104 



It will be noted that according to these estimates the area 

 is only 27,914 square miles ; also that our plankton station, lo- 

 cated about two miles above the mouth of Spoon River and 

 slightly above that of Quiver Creek, is just within the lower 

 half of the basin. The water passing this point is derived from 

 an area of 16,250 square miles (15,363 according to Cooley), or 

 55% of the total basin, no account being taken of additions to 

 the stream by the pumping works at Bridgeport or by the 

 Drainage Canal. 



A glance at the map (PI. I. ) shows that the basin of the 

 Illinois River extends diagonally across the center of the 

 state from the southwest to the northeast as a broad belt one 

 hundred miles in width. The head-water region is spread out 

 in a Y-shaped area which embraces the southwestern portion 

 of Lake Michigan. The northern arm of the Y is formed by 

 the basin of the Des Plaines, which extends northward into 

 Wisconsin for a distance of 50 miles. It is separated from 

 Lake Michigan by a narrow strip of territory 3 to 22 miles in 

 width which is tributary to the Lake. The eastern arm is the 

 more extensive basin of the Kankakee, which extends for a 

 distance of over 90 miles into Indiana. 



The river does not take its course through the middle of 

 the basin but lies to the westward, being 60 to 80 miles 

 from the southeastern watershed and but 20 to 40 miles from the 

 northwestern. In the last 50 miles of the course the western 

 watershed is within 10 miles of the stream. Such a deflection 

 of the main stream toward one side of its basin is not infre- 

 quent, and has been attributed in many cases to the influence 

 of the rotation of the earth. Russell ('98) says: "Thus, in 

 the northern hemisphere the tendency of the earth's rotation 

 is to cause the streams, no matter what the direction of flow, 

 to corrade their right more than their left banks. * * * 

 There is thus a tendency, due to the earth's rotation, for them 

 to excavate their right more than their left banks, and to mi- 

 grate to the right of their initial courses. This tendency is 

 slight, but all the time operative." Streams flowing south- 



