138 



per second, while the bank-full capacity is estimated at double 

 this amount. In either case complete overflow stages appear 

 more readily than they do in the majority of streams. 



The impounding action of the bottom-lands, on the other 

 hand, begins with every rise of the river, for as the water rises 

 large amounts are drawn off from the main stream by the 

 adjacent lakes and bayous, many of which retain their connec- 



Bank-full Capacity of Lower Illinois River. 



tion with the river even at the lowest stage. In the vicinity 

 of Havana, for example (see Plate II.), Quiver, Thompson's, and 

 Matanzas lakes respond at all times to fluctuations in the river. 

 Flag Lake is invaded at about the stage of 3 feet, and at 5 to 6 

 feet the water begins to overflow the bottom-lands between Flag 

 and Thompson's lakes. It is not, however, until the river has 

 reached a stage of 8 to 9 feet that the water enters Phelps Lake. 

 The wooded bottom-lands to the east of Flag Lake are not en- 

 tirely submerged until the gage reads 12 feet, while those below 

 Spoon River and adjacent to the main stream do not disappeai* 

 until the water has reached 16 feet. Thus the impounding 

 action of the bottoms is at its greatest as soon as the water 

 reaches the condition of complete overflow, though it begins 

 at the first stages of a rise above low water. 



In his discussion of this subject Cooley ('91), speaking of 



