XXXVl FISHES OF ILLINOIS 



of Chicago and to the principal towns upon its banks in conveying 

 away their Hquid wastes, which it renders harmless by decompo- 

 sition and useful by converting them more or less directly into a 

 food supply for fishes. 



The Illinois may be regarded as in many respects a typical stream 

 of the central prairies of the Mississippi Valley, peculiar now, how^- 

 ever, in the enormous amount of sew^age which it carries — mainly 

 received from Chicago by way of the drainage canal — together with 

 the large amiount of refuse from distilleries and cattle-yards along its 

 course. It flows, in most of its length, down the bed of an ancient 

 outlet of Lake Michigan, by which the waters of that lake were con- 

 veyed to the Mississippi River. AVithin this bed it has excavated 

 its own present channel, with its present bottom-lands or "first 

 bottoms," subject to overflow at high water. Its second bottoms, 

 above the reach of high water, are the flood-plain of the former out- 

 let of the lake. This ancient channel varies in width from 1^ to 6 

 miles, or, if the flood-plain of the older river be also included, to a 

 maximum width of 20 miles, the bluffs on either side ranging in 

 altitude from 450 to 800 feet. The highest points of these bluft's are 

 near Peoria, and near the mouth of the river in Calhoun county. 

 The watersheds bounding the river basin range in height from 700 

 to 1,000 feet above the sea, the average elevation being 600 or 700 

 feet. 



The length of the Illinois from its origin in the junction of the 

 Kankakee and the Des Plaines is approximately 270 miles; or, if its 

 longest tributary, the Kankakee, be added, the total is 505 miles. 

 The length of the stream itself is 28 per cent, greater than that of a 

 straight line from its origin to its mouth — an unusually small per- 

 centage for the tributaries of the Mississippi. It takes, in other 

 words, an uncommonly direct course. The area of its basin is 

 approximately 29,000 square miles, nearly 25,000 of which lie with- 

 in Illinois, approximately 1,000 square miles in Wisconsin, and 

 3,200 in Indiana. Its basin thus comprises about three sevenths of 

 the area of the state. It extends diagonally across the center of 

 Illinois from the northeast to the southwest as a broad belt about a 

 hundred miles in width, the upper end of which expands in a Y- 

 shaped area to embrace the southwest part of Lake Michigan. The 

 northern arm of the " Y" is formed by the basin of the Des Plaines, 

 and the eastern arm by the more extensive basin of the Kankakee. 

 From its origin, fifty miles southwest of Chicago, it runs almost due 

 west some sixty miles to a point not far above Hennepin, where it 

 turns abruptly towards the left, flowing southwest by south a hun- 



