82 American Fisheries Society 



over the dam comparatively clear of sediment. Such sus- 

 pended organic materials as continued in it were pulverized 

 at the dam and the water was thoroughly mixed with air, 

 so that an eighth of a mile below, instead of the 7.5 per cent 

 of oxygen found above, Spaulding's analyses gave us 65.4 

 per cent. A large part of this was really contained in air 

 mechanically mixed with the water but not actually dis- 

 solved; but three-fourths of a mile below this free air had 

 escaped, and the permanent content of dissolved oxygen 

 there was 24.8 per cent. From that on down the river as 

 far as we went the dissolved oxygen increased, under ordi- 

 nary conditions, until, at Chillicothe, the lowest point at 

 which we made collections, a maximum of 49 per cent was 

 reached. Notice, now, that at the lower end of this ninety- 

 three mile section of the river, we obtained less than 50 per 

 cent of saturation with oxygen, while just above the upper 

 end, in the mouth of the Kankakee, we got 112 per cent, 

 showing that the water did not return to anything like its 

 normal stage of oxygen saturation through these ninety- 

 three miles. 



To this rather long and complex account 1 will add only 

 a remark or two as to the effect of these conditions on the 

 fish life of the stream. 



In the first place, the upper twenty-six miles of the Illi- 

 nois was at this time practically a great septic tank for the 

 city of Chicago and its suburbs. Our studies were made, 

 however, in a season of continuously low water and unusual 

 heat. What the facts would be under other conditions it 

 will take further investigation to determine. At the foot of 

 the dam below Marseilles, where the water has been aerated 

 and partially purified, carp are said to appear almost con- 

 tinuously, and black bass are caught here occasionally. Not- 

 withstanding the deficiency of oxygen, fish were found all 

 along down the stream in increasing numbers as we went 

 farther south. Fortunately, the upper end of this river is 

 not the fish-producing end. There were some minor fish- 

 eries here originally, but all the important commercial fishing 



