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far-reaching effect that the turmoil of flood has upon the 
quantity and the distribution of the plankton, there are intro- 
duced other factors whose influence, though perhaps more 
subtle, is of no less importance. Such factors are changes in 
the chemical constituents dissolved in the water, in its temper- 
ature, in its transparency, and in the relative proportions of 
plant and animal life. 
The run-off. of the rainfall of the catchment-basin of the 
Illinois is influenced by a variety of conditions, all of which 
are more or less variable. These are the amount and distribu- 
tion of the rainfall, the slope, the perfection of drainage lines, 
the geological structure, the amount of vegetation, and the 
temperature. As has already been stated, the rainfall, amount- 
ing on an average to 37.858 inches, is distributed with consider- 
able uniformity—at least in the northern basin, with which we 
are most concerned. Purely local excesses and deficiencies of 
rainfall occurring within this area are rarely of sufficient pro- 
portions or duration to affect profoundly the customary regi- 
men of the stream as a whole. 
According to Leverett (’96) the slope of the stream beds of 
the principal tributaries in the northern basin, the Des Plaines, 
Kankakee, Fox, and Vermilion rivers, is on an average in their 
lower courses several feet per mile, while in the lower two 
hundred and twenty-five miles of the Illinois itself the slope is 
only thirty feet or .13 foot per mile. In much of the state the 
principal streams have an average slope of about two feet per 
mile; and the small streams, of five to ten feet, excepting the 
head waters. In the main, therefore, the slope of the stream 
beds is such as to favor a very moderate run-off. The slope of 
the general surface is also very moderate. It ranges from ten 
to twenty feet per mile, being somewhat greater in the newer 
drift, where moraines are more abundant, than it is in the 
older drift of the southern basin. The steeper slopes of the 
newer drift are, however, counterbalanced by the much in- 
ferior development of drainage lines within its area. There are 
large tracts of land at the head waters of the Vermilion and 
