110 
THE RIVER. 
As previously stated, the course of the Illinois as compared 
with that of other streams of the Mississippi basin is remark- 
ably direct. The present stream has not as yet developed in its 
bottom-lands—the bed of the ancient stream—a meandering 
course like that of the neighboring Mississippi. The position 
of its channel in the bottom-lands is often determined largely 
by the deposits of its tributaries, those made by the streams 
from the east, as the Mackinaw and the Sangamon, forcing the 
river towards the western bluff, while those from its western 
confluents, as Spoon River and Crooked Creek, crowd it against 
the eastern bluff. The width of the river at low water gradu- 
ally increases from 536 feet at Peru to 1,040 feet at its entrance 
to the Mississippi. The expanse in Peoria Lake is over a mile 
in width, and further down, in Havana Lake, it is about 2,500 
feet. 
The undeveloped condition of the flood-plain of the Illinois 
River and the consequent large areas of the marshes, lagoons, 
and lakes, affect the plankton of the river most fundamentally. . 
In the first place the flood-plain serves, to an unusual extent, 
as an impounding area in which the flood-waters are stored, the 
barriers, natural and artificial, in the bottom-lands combining 
with the low gradient to delay the run-off. This delay is still 
further prolonged in most years by high water in the Missis- 
sippi caused by the run-off from districts of more northerly lat- 
itudes and higher altitudes and thus occurring after the spring 
run-off in the basin of the Illinois. In a few instances the 
backwaters from the floods of the Mississippi have been known 
to flow up the Illinois for a distance of one hundred miles, 
and the slope of the stream is such that the impounding 
effect might, under suitable conditions, extend even farther. 
The result of this combination of factors is to increase to a 
marked degree the volume of water, and to add greatly to the 
diversity of the environment at the time of the maximum de- 
velopment of spring plankton. It thus profoundly affects both 
the total product of plankton and its diversification. 
