98 



The Geological and Hydrographic Features of the 

 Illinois River Basin. 



These subjects have received elaborate treatment, in con- 

 nection with the problem of the disposal of the sewage of the 

 city of Chicago, by Cooley ('89 and '91); and by Leverett ('96) 

 in a report to the U. S. Geological Survey upon the "Water 

 Resources of Illinois." The following discussion of those 

 physical features which are more or less directly related to the 

 ^heme of this paper has been, to a considerable extent, com- 

 piled from these papers, with such supplementary data as 

 could be gleaned from the reports. of the U. S. Engineers and 

 from the observations and records made by the biological 

 station staff at Havana and at other points along the river. 



In many respects the Illinois River may be regarded as a 

 typical stream of the prairie region of the North-Central 

 States; and its basin, in the glaciation of its surf ace,, the level 

 character of the land, the fertility of the soil, the absence of 

 extensive forest areas, the amount of rainfall, the general 

 climatic conditions, and its central position, might well be 

 called a typical one for the central region of the Mississippi 

 Valley. On the other hand, in several very important respects 

 the river presents features and combinations of features that 

 are exceptional and even unique. Foremost among such 

 features is the very large amount of sewage received, an 

 amount largely increased by the opening of the Drainage 

 Canal in December, 1899, by which almost the entire sewage 

 of a metropolis of about two million inhabitants enters the 

 river. This, together with the large amount of organic refuse 

 from the distilleries and cattle-yards along the course of the 

 river, adds immensely to the fertility of its waters, especially 

 when the river is low. Again, the present river is a babe in a 

 giant's bed. The channel and the bottom-lands of the present 

 stream lie in the bed of an ancient outlet of Lake Michigan 

 whose flood-plain constitutes the fertile "second bottom." In 

 this channel of its predecessor the Illinois River is now rapidly 

 building up its flood-plain, the low gradient of the former o.c- 



