117 



high water (16 feet), tests made about half a mile below the 

 mouth of Spoon River with a steam launch indicate a current 

 at that point of about 3 miles per hour. The figures quoted from 

 the report of the Engineers give the mean velocity for the whole 

 stream at the place of observation, while our tests were made 

 in mid-channel and refer to that point alone. The results of 

 our own tests and those of the Engineers both indicate that the 

 current at low water is but one fourth as rapid as at times of 

 overflow. The current in the bottom-lands at time of overflow 

 is much slower than it is in the channel, being retarded by the 

 vegetation and by the natural and artificial irregularities of the 

 surface, such as the deposits of tributary streams, the embank- 

 ments of railroads, and the levees about cultivated lands. The 

 bottom-lands thus serve as impounding areas from which the 

 water, except at times of maximum flood, is slowly drawn off 

 through the main channel. 



The rate at which the current moves down the stream is 

 not, however, an index of the rate at which the crest of a 

 flood traverses the same distance. For example, the crest of 

 the flood of May, 1892, as shown in the appended table, passed 

 from Morris to the mouth of the river in fifteen days, while the 

 current in the main channel, under these flood conditions, 

 would traverse this distance in four or five days — less than a 

 third of this time. 



Flood Movement in the Illinois River. 



Place 



Date 



Distance 

 Miles below Morris 



Rate 

 Miles per hour 



Morris 



May 6 

 May io 

 May 16 

 May ig 

 May 21* 



124.3 

 230.0 



259-2 





Copperas Creek 



1.29 

 0.41 

 0.63 

 0.61 





Kampsville 



Mouth 







*Estimated. 



As stated on a previous page, the current at high water ranges 

 from two to three miles per hour, at which rate, as above shown, 

 water in the main channel would pass from Morris to the 

 mouth of the river in four or five days. This flood was sudden 



