137 



Havana and the latter 22 miles below. During the summer 

 season, when local rains occur, it sometimes happens that a 

 storm is confined to the basin of a single tributary, especially 

 as the Sangamon and Spoon rivers lie, according to Cooley 

 ('91), in different storm-tracks. In such cases the effect may 

 be very evident at one gage, but be dissipated in large part be- 

 fore it reaches the other. 



The progress down stream of floods which originate in the 

 upper valley varies with the abruptness and extent of the rise and 

 with the stage of water in the lower Illinois. As stated on page 

 117, it took 15 days for the crest of a flood to pass from Morris 

 to the mouth of the Illinois, a distance of 259.2 miles. Compar- 

 isons of the gage-readings at Copperas Creek and LaGrange 

 show that the progress of the crest of the flood between the 

 two dams is subject to great variations in duration. In some 

 instances the culmination is reached upon the same day at both 

 dams; in rare instances it is reached at LaGrange several days 

 before it is at Copperas Creek, probably as a result of excessive 

 flood water from the Sangamon. In the majority of cases, 

 however, the maximum height is reached at the upper dam in 

 from 2 to 3, or even as high as 7, days before it is at the lower 

 dam. This delay is due to a variety of causes, of which one 

 of the principal ones is the impounding action of the 

 bottom-lands. 



FLOODS AND THE IMPOUNDING ACTION OF THE BOTTOM-LANDS. 



Owing to the slight development of its flood-plain, overflows 

 occur at early stages of the rising river. The appended table, 

 adapted from Cooley ('91), gives data pertaining to bank height 

 and bank-full capacity of the river at various points along its 

 course. 



It will be noted that the bank-full capacity at Kampsville 

 is 40,000 second-feet, only one third more than Greenleaf s ('85) 

 estimate of the average discharge at that point. At Copperas 

 Creek, on the other hand, the more moderate estimates of 

 Cooley ('91) place the average discharge at 10,500 cubic feet 



