Forbes. — Survey of Illinois River 81 



Illinois towns, it was impossible even for an expert to tell 

 which of them came from the sewage system and which 

 from the Illinois River at Morris and Marseilles. 



The gases of the water itself are, of course, more essen- 

 tial, since it is upon these that the fishes must depend for 

 respiration. We found that at Morris the dissolved oxygen 

 of the water amounted to an average of 9.8 per cent of 

 saturation, the ratios ranging from ]/? per cent to 3^4 per 

 cent of saturation July 22, and from 9 per cent to 21 per 

 cent from July 28 to August 31. By "saturation" we 

 mean the amount of oxygen which the water will take from 

 the air by simple contact with it. so that oxygen equilibrium 

 is established between the air and the water. Sometimes the 

 water will contain an excess of oxygen, through living 

 plants immersed in it and giving off oxygen in the sunlight. 

 When we found at Morris the ratios of gas mentioned 

 above, we found in the waters of the Kankakee, nine miles 

 above, 112 per cent of saturation. There was more oxygen 

 there than the water would take directly from the air. 

 These comparisons show that from 5/6 to 99/100 of the 

 oxygen of the river water was being used up, at this time, 

 by decomposition processes going on within it at this point. 



There were, of course, no fishes here, or any other ani- 

 mals requiring oxygen. Fishes were abundant in a small 

 tributary of the river at Morris known as Mazon Creek, and 

 in a slough at its mouth. Carp were especially numerous in 

 this slough, but they did not venture into the river under 

 these midsummer conditions. 



Going down the river from Morris sixteen miles to the 

 first dam crossing it. at Marseilles, we found an average of 

 only 7.5 per cent of oxygen-saturation, and, of course, there 

 were no fishes anywhere in this section of twenty-six miles. 

 At the Marseilles dam the water had a fall of from 12 to 14 

 feet, and above this dam, of course, there was a semi-stag- 

 nant pool through which the water flowed very slowly. 

 This pool served as a kind of settling tank, such that the 

 larger organic particles fell to the bottom and the water went 



