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 Illi- 

 nois pursues a course almost due west for a distance of sixty 

 miles, to a point a few miles above Hennepin, where it turns 

 abruptly toward the left, and flows southwest by south in quite 

 a direct line for a 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 48% 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 1,317 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- 



