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 
shghtly 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 
50% 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 hes 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- 
