THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS xlv 



termine the character of the plankton developing in its waters. 

 The resulting contributions may thus differ greatly in amount 

 and component organisms, and accordingly tend to diversify 

 the river plankton of low water to a degree even more marked 

 than that of high water. 



"With the confinement of the river waters to the channel 

 goes a marked condensation of the sewage, which, under con- 

 ditions of uninterrupted low water, leads at times to an excessive 

 development of the plankton, or, if the river is closed by ice, to 

 stagnation conditions. But few years, however, offer such op- 

 portunites; for, as a rule, in most low- water periods sudden and 

 heavy rains are wont to occur, which flush the stream, wash 

 away the sewage and plankton-laden waters, and store anew 

 the reservoir lakes without causing any considerable overflow. 

 After each catastrophe of this sort the decline of the flood affords 

 a new and favorable opportunity for the development of the 

 plankton."' 



The effects of change of temperature, of differences of 

 turbidity, of chemical conditions of the waters of the stream, 

 and the like, are discussed at length in Dr. Kofoid's report. f 



As a framework to this sketch of the Illinois River and the 

 waters of the Illinois basin generally, an outline of its geological 

 surroundings is essential. From its source to Peoria the river 

 flows through a district covered by the Wisconsin drift. From 

 Peoria to southern Pike county the outlines of its western border 

 are covered by the Illinois drift capped by loess. From thence 

 southward they are nearly free from glacial drift, but are heavily 

 coated with loess, while those on the east have a moderate 

 covering of Illinois drift capped by loess. Within the Wisconsin 

 drift the marshes, bogs, and lakes are of small extent, but the 

 drainage lines are, on the whole, rather imperfectly developed. 



The portion of the basin lying in the Illinoisan is much 

 better drained. There are almost no marshes or swamps in it, 

 except those in the bottoms of the river itself, but there are 

 numerous shallow valleys which are poorly drained. 



The Illinois, as stated before, follows the old " Chicago 

 outlet" as far as its curve near Hennepin. In the 41 miles 

 from the junction of the Des Plaines and the Kankakee down 

 to Utica, where apparently a small preglacial tributary of the 

 Illinois is entered, the course of the present Illinois is independent 



*Bull. 111. State Lab. Nat. Hist., Vol VI.. Art. II.. pp. 151-156. 

 fLoc. cit., pp. 168-252. 



