the river was more heavily charged 
with organic matter before 1900 than in 1914, and this headwaters pol- 
lution diminished rapidly downwards, the river returning to practically 
normal conditions much farther up-stream. 
11. In our earlier paper it was shown that the tumbling of the 
water over the dam at Marseilles had a marked effect in 1911 to increase 
the content of dissolved oxygen, the additional oxygen being obtained, of 
course, from the air mixed with the water by the fall; and it was also 
shown that this increase of oxygen differed with the stage of water in 
the stream, being greatest when the river was low and the head of the 
falls was consequently highest. From this it follows (although we have 
no data on the influence of the dams before 1900) that this oxygenating 
effect must have been greater at the relatively low stages of that period 
than it is now. The removal of the upper dams, especially that at Mar- 
seilles—a measure strongly advocated in the interests of navigation— 
would have the effect to carry the polluted waters farther down-stream 
and to postpone their renovation both in distance and time. 
12. It is also obvious, from what has already been said, that the 
organic contents of the sewage, unfit, as they are delivered to the stream, 
to serve at once as food.for its normal clean-water animals and plants, 
must become first available to them farther down-stream than before, and 
that, as a consequence, such normal organisms will find themselves ex- 
cluded, at least at times of greatest pollution, from parts of the upper 
river where they have previously found food. Just how far down-stream 
this change of conditions has progressed is a question difficult to answer 
exactly, especially as we have no collections of aquatic plants and animals 
from the upper river made before 1900 in a way to bring them into com- 
parison with those made by us since that date and thus to give us direct 
