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 fallin August, September, and October,—and occasion- 
ally in the early winter months. 
THE REGIMEN OF THE ILLINOIS RIVER. 
Leverett (796) 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 
