xl FISHES OF ILLINOIS 



in other words, an uncommonly direct course. The area of its 

 basin is approximately 29,000 square miles, 28,100 of which lie 

 within Illinois, 1,020 square miles in Wisconsin, and 3,140 in 

 Indiana. Its basin thus comprises about three sevenths of the 

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

 nois 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 aboA^e 

 Hennepin, where it turns abruptly towards the left, flowing 

 southwest by south a hundred and sixty-five miles (two hundred 

 and five by river) to its union with the Mississippi, twenty-five 

 miles above St. Louis. Its bottom-lands have an average width 

 of 3.1 miles, from Utica to the mouth of the river. The immediate 

 banks of the stream are usually higher than the adjacent sur- 

 faces, and the same ma}^ be said of its tributary streams where 

 they flow through the bottoms of the Illinois. Bayous, lagoons, 

 marshes, and temporary ponds occur along the course of the 

 river, especially in its central portion from Hennepin to Mere- 

 dosia, all subject to invasion or obliteration by the river in times 

 of flood, but filled, at low water, either from springs or from the 

 general drainage of their basins. Spring-fed lakes are rather 

 common along the eastern side of the river, from Pekin to its 

 mouth, deriving their waters from the rainfall collected by the 

 second bottoms, at whose margin they usually lie. 



This large area of marshes, lagoons, and lakes affects the 

 life of the river in many important ways. v The flood-plain serves 

 as a storage area for the waters of overflow, greatly delaying the 

 run-off at times of flood. This delay is still further prolonged, 

 in many years, by high water in the Mississippi, which often 

 extends far up the Illinois — in a few instances as much as a 

 hundred miles. As a result of these conditions the average 

 volume of water in the stream throughout the year is greath r 

 increased, and a wider range and breeding ground and a greater 

 food suppty are afforded to the fishes of the stream. 



The fall in the Illinois River is but slight — an average of .267 

 of a foot per mile of its total length. Fifty and seven tenths feet 

 of this fall occur in the first forty-two miles of its course, and 

 from Utica to the mouth of the river the total fall is but 31 feet, 



