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 riseand 
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 Greenleatf’s (’85) 
estimate of the average discharge at that point. At Copperas 
Creek, on the other hand, the more moderate estimates of 
Cooley (791) place the average discharge at 10,500 cubic feet 
