545 



General Summary of Chemical Features 



The foregoing general discussion of the assemblage of conditions, 

 chemical and biological, found at each of our observing stations in 

 succession for the entire period of our observations, may well be 

 supplemented by a separate recapitulation of the more important 

 chemical features of the river situation made to include oxygen de- 

 terminations from the middle and lower Illinois as well as from the 

 upper part of the stream, and from the Mississippi near its mouth — 

 a form of discussion which will bring out a few conclusions not 

 easily arrived at in any other way. 



July and August, ipii. — The lowest ratios of dissolved oxygen 

 in the upper Illinois were found in July and August, 191 1, When 

 determinations at the mouth of the Kankakee varied but little from 

 10 parts per million, those from the midstream at Morris, nine miles 

 below, ranged from .24 to 1.78; at Marseilles, above the dam, seven 

 miles farther down, they averaged .67, and below the dam 2.18; and 

 at Starved Rock, 3.18. At Chillicothe the ratios varied on ditTerent 

 days from 2.10 to 4.21 parts per millon, with an average of 3.47. 

 Single determinations at this lowest of our river stations yielded as 

 little as 19.5 per cent, of saturation, and the average of all our mid- 

 summer determinations here for 191 1 was but 40.8. As the Kan- 

 kakee River average at this time was 112.2, it appears that during 

 the upper ninety-three miles of its course the Illinois did not regain 

 much more than a third of the oxygen lost to the Chicago sewage. 



Heavy general rains from the loth to the 12th in the upper Illi- 

 nois basin disturbed river conditions materially, bringing the Kan- 

 kakee up about six inches at its mouth and the Illinois eight inches 

 at Marseilles and raising the levels below. Our trips for collections 

 and chemical analyses were made on three rounds from Dresden 

 Heights to Chillicothe, the first ending a week before these rains be- 

 gan, the second beginning at Dresden Heights on the date of the 

 principal rainfall, and the third following ten days after the rains 

 were over. Our oxygen tests are comparable for these trips at the 

 following five points : Dresden Heights, Morris, Marseilles above 

 the dam, Marseilles below the dam, and Chillicothe. At all these 

 places except Morris the oxygen ratios were considerably lower after 

 the rain than before, the decline being greatest (50 per cent.) at 

 Chillicothe, nearly a week after the rains. Averaging the readings 

 for these five points on the three successive trips, we got the same 

 mean oxygen ratio, of 3.64, for both the first and the last trips, and 

 3.17 for the intermediate one, indicating a decline of 13 per cent, in 



