509 



saturation with oxygen for the southern, or Kankakee, side were 

 113 per cent, larger in September, 191 1, than those of the northern, 

 or Des Plaines, side; 34.5 per cent, larger in November; and 13.9 

 per cent, larger in February and March, 19 12. Flooding rains, on 

 the other hand, tend to increase the difference and to carry it further 

 down the stream. The clean Kankakee being a much larger river 

 than the polluted Des Plaines, the eft'ect of a strong rise in these 

 streams is to increase the ratio of south-side unpolluted water to 

 north-side polluted water, and to carry a larger volume of the former 

 down to Morris and even to Marseilles, with less admixture than at 

 low-water stages. This condition was illustrated by observations 

 made November 3, 191 1, when recent heavy rains had brought the 

 river up about three feet. Midstream ratios, samples for which were 

 taken from what was obviously Des Plaines River water, averaged 4.6 

 parts per million of oxygen, while south-shore samples (Kankakee 

 water) contained 10.8 parts per million. The water temperature at 

 this time was 41° F. 



All our midstream samples were taken for analysis as near the 

 center of the river as practicable, and usually from a depth of eight- 

 een inches or two feet below the surface. Midstream ratios aver- 

 aged 7.3 per cent, of saturation in July, 191 1, 16.37 P^i* cent, in 

 August, 28.5 per cent, in November, and 48.76 per cent, in February, 

 191 2. In September, 19 12, they stood at 17.9 per cent., and No- 

 vember 14 at 57.2. This is the highest of our midstream readings; 

 but a still higher one might have been obtained March 19, 19 12, 

 when even the north-shore ratio was 65.4 per cent., or 9.2 parts per 

 million, with a temperature of 34.7° F. 



Oxygen determinations were made on the same date from both 

 sides and from the center of the river on only three days — February 

 16, September 27, and November 14, 19 12. On these days the mid- 

 stream percentages were much nearer to those of the northern or 

 heavily polluted side than to those of the southern or lightly con- 

 taminated side, the mean dift'erence being 3.7 per cent, in the first 

 case and 48.4 per cent, in the second. In this we may probably see 

 the effect of the wing-dam at the mouth of the Des Plaines in forcing 

 the water of that stream well across the Illinois, leaving a compara- 

 tively narrow strip of imperfectly mixed Kankakee water along the 

 south bank. 



Bxtremc Midsummer Conditions. — The lowest ratios of oxygen 

 at Morris were those of July 22, 191 1, when samples were collected 

 from the middle of the stream and from the soiith bank only, the 

 former at a depth of five feet, and the latter at three feet below the 



