136 
peras Creek and LaGrange exhibit certain general differences 
which are not thus explained, but result from some more widely 
operative cause. As a rule the extremes of high and low water 
are more pronounced at LaGrange than at Copperas Creek. This 
is apparent in the hydrographs of 1883, 1893, and 1896-98. ‘It is 
also expressed in the tabulation (Tables I. and II.) of the ex- 
tremes of high and low water at Copperas Creek for twenty-one 
years (1879-1899) and at LaGrange for seventeen years (1883- 
1899). The average range between highest and lowest water 
at Copperas Creek is 13.14 feet, at LaGrange 14.68 feet—an in- 
crease of 1.54 feet or 11.5%. In lke manner the average of 
the total -- and — movements per year of the river level at the 
upper dam is 50.5 feet to 59.15 feet at the lower one—an in- 
crease of 17.1% in the fluctuations of the stream at that point. 
The greater fluctuations probably result from the fact that 
in the lower basin, within which the lower dam lies, the rainfall 
is greater, the drainage lines better developed, and the run-off 
more rapid. The reservoir action of the Des Plaines, the Kan- 
kakee, and the pumps at Bridgeport are also less effective in 
regulating the flow of the stream at this point, owing to their 
distance and to the reduction in their relative contributions. 
The distance, by river, from Copperas Creek to Havana is 
16.8 miles; from Havana to LaGrange is 42.7 miles—a total of 
59.5 miles. The fall between the two dams is given by Cooley 
(91) as 8.5 feet at natural low water. Our plankton station 
thus hes in the upper and more evenly regulated portion of the 
LaGrange pool, and probably in the most stable portion of the 
river between Utica and its mouth. The dam at LaGrange 
is estimated by the engineers to raise the water 8 feet at 
LaGrange, 2.4 feet at Havana, and 2 feet at Copperas Creek. 
As has been stated on page 113, prolonged low water will lower 
the level at Havana to 2 feet or even less. At such times the 
river between the two dams is practically a slack-water pool, 
which responds quickly to flood water from any source. Two 
principal tributaries, Spoon River and the Sangamon, enter this 
pool from opposite sides of the river, the former 4 mile above 
7] 
