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. 8. 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 Gulf of 
Mexico,a distance of 1,097 miles by river, it has, according to the 
most recent surveys, an average slope of .24 of a foot per mile— 
about twice that of the Illinois from Utica to the mouth. 
