134 , 



pounding action of the bottom-lands, a phenomenon which will 

 be discussed later. This discharge occurs most often in March 

 or April, or in June, and rarely, if ever, in the late summer or 

 fall, from July to December. 



The minimum discharge of the Illinois is, for a stream of its 

 size, extremely small, indicating the impermeability of the 

 strata of its basin and the slight contribution of ground water 

 to its volume. According to Cooley ('91) the discharge over 

 the dam at Copperas Creek for a period of low water continuing 

 for twenty days was less than 500 cubic feet per second, or 200 

 cubic feet less than the amount sent through the Illinois and 

 Michigan Canal by the operations of the Bridgeport pumping 

 works. In conclusion Cooley says: "It is probable that since 

 the Bridgeport pumps were erected, in 1883, over half the min- 

 imum discharge above Havana has come from Lake Michigan, 

 and one third of the minimum below the Sangamon. For the 

 purposes of calculation, the normal low-water volume is taken 

 at 600 cubic feet per second for the upper section of the river 

 and at 1,200 cubic feet for the lower section." The minimum 

 discharge occurs most frequently in the late summer and 

 early fall, — in August, September, and October, — and occasion- 

 ally in the early winter months. 



THE REGIMEN OF THE ILLINOIS RIVER. 



Leverett ('96) gives the following as the usual regimen of 

 an Illinois stream: 



"During the winter, when the ground is frozen and precip- 

 itation is comparatively light, the streams are low. In early 

 spring the thawing of the ground and the greater precipitation 

 lead to a spring freshet, when the streams are often bank-full, 

 or even overflowing. This freshet usually occurs in March or 

 early in April. For a few weeks after this freshet the streams 

 are at a moderate stage, slightly above the normal. This is 

 followed by the ' June rise,' occasioned by the great rainfall 

 which occurs in that month, when the streams often reach as 

 high a stage as in the spring freshet. After the June rise the 



