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 like 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 lies 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 ^^ mile abov^e 



