133 
mile, or 10,570 cubic feet per second—a total, for the year, of 
333,563,832,000 cubie feet, or 2.27 cubic miles. The run-off in 
an ordinary year is equivalent toa 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 .64 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 I]linois 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 lessthan 
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- 
