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stream with all the elements of its fertility strained out; but such a self- 
contained and self-sufficient system is far from anything we actually find, 
and there must be a heavy loss of potential productivity at the mouth of 
every normal stream. 
It may be noted, however, that a river, notwithstanding its continuous 
losses, may actually maintain a heavier plankton than a lake, even where 
both derive their food materials from the same source, as from the 
waters of a spring flood. The river being continuously fertilized while 
the lake has only the fertility left to it when the flood subsides, if the 
former has a slow current, as in low water, and a long course, its plank- 
ton may multiply to a number which the fixed fertility of the lake can not 
maintain. 
EFFECTS ON THE FISHERIES 
The apparent effects upon fish production of some of the environ- 
mental changes here discussed were treated in considerable detail in 1913 
in our paper on the biology of the upper Illinois River. In this it was 
shown that the fishes normal to the river were virtually driven out of it 
during the midsummer low water of 1911 and 1912 by pollutional condi- 
tions extending as far down as Ottawa, 34 miles from the source, and 
that it was not until Hennepin was reached, 32 miles still farther down, 
that the fish population of the river was made up of substantially the 
same species as were to be found farther down. It was, however, in the 
Henry-Chillicothe section of the river, 77 to 93 miles down-stream, that 
the relative numbers of the several species first became virtually normal, 
and that all ichthyological effects of the Chicago contamination had 
obviously disappeared. 
After the above paper appeared we compiled statistics of production 
for the whole river from every available source, and these were placed, 
in 1915, at the disposal of Alvord and Burdick, civil engineers, who were 
making at the time for the Rivers and Lakes Commission of the state an 
extensive study of the Illinois River and its bottom-lands with special 
reference to problems of fisheries, agriculture, and flood control. Their 
report on the status and possibilities of the river fisheries embodies sub- 
stantially all our information bearing on the subject at the time, and we 
find their reasoning convincing and their conclusions sound. Our objects 
and point of view are, however, somewhat different from theirs, and we 
find it necessary to develop some parts of the discussion more fully than 
was done by them in their report. 
That the actual harvest of the Illinois River fisheries was greatly 
increased from 1900 to 1908 as compared with the yield from 1894 to 
1899 is shown by statistics collected by the Illinois Fishermen’s Associa- 
tion, the Illinois State Fish Commission, and the U. S. Bureau of Fish- 
eries; but that a marked decline of production followed, at least until 
1913, is made evident by data of shipments from Havana, one of the 
principal fishing points on the river, obtained by the junior author of 
this paper for the six years following 1907. From 1894 to 1899 the 
