130 



constitute a very important element in the fluctuations of the 

 main stream, and its contributions of detritus are extensive. 



The basin of the Kankakee, occupying 5,146 square miles, 

 lies in a single belt of latitude, extending 216 miles in an east 

 and west direction. About 700 square miles of the lower part 

 of the basin have a slope sufficient to afford a rapid run-off, and 

 of the remainder fully one half is swamp and marsh, the other 

 half being flat or slightly rolling, but capable of cultivation. 

 The drift, except in the lower portion, is of considerable depth, 

 and in much of the marshy region extensive deposits of sand 

 are found. The lower stream is also bordered by extensive 

 sand deposits, and these afford a storage basin for the waters 

 derived from the adjacent slopes. Cooley ('89) estimates the 

 mean of the extreme flood-discharges of the Kankakee at its 

 junction with the Des Plaines at 31,200 cubic feet per second, 

 and the mean of the extreme low-water discharges at 500 cubic 

 feet per second. Flood water at the mouth of the Kankakee 

 has been known to reach sixteen feet above low-water level. 

 Aside from the uniformity in latitude, the physical features of 

 the Kankakee basin, under present conditions, favor a gradual 

 run-off, with floods which rise slowly to a moderate height and 

 continue for a considerable period. Owing to the storage facil- 

 ities of the basin the stream maintains a relatively large flow 

 even in periods of prolonged drouth. The Kankakee is thus an 

 important factor in moderating the extremes of high and low 

 water in the Illinois. Its contributions of silt are but slight. 



The northern part of the basin of the Fox River is similar to 

 that of the Des Plaines, and acts as a storage reservoir; but the 

 southern part, which is of greater extent, has steep slopes, and 

 the rapidity of the run-off is thereby heightened. Leverett ('96) 

 states that its flood waters reach a level of flfteen feet above the 

 normal, and that the discharge, presumably at low water, is 526 

 cubic feet per second. Cooley ('91) gives the discharge of the 

 Fox in the flood of February, 1887, as 13,680 cubic feet per sec- 

 ond. The conditions of its basin are such as to aggravate the 

 fluctuations and to increase the amount of silt in the Illinois. 



