135 
streams usually drop to a low stage and remain low through 
the heated term, evaporation and absorption being so great as 
to dispose of nearly all the rainfall. In the autumn, about the 
autumnal equinox or a little later, heavy rains occur, which 
cause the streams to become swollen for a few days, or even 
weeks, but which seldom cause them to overflow their banks. 
In some years these seasonal variations are slight, and the 
streams show but little change in volume, but such years are 
exceptional. The rainfall is seldom sufficient to cause freshets 
to last more than a few days. The moderate and low stages 
are estimated to generally cover ten months of the year, and 
occasionally eleven months.” 
In the Annual Reports of the Chief of Engineers of the U.S. 
Army for the years 1890 and following, Captain Marshall has 
published the readings of the river gages located at the govern- 
ment dams in the Illinois River. The readings at Copperas 
Creek, 16.8 miles above Havana, begin in 1879, and those at 
LaGrange, 42.7 miles below, in 1883. The dam at Copperas 
Creek was completed in 1877 and the one at LaGrange in 1889. 
On Plate VII. will be found hydrographs plotted from the daily 
readings at the gages below these dams from 1879 and 1583, re- 
spectively, to 1900. The heavy sinuous lines represent the 
fluctuations of the river, referred in the plot to the low-water 
level of 1879. The mean annual curve was plotted from the 
mean monthly readings at the above-named localities from 
1879 and 1883, respectively, to 1900. Deficiencies in the records 
at the two points named have been supplied, by estimate, from 
records at nearest point of observation, in a few cases from our 
Havana records. 
A comparison of the hydrographs of the gages at Copperas 
Creek, Havana, and LaGrange reveals a close correspondence 
in the fluctuations, and suggests that these main movements 
are due to general rains or spring thaws coincident in the 
greater part of the drainage basin. The differences are of a 
minor character, and many of them are consequent upon local 
conditions of rainfall. On closer inspection the records at Cop- 
