128 



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 



