105 
ward in the northern hemisphere thus tend to shift toward the 
western watersheds of their basins. The position of the Illi- 
nois River in its basin seems to afford an illustration of this 
application of Ferrel’s law to the flow of rivers. A reference 
to the contour maps of the state which Leverett (96) gives, 
shows that the bed of the ancient stream which the Illinois 
River and its bottom-lands now occupy lies toward the western 
side of the flood-plain of the stream; in other words, from 
Hennepin to the Mississippi the second bottoms are of much 
greater width upon the eastern than upon the western side of 
the river. 
From its origin, fifty miles southwest of Chicago, the [lli- 
nois pursues a course almost due west for a distance of sixty 
miles, to a pointa few miles above Hennepin, where it turns 
abruptly toward the left, and flows southwest by south in quite 
a direct line fora distance of 165 miles (205, by river) to its 
union with the Mississippi, 25 miles above St. Louis. 
The tributaries of the Illinois River are distributed in a 
somewhat unusual manner. Dividing the river into three 
regions, the upper, middle, and lower, terminating respectively 
at Ottawa, at our plankton station, and at the mouth, we find 
a very unequal distribution of tributary areas. The upper 
river, although but 12% of the total length, drains 37% of the 
basin owing to the fact that it receives three large tributaries, 
—the Kankakee, the Des Plaines, and the Fox,—besides a con- 
siderable number of smaller streams. The middle river, on the 
other hand, constitutes 45% of the total length but drains only 
18% of the basin. This is due to the fact that aside from the 
Vermilion and the Mackinaw there is no tributary of impor- 
tance in the 117 miles of its course. These two tributaries have 
basins of 1,517 and 1,217 square miles respectively and rank in 
area as seventh and eighth in the list of tributaries. The 
lower river constitutes about 45% of the total length, and 
drains a corresponding per cent. of the total basin, since it re- 
ceives the Spoon and Sangamon rivers and, in addition, a num- 
ber of creeks of considerable size. The location of the plank- 
