148 



but appears occasionally (nine times in twenty-one years) in 

 the annual ones as a well-defined flood. In the majority of 

 years, however, it appears as a slight interruption in the decline 

 of the earlier spring flood. In a few cases these fluctuations 

 occur late in May or early in July. The fact that this June 

 rise is so little felt in the Illinois, while it is so prominent in 

 other streams of the state, is explained by the fact that it often 

 occurs within the period of overflow, when large accessions of 

 water produce relatively slight rises of river levels. A compar- 

 ison of the hydrographs of 1889 and 1892 will illustrate the 

 points in question, the June rise of the latter year appearing as 

 a slight fluctuation in the declining flood, while in the former 

 year it stands out as a well-marked rise, owing simply to the 

 previous low water. 



As compared with Leverett's normal regimen for an Illinois 

 stream, we find in the case of the Illinois River that the high- 

 water period exhibits a considerable range; that it extends over 

 a much longer time; and that the phenomenon of the June rise 

 is less pronounced, — all of which deviations may be explained 

 by the impounding action of the slightly developed flood-plain 

 of the Illinois. 



Following the period of high water comes an equally pro- 

 nounced period of low water, extending through the summer 

 months until the late autumnal or winter rise. As shown by 

 the averages, this low-water period (below four feet) extends 

 from August to November, inclusive. It varies, however, with 

 the high-water period, appearing even in May, as it did in 1895, 

 and frequently continuing through the fall and winter till late 

 in February, as it did in 1891, 1893, and 1894. The average 

 time during which the water was below 4 feet for the 21 years at 

 Copperas Creek is 134.1 days, two days more than the high- 

 water period. This low-water stage is quite variable in its du- 

 ration, ranging from 260 days in 1895 to 5 in 1885. The low- 

 water period, which Leverett estimates as continuing at least 

 ten months in normal Illinois streams, is thus much shortened 

 in the Illinois River. The lowest levels of the year are reached 



