THE TOPOGRAPHY AND HYDROGRAPHY OF ILLINOIS XXXi 



The lower part of the river occupies a preglacial valley, the south- 

 ward continuation of the preglacial valley occupied by 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 southwestward 

 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 excava- 

 tion being almost entirely in beds of drift except for about 1 5 miles 

 between Lemont and Joliet and 40 miles between Morris and Peru, 

 where rock strata have been eroded. Throughout 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 extending 

 north and south a distance of 90 miles from Kenosha county,. Wis- 

 consin, to the head of the Illinois in eastern Grundy county, Illinois. 

 The whole drainage basin covers an area of about 1,758 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 containing many smaller moraines which have 

 prevented the formation of good natural drainage-lines. The land 

 is, consequently, 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 measurements 

 by the U. S. Geological Survey gives for the average discharge 1,100 

 cubic feet per second. The water of the northern section is moder- 

 ately clear, but becomes more turbid and polluted lower down. The 

 bottom of the river and its tributaries is largely sand and gravel, 

 with rock in its portions of swiftest descent. 



The Des Plaines has its source in an extended marshy valley in 

 Kenosha county, Wisconsin. This valley is so nearly level that at 

 times it is very difficult to tell which way the water flows. It stands 

 112 feet above Lake Michigan (Leverett) and drains northward into 

 Root River as well as southward into the Des Plaines. The Des 

 Plaines flows nearly parallel with the shore of Lake Michigan to a 



