154 
ing, since the river now hurries its burden downward at a rate more rapid 
than it did before it was shut out from so large a part of its valley. 
This correlation of production with the area of overflow is well 
brought Out by Alvord and Burdick in the following diagram showing 
the relation, from 1894 to 1914, between the yield of fishes from the 
Illinois, on the one hand, and the area of the river valley which was 
under water for half the year or more on the other. 
200 
: 
i BS Ge as ae Ee ee ee 
Se a re EN | a TN Nee 
1} +++ 
WATER AREA- THOUSANDS OF ACRES 
ANNUAL FISH YIELD-MILLIGNS OF POUND 
"That the various important changes in the river environment should 
take unequal effect on the various species of fish, is of course to be 
expected. Those most sensitive to pollution will be the first to disap- 
pear from the upper river as pollution increases and the last to reappear 
as one goes down the stream. As the river fisheries are now directed 
chiefly to the catching of carp, native fishes of similar range and habits 
to the carp are most likely to be taken in excessive numbers and to 
dwindle year by year as a consequence; and as the ratio of shallow along- 
shore water to the deeper parts of the river and lakes is diminished by 
diking and draining of bottom-lands, the species which feed and breed 
alongshore will feel the effects of the change most deeply. In our col- 
lections of 1911 and 1912 a single minnow known as the shiner (Notropis 
atherinoides) was especially notable for its indifference to pollution, and 
this was the only fish found in the main current in midsummer, in more 
than accidental frequency, as far up the stream as Marseilles, 26 miles 
from the source. A number of other species were taken at this place, 
it is true, below the dam, but only at the mouths of small tributaries 
which were diluting the river water with a cleaner flow. At Ottawa, 34 
miles from the source, single specimens of gizzard-shad, red-horse, and 
European carp were taken from the undiluted river among 79 fishes 
captured in various ways, 69 of which, however, were the shiner above 
mentioned. At Starved Rock (42 miles), the black bullhead and the 
large-mouthed black bass were added to the list, and at Hennepin (66 
