425 



ice was present during a prolonged period of low water, and it was ac- 

 companied by the almost complete disappearance of the plankton organ- 

 isms and by the death of large numbers of fish. Although this occurred 

 before the Chicago Drainage Canal was opened, it must be remembered 

 that the Illinois River was even then a heavily polluted stream. At that 

 time the volume of water carried was much smaller than at present and 

 the rate of flow was slower. It seems probable that the river because 

 of its slower current was more likely to freeze over then than now, but 

 that this tendency was partially counteracted by greater changes in level 

 which tended to break up the ice and keep the channel open. 



During January and February, 1925, dissolved oxygen determina- 

 tions were made in the Illinois River and related waters between La Salle 

 and Meredosia. Table I shows the results of these determinations to- 

 gether with notes on ice conditions and temperature of the water. Since 

 the river froze over before Christmas, it seems probable that when the 

 field work was begun, January 2;5, dissolved oxygen concentration had 

 reached its lowest point. Sami^les taken on January 23 and 21 showed 

 about 1.7 parts per million of dissolved oxygen at Peoria Narrows, Pekin, 

 and Havana, but those from Henry and La Salle showed about 6 parts per 

 million. This is probably not because of better aeration up-stream but 

 becau.se the initial charge of oxygen dissolved in the water that diluted 

 the sewage had, on account of the low temperature, not yet been consumed. 

 It is quite probable that at this time the very low oxygen prevailed from 

 Peoria Narrows to the mouth of the river at Grafton. On I'ebruarv 2. 

 oxygen concentrations as low as 0.4 part per million were found in Mud 

 Lake, which is partly fed by river water. Records of bullheads dying 

 in the traps and seines in Treadway Lake and Coleman Lake indicate 

 tbat low dissolved oxygen concentrations prevailed also in those back- 

 waters that are fed by the river. A sample taken in the middle of Quiver 

 Lake showed 2.3 parts per million of dissolved oxygen. This compara- 

 tively high figure is due to the highly oxygenated water coming out of 

 Quiver Creek. On February 4 samples taken in Treadway Lake and 

 Coleman Lake showed over 5 parts per million of dissolved oxygen. This 

 high figure is brought about by the dilution of these lakes with Sanga- 

 mon River water. By February 5, when the river channel was partly 

 o])en, sam])les at Beardstown showed 3.3 and 3.7 parts per million of dis- 

 solved oxygen in mid-channel with 4.6 and 4.1 near the east bank, due 

 to the better water coming out of Sangamon River. On February 6 and 

 7 there was much open water, but the water temperature w-as still between 

 0° and 4° C. Samples taken at Henry and Chillicothe showed about 

 5 parts per million of dissolved oxygen, which is somewhat lower than 

 the up-stream samples taken two weeks previously ; but the samples taken 

 at Peoria Narrows and Pekin gave about 3 parts per million, almost twice 

 the amount found on the earlier trip. The thawing continued until the 

 channel was free of ice by February S. On February 10 at Browning 

 the channel showed 5.8 parts per million of dissolved oxygen. The next 

 day at Beardstown it was 6.6. 



