XXXIV FISHES OF ILLINOIS 



sisting of that portion of the river above the turn at Hennepin; 

 and the lower Illinois, below this point. The lower part of the 

 river occupies a preglacial valley, the southward continuation 

 of the preglacial valley occupied b} r Rock River in southern 

 Wisconsin and northern Illinois. The upper Illinois, however, 

 flows through an interglacial and postglacial valley, the old 

 "Chicago outlet." This outlet was the line of south westward 

 discharge from the basin of Lake Michigan across the low divides 

 near Chicago and thence down the Des Plaines and Illinois to the 

 Mississippi. It has a depth ranging from 20 to 70 feet, the 

 excavation being almost entirely in beds of drift except for 

 about 15 miles between Lemon t and Joliet and 40 miles between 

 Morris and Peru, where rock strata have been eroded. Through- 

 out its entire length the bluffs are steep like river banks, and 

 the deposits made by side streams on the edge of the valley 

 are very meager — a feature which indicates that the stream had 

 great volume, probably filling the channel from bluff to bluff, 

 and a current sufficiently strong to carry nearly all of the detritus 

 brought into it by the side streams. 



Since the Illinois is formed by the union of the Des Plaines 

 and the Kankakee, it may be best to describe those streams first. 



DES PLAINES RIVER 



The Des Plaines drains a narrow intermorainic strip ex- 

 tending north and south a distance of 90 miles from Kenosha 

 county, Wisconsin, to the head of the Illinois in eastern Grundy 

 county, Illinois. The whole drainage basin covers an area of 

 about 1,366 square miles, its greatest width being scarcely 25 

 miles. This region all lies within the Wisconsin drift, between 

 two rather large moraines to the east and west of it, and con- 

 taining many smaller moraines which have prevented the 

 formation of good natural drainage-lines. The land is, conse- 

 quently, very imperfectly drained, and contains numerous small 

 lakes and marshes, although this condition has been much 

 changed by extensive systems of tiling. A series of measure- 

 ments by the U. S. Geological Survey gives for the average 

 discharge 1,100 cubic feet per second. The water of the northern 

 section is moderately clear, but becomes more turbid and 

 polluted lower down. 'The bottom of the river and its tribu- 

 taries is largely sand and gravel, with rock in its portions of 

 swiftest descent. 



The Des Plaines has its source in an extended marshy valley 



