ArtIcLE VI.—Some Recent Changes in Illinois River Biology.* By 
STEPHEN A. ForBes AND Ropert EARLE RICHARDSON. 
The principal causes of change in the biological environment of the 
Illinois River which have come in since 1899 are the opening of the drain- 
age canal of the Sanitary District of Chicago in January, 1900, a great 
increase in the amount of Chicago sewage emptied into the stream, and 
an extensive reclamation of the river bottoms for agricultural uses; and 
it is our present purpose to trace the principal effects of these changes 
upon the life of the stream. Although this study has had primarily a 
purely scientific motive, it has bearings of a practical character which we 
have followed out in some cases beyond the boundaries of our proper 
inquiry, with a view to making our results available, or at least suggestive, 
to those who have to deal with problems of sewage disposal, fisheries, 
bottom-land agriculture, and the like. 
The principal changes due to the three causes above mentioned are 
presented summarily in the following numbered sections :— 
1. The opening of the drainage canal of the Sanitary District of 
Chicago, January 17, 1900, admitted a flow of sewage-laden lake water 
which greatly increased the average depth and flow of the Illinois and 
lengthened the period and extended the range of its overflows. During 
the ten years preceding 1900 the average flow of the river at Peoria, 110 
miles down-stream, was 8,391 cubic feet per second (or ‘second feet”) +; 
but the flow of the sanitary canal alone in 1913 averaged 7,193 sec. ft., 
or 85.7 per cent. of the Peoria flow of the original river.t 
If we take account of some 600 sec. ft. of Lake Michigan water 
coming into the Illinois River by way of the old Illinois and Michigan 
Canal, which is, of course, additional to the contribution of the sanitary 
canal, we have left 7,791 sec. ft. as the average flow from the natural 
watershed of the stream above Peoria during the decade just preceding 
1900. Combining the increments from the two canals, we find just halt 
the average Peoria flow derived in 1913 from Lake Michigan. 
At average low water of the above-mentioned decade just preceding 
1900, the flow: at Peoria was only 17 per cent. that of the canal in 1913, 
and at the Jowest rate of discharge (723 sec. ft.) it was only 10.5 per 
cent. At the highest water of the period on the other hand, the canal, if 
open at the time, would have increased the flow of the stream by less 
than 10 per cent. The ratio of canal to river water at the lowest river 
level was over eight times that at the highest. The effect of the canal 
upon the rising river is therefore to hasten the onset of the overflow and 
to increase the area greatly in the beginning, but this effect diminishes as 
"The present brief paper is an abstract only of some of the more general and 
significant parts of the product of a long series of studies of the biology of the Illinois 
River and its dependent waters which will be reported in detail in a number of articles 
now in course of preparation. 
7 Jacob A. Harman, in the Report of the Illinois State Board of Health on Sani- 
tary Investigations of the Illinois River Tributaries, in 1901. 
¢ Alvord and Burdick, Report on the Illinois River and its Bottom-lands (1915), 
p. 37. 
