114 



The records during periods of low water both at Havana and 

 Copperas Creek thus indicate that the effect of the dam at La- 

 Grange is to raise the water in the upper end of the basin 

 somewhat less than 2 feet — about 1.7 feet at Havana, according 

 to the Havana gage. This gage was established in 1875 by Mr. 

 R. A. Brown, a U. S. Army Engineer, and is based on the low- 

 water record of 1873. 



The fall in the Illinois River, owing to the lack of devel- 

 opment of the relief of its basin, is but slight. The difference 

 between the elevation of its highest watershed and the low- 

 water level at its mouth is only about 600 feet, or an average 

 fall of 1.2 feet per mile of the total course. The Illinois 

 proper, from the union of the Kankakee and Des Plaines to the 

 mouth, has, according to Cooley ('91, App. I.), a total fall of 

 81.7 feet, or an average of .267 feet per mile. Of this fall 50.7 

 feet occur between the mouth of the Kankakee and the head 

 of the pool of the Henry dam at Utica in a distance of 42.6 

 miles. From Utica to the mouth, a distance of 227 miles, the 

 fall is but 31 feet, or an average of .137 of a foot per mile. 

 According to Rolfe the altitude of the low-water level at 

 LaSalle, three miles below Utica, is 440 feet, while at the 

 mouth of the Illinois it is 402 feet, thus affording a total fall 

 between these places of 38 feet and an average of .167 of a foot 

 per mile. The elevations given by Professor Rolfe are based 

 upon Illinois and Michigan-Canal levels, while those given by 

 Cooley are derived from later surveys. Accepting either figures 

 the fall in the main stream from Utica to the mouth is but 

 slight, exceptionally small, indeed, in comparison with the gradi- 

 ent of other rivers of the Mississippi system. For example, the 

 Mississippi at Cairo has a slope of .666 of a foot per mile, almost 

 five times that of the Illinois, while from Cairo to the Grulf of 

 Mexico, a distance of 1,097 miles by river, it has, according to the 

 most recent surveys, an average slope of .24 of afoot per mile — 

 about twice that of the Illinois from Utica to the mouth. 



