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. S. 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 Greenleafs 

 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 



