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mile, or 10,570 cubic feet per second — a total, for the year, of 

 333,563,832,000 cubic feet, or 2.27 cubic miles. The run-off in 

 an ordinary year is equivalent to a trifle over eight inches of 

 rainfall in the whole watershed, or 21% of the total rainfall for 

 the year. Greenleaf ('85) states that the area of the entire 

 catchment-basin of the Mississippi is 1,240,039 square miles, and 

 that the average discharge per second is 675,000 cubic feet, or 

 only .54 cubic foot per second per square mile. The Illinois, 

 with one forty-third of the catchment-basin, thus contributes 

 one twenty-third of the discharge. Greenleaf's estimate of the 

 discharge of the Illinois, even when reduced to 0.908 second- 

 foot per square mile, places the stream in the category of the 

 St. Lawrence River, whose discharge is slightly in excess of one 

 second-foot, rather than with the Mississippi River, whose total 

 discharge is but about half that amount. The average dis- 

 charge from the Illinois is thus somewhat less than that of the 

 Connecticut, of the Hudson, or of the Seine ; is about the same 

 as that of the Delaware and of the Elbe ; but is much less than 

 that of the Loire and of the Po, and relatively less than that of 

 many European streams. 



The maximum flood discharge of the Illinois has been vari- 

 ously estimated. On the basis of normal basin ratios for streams 

 of like climatic conditions it should be equal to the two-thirds 

 power of the area (A % ), which would be about 123,000 cubic 

 feet per second. The exceptional flood of May, 1892, the crest 

 of which at Kampsville had a height of 22.8 feet above low 

 water, was reported by the engineers of the Chicago Drainage 

 Commission to have discharged only 94,760 cubic feet per second 

 at the mouth of the river. Cooley ('97), basing his estimate 

 upon the discharge curves of the Mississippi at Hannibal and 

 at Grafton, states that the maximum discharge seldom exceeds 

 70,000 to 80,000 cubic feet per second, and that a flood of 16 feet 

 — a height which ordinary floods rarely exceed — would approx- 

 imate only 55,000 cubic feet per second. The maximum dis- 

 charge is thus considerably below what is to be expected, and 

 the explanation lies in the delay in the run-off due to the im- 



