518 



Ilunois River at the Marseilles Dam 



Above the Dam. — In midsummer, 191 1, the water at the Mar- 

 seilles dam had a grayish look and a disagreeable odor, but with 

 perceptibly less material in suspension than at Morris. There was 

 no bubbling of gases from a bar of sludge at the point of the island 

 half a mile above the dam, but chunks of sludge were floating to 

 the surface on the warmest days. The odor of this bottom sediment 

 was the same as at Morris and from the sanitary canal at Lock- 

 port. The surface temperature was 71° F. July 30, 72° August 11, 

 and 71.9° August 24. 



The situation a quarter of a mile above the dam may best be 

 described by a comparison with that' at Morris, these points being 

 but seventeen miles apart, with no important tributary of the river 

 or other modifying factor coming in to interfere with the spontane- 

 ous development of conditions within the streani itself. Another 

 important and interesting comparison is that of chemical and bio- 

 logical data from above and from below this dam, since this com- 

 parison will show us what and how great are the effects of the fall 

 upon a polluted water. 



We may notice first that the mixture of Kankakee and Des Plaines 

 water was evidently complete above the dam at ordinary stages, the 

 two shores and the middle of the stream differing but little in respect 

 to their ratios of oxygen and carbon dioxide, and not always in the 

 same direction as at Morris. The north, or Des Plaines, shore water 

 at Marseilles was, indeed, somewhat more oxygenated than that of 

 the south, or Kankakee, shore water, both in February and in August, 

 191 2 — the only months when tests available for this comparison were 

 made. More precisely, the north side oxygen ratios were 3 per cent, 

 higher than those on the south side February 20 to 22, and 11 per 

 cent, higher August 21; while the midstream percentages were a 

 little lower than those of either shore in February, and a little higher 

 than either in August. Such differences may be regarded as either 



