Forbes. — Survey of Illinois River 83 



is now carried on far down the stream, the first important 

 point being Hennepin, fifty miles below the mouth of the 

 Des Plaines. 



DISCUSSION 



Dr. George W. Field, Boston. Mass. : Did you make any observa- 

 tions on the rapidity of nitrification in the stream? 



Dr. Forbes : We did not. 



Dr. Field : What is the condition at the dam at the mouth of the 

 Kankakee ? 



Dr. Forbes : There is an unfinished dam at the mouth of the Des 

 Plaines. Some years ago the Economy Light and Power Company 

 began to build a dam at this point with a view to creating a water 

 power there, but litigation arose in resistance to this procedure, and 

 the work was stopped and the dam remains unfinished. It extends 

 effectively across a part of the stream, however, and thus acts as a 

 wing dam to concentrate and to hasten the current of the Des Plaines, 

 throwing it with some force to the other side of the Illinois, and 

 mixing the waters of the Des Plaines and the Kankakee more thor- 

 oughly than would be the case if they were allowed to flow down gently 

 side by side. They are not completely mixed, however, even as far down 

 as Morris, some nine miles below, as was shown by a comparison of 

 the oxygen content of the water at the two sides of the river. That 

 which would naturally be Kankakee water, if left uncontaminated, con- 

 tained, at Morris, twice as much oxygen as that from the other side 

 of the river, corresponding to the Des Plaines. Nevertheless, this min- 

 gling of the waters and consequent dilution of the sewage content 

 doubtless works to the advantage of the stream by bringing about a 

 more rapid decomposition of its organic contents than if its sewage- 

 laden waters flowed down undiluted, side by side with a comparatively 

 clear current. 



One other point of special interest, to which I have not alluded, 

 came out in the course of our analyses. At one time during this period 

 a heavy rain fell which brought the river up at Morris some six or 

 eight inches. Fortunately, we had begun our chemical determinations 

 some time before this rise, and we continued them through it and for 

 a considerable time thereafter, so that we were able to see what the 

 effect of these flooding rains was upon the oxygen content of the water 

 of the river. This went down about 50 per cent the whole length of 

 the stream as a consequence of this flood, the difference between the 

 former oxygen content and that after the rain growing greater as we 

 went down the Illinois, so that it was greatest at Chillicothe. 



This was a rather surprising fact. Here was a great and sudden 

 afflux of flood water which one would suppose to be comparatively 

 free from organic matter, in which, consequently, there would be little 

 decomposition in progress, and the oxygen content of which would 



