118 



and exceptionally high, — reaching 18.80 feet at Copperas Creek, 

 a height surpassed in recent years only by the flood of 1883 

 (19.25 feet), — and would thus cause a current at least as rapid 

 as the estimate above given. 



This retardation of the flood is due, in part at least, to the 

 overflow of the bottom-lands, and it is much more pronounced 

 in the Illinois River than it is in ordinary streams owing to 

 the imperfect development of its flood-plain and the conse- 

 quent early inception of overflow stages on a rising river. 



The current is an important factor in the environment of the 

 plankton. In the first place it largely determines the amount 

 of silt in suspension in the water, for upon its speed depends 

 not only the amount of material eroded from the banks and 

 carried on by the flood, but also the rate of deposition of the 

 silt delivered to it by the more rapid tributary streams. 



The most important relation of the current to the plankton 

 lies in the fact that it is a large factor in determining the 

 length of time in which the plankton can breed, and thus 

 curtails or extends the possible number of generations of the 

 planktonts. The table on page 115 indicates that at the low- 

 water stage over twenty-three days elapse before the water 

 which enters the river at LaSalle, at the upper end of the pool 

 of the Henry dam, joins the Mississippi at Grafton. At the 

 stage of overflow the interval is reduced to less than five and 

 a half days, or about one fourth of the time at low water. It 

 is evident that the possible number of generations increases in 

 arithmetical progression as the current declines, while the 

 number of individuals may increase in geometrical progres- 

 sion. A concrete example will illustrate. A given organism 

 which multiplies by fission and in which a new generation ap- 

 pears each twenty-four hours will, at times of low water, reach 

 the twenty-third generation as it floats from LaSalle to the 

 mouth of the river, and the total possible number of individ- 

 uals of the last generation would be 8,388,608, while in the 

 more rapid current at the stage of overflow the number of gen- 

 erations could be but five and a half, and the number of indi- 



