544 



to the moment and to the place of the collection of the sample tested, 

 Hennepin to Henry. — At Hennepin it may be fairly said that vir- 

 tually normal conditions were found, except for the state of the bot- 

 tom in winter time ; although it must be confessed that our chemical 

 data, especially those for the midsummer low-water of 191 1, were 

 hardly consistent with this statement. Only 22 to 28 per cent, of 

 oxygen saturation, with carbon dioxide running up to 7.5 parts per 

 million, are ratios very far from those normal to an uncontaminated 

 stream; and the highest oxygen ratio found here at any time was 82 

 per cent, of saturation in November. Nevertheless, the greenish 

 tint of the water caused by the chlorophyll-bearing plankton, the vir- 

 tually complete disappearance of septic and pollutional organisms, 

 the inoffensive odor of the bottom sediments, except in winter, the 

 great predominance of living over dead specimens of the twenty-two 

 species of mussels taken here, and the appearance, even in hot sum- 

 mer weather, of suckers, crappie, warmouth, and blue-gill sunfish 

 in the products of our seines, showed that biological tests must be 

 added to those of chemical analysis if we are to have a fair picture 

 of the stages of recovery in a heavily polluted stream. 



Henry to Chillicothe. — In this, the last section of the upper Illi- 

 nois systematically studied by us in 191 1 and 1912, the process of 

 renovation is simply carried a little farther on than in the Henry- 

 to- Hennepin section just above. It is only in the winter time that 

 the effects of pollution are manifest here to the senses, in the more 

 or less rank and repellent odor of the sediments at the Henry dam, 

 and even of those from the bottom of the open channel at Chilli- 

 cothe ; and it is perhaps in part to this condition that we must attrib- 

 ute the reported great reduction in numbers of large catfish and of 

 buffalo from this part of the stream. 



There are, indeed, many species of fishes, most of them charac- 

 teristic bottom-feeders, which were formerly common in the upper 

 part of the Illinois River, taken in quantity there by fishermen sev- 

 eral years ago, but which are now either wanting, rare, or greatly 

 reduced in numbers. The large catfishes, the red-horse, the buffalo, 

 and the sheepshead are examples ; and even the bullheads are said 

 to be less common at Henry and Depue than in former years. On 

 the other hand, the sunfishes, crappies, and bass are likewise reported 

 to be decreasing of recent years, at least at Henry and Depue. In- 

 deed, the reported recent reduction in numbers of the more abundant 

 food fishes clearly becomes more pronounced as we go up stream 

 from Chillicothe to Depue, although fishing operations are less active 

 northward — a fact which points to unfavorable river conditions as 

 a probable cause of this diminished vield. 



