542 



way into the stream, particularly along the south side in the vicinity 

 of the mouths of creeks. In spring and in fall, bubbling from the 

 bottom ceases, the odor of the water is no longer repellent, a few 

 invertebrate animals reappear, and the oxygen ratios rise to a con- 

 siderable fraction of those normal to the Kankakee. The extent of 

 this seasonal oscillation depends, of course, upon the rainfall and 

 temperature; and the opposite extreme is reached in winter, when 

 midstream oxygen ratios may be fully as high as those of the summer 

 time for Chillicothe and Peoria. 



The Marseilles Dam. — The waters of the stream change but little 

 between Morris and the Marseilles dam, the oxygen content being 

 sometimes a little higher at one point and sometimes at the other. 

 The mean of eight sets of determinations in six different months was 

 4.5 parts per million for Morris and 4.3 at Marseilles above the dam. 

 The water is usually somewhat freer of sewage organisms at Mar- 

 seilles than at Morris. This is evidently owing in part to sedimenta- 

 tion of suspended particles, especially the larger ones, in the slack- 

 water above the dam, where the current, even at flood stages, was 

 only about half a mile an hour. Both plants and animals are a little 

 more varied and abundant at Marseilles in sheltered pockets along 

 the banks; and a larger variety of fishes was obtained at the edge 

 of the river close to the mouths of creeks. 



The interesting feature oi the Marseilles situation is the effect 

 produced by the fall over the dam* at low water. In samples taken 

 far enough below the fall to give the air caught in the water ample 

 opportunity to escape, we found the ratios of dissolved oxygen in 

 July and August, 191 1, more than three times as great as those above, 

 from one and a half to two times as great in August and September, 

 1 91 2, when the water was cooler and the river higher than in the 

 midsummer of the previous year, and 13 to 14 per cent, greater 

 under winter conditions in February and March. This difference in 

 oxygen doubtless had its effect upon the abundance of fishes, al- 

 though these might well be expected to be more numerous below 

 the dam than above, even if the oxygen supply were the same. The 

 fish population was, however, small and scanty, if we may judge by 

 our collections, consisting mainly of carp, bullheads, and the ever- 

 present shiner in the main stream, with a considerable variety of 

 species in shallow water near the mouths of tributaries. Black bass 

 were reported to us to come up to the dam only in the highest water. 



Ottawa and Starred Rock. — At Ottawa and Starved Rock a con- 



*This dam is about 710 feet in length, and its height is about ten feet above 

 the rock-bed of the river. (E. H. Heilbron, in letter.) 



