132 
per square mile, while for some of the reservoir tributaries of 
the upper Mississippi, which include regions of less rainfall, it 
is estimated at about 0.75 cubic foot per mile. 
There have been as yet no-adequate measurements of the 
flow of the Illinois River at its mouth, and no extended gagings 
at any point on its course aside from the observations made by 
the U. 8. Army Engineers at LaGrange, and a few isolated 
records and estimates at other points. 
The average flow of the river has been estimated by Cooley 
(97) to be under rather than over one cubic foot per second 
per square mile of watershed. Upon this basis it would be 
somewhat less than 29,000 cubic feet feet per second, or 915,- 
170,400,000 cubic feet or 6.2 cubic miles per year. 
Measurements made in 1882 at Hannibal and at Grafton in 
the Mississippi River above and below the mouth of the Illinois 
indicate that the discharge of the Illinois River ranges from 
11,000 to 80,000 cubic feet per second, with an average of 30,- 
000 cubic feet. This is equivalent to a maximum run-off of 
nearly 3 second-feet per square mile, a minimum of 0.4, and an 
average of 1.1. In 1882, one of the wet-year series, the river 
was out of its banks during the greater part of the year owing: 
to the heavy rainfall which, throughout the state as a whole, 
was 18 per cent. in excess of the average. A reduction of 18 
per cent. from the average flow of 1882 leaves 24,600 cubic feet 
per second, or 0.902 of a second-foot per square mile, as an esti- 
mate of the mean annual flow of the Illinois River. Cooley’s 
estimate (’97) of the average run-off of the upper 15,250 square 
miles of the basin, based upon the gage-readings at Copperas 
Creek for eleven years (1879-1889), is 10,500 cubic feet per 
second, or 0.688 second-foot per square mile. This is 25 per 
cent. below the above-given estimate, based on Greenleaf’s 
measurements at the mouth of the stream. Accepting these 
two estimates as approximately the average, and adopting the 
calculation for the river at Copperas Creek as applicable to 
Havana, eighteen miles below, we have the average run-off at 
our plankton station approximately 0.688 second-foot per square 
