496 



the larger river, the Des Plaines water on the north side and that 

 of the Kankakee on the south, and are not completely mingled until 

 the first dam is reached, at Marseilles, twenty-six miles below. They 

 are less distinct, however, than they would be if it were not for an 

 unfinished dam at Dresden Heights, which acts as a wing-dam to 

 concentrate and hasten the current of the Des Plaines, throwing it 

 with some force towards the other side of the Illinois, thus arti- 

 ficially mixing, to some extent, the waters of the Kankakee and the 

 Des Plaines at their junction. These continue sufficiently separate, 

 however, as far down as Morris, nine miles below the junction, to 

 .make a notable difference in contamination between the tw^o sides 

 of the stream. 



The effect of the dam at Marseilles is to check the current above 

 sufficiently to permit a considerable precipitation of suspended mat- 

 ter, which accumulates there in a deep bed of more or less putrid 

 sludge. In going over the dam the water is thoroughly mixed and 

 aerated, and the softer masses suspended in it are pulverized, and 

 thus made ready for more rapid decomposition. 



Between Dresden Heights and Morris the waters of the Illinois 

 are slightly diluted by contributions from the Au Sable on the north 

 and Mazon Creek on the south. At Ottawa, the Fox River comes 

 in from the north — a stream a hundred and fifty miles long with an 

 estimated low-water discharge of about two hundred and forty cubic 

 feet per second.* On the southern side is the Vermilion, ninety 

 miles long, emptying opposite La Salle. 



These are the only tributaries of the upper river of sufficient 

 importance to be noticed as influencing sensibly chemical or biolog- 

 ical conditions in the main stream. A cluster of bottom-land lakes 

 of some importance is found above Hennepin, and Senachewine Lake 

 has its outlet into the river between that town and Henry. At the 

 latter point is a dam which repeats, to some extent, the effects of 

 the upper dam, at Marseilles. Chillicothe, which was the lower limit 

 of most of our operations, is at the upper end of that expansion of 

 the Illinois known as Peoria Lake. 



The Microplankton, Summer of 191 i 



Three successive rounds were made in the summer of 191 1 to 

 the selected list of stations between Dresden Heights and Chillicothe, 

 by a chemist (Mr. Charles H. Spaulding) and a biologist (the junior 



*Chemical and Biological Survey of the Waters of Illinois. Report for igilr 

 p. 162. 



