429 



A sample taken February 17 at Peoria Narrows with some new 

 shore ice present showed 7.6 parts per miUion of dissolved oxygen, and 

 a sample on February 18 showed 7.2 at the same place, with no change 

 in temperature. On February 19 at Water-works Point, where there 

 was still some shore ice. samples showed 8.9 parts per million of dis- 

 solved oxygen. For the remainder of February the dissolved oxygen 

 figures ranged from 8 to 10 parts per million in the vicinity of Beards- 

 town and Meredosia, with no ice and with the water temperature abotit 

 4° C. 



From late autumn until late spring the water of the Illinois River 

 is cold enough to slow down the oxygen consumption of the organisms 

 that utilize the sewage and other organic matter in the river to a point 

 where the rate of aeration balances or exceeds the rate of oxygen con- 

 sumption. Under these conditions the amount of oxygen present is suffi- 

 cient for the ordinary river animals. If, however, aeration is stopped 

 by the freezing over of the river the dissolved oxygen is gradually ex- 

 hausted until it becomes so low in Upper Peoria Lake that fish show 

 signs of distress. When this condition is established, it does not change 

 until the ice goes out or until it is diluted with a large amount of water 

 highly charged with oxygen. When the ice goes out of a small fraction 

 of the channel there is a marked increase in the amount of oxygen dis- 

 solved in the water, and in about a week the normal cold water concen- 

 tration of dissolved oxygen is established. 



The Effect of Low Dissolved Oxygen 

 Concentrations on the Fishes of the Illinois River 



Fishes, like all other living things, require a more or less constant 

 supply of oxygen to maintain the activities of life, but the minimal 

 amount may vary greatly for the different forms of life. A striking 

 example of this is the problem in hand. The minute organisms which 

 feed on sewage and other organic matter can live and consume oxygen 

 in concentration too small to support the life of fishes. Some of them, 

 indeed, can live in a total absence of dissolved oxj'gen, since they are 

 able to derive oxygen from complex organic compounds. 



A fact of fundamental importance to the existence of fishes is their 

 ability to avoid oxygen concentrations that would be fatal to them. When 

 low dissolved oxygen concentration obtains in a body of water, such as 

 the Illinois River, the fishes retreat into backwater lakes, spring holes, 

 mouths of tributaries, and other places where stagnation has not de- 

 veloped. 



A case of this kind recently occurred near the town of S[)ring Bay 

 on the east shore of upper Peoria Lake, where there is a lagoon fed by 

 springs. At rather low stages of the river this lagoon, which has an 

 area of about two acres, is filled with clear, well-aerated water, warm 

 enough to j)revent freezing. Some time after the river was completely 

 frozen over in January, 1925, the fishes began to crowd into this spring 



