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lies primarily in the length of time afforded for breeding. The 

 rate of the current is but one of the elements determining this 

 time. If this be true, the expression of this relation should 

 take another form, as, for example, the volume of plankton 

 present in any body of fresh water varies with the length of 

 time afforded for breeding. 



THE DISCHARGE. 



The total annual production of plankton in a given body of 

 water can be estimated only when the total discharge for that 

 time is known. The seasonal fluctuations of the discharge also 

 profoundly affect the local and seasonal distribution of the 

 plankton, and modify alike its quantity and its constituent or- 

 ganisms. For these reasons the consideration of this element 

 in the environment of the plankton is of prime importance. 

 The discussion of the subject naturally falls under two heads, 

 namely, the rainfall and the run-off. 



The Rainfall. — The political boundaries of Illinois do not 

 coincide with the watersheds of the Illinois River basin, though 

 almost half the area of the state lies within this basin, which, 

 moreover, extends through more than two thirds the length of 

 the state, and is fairly typical of four fifths of its area. The 

 small portion of the basin (4,287 square miles) which lies out- 

 side of the state does not present conditions of rainfall which 

 materially differ from those of parts of the basin within the 

 state. The northern part of Indiana, in which the Kankakee 

 basin lies, has a mean annual rainfall, according to Leverett 

 ('97), of 35.49 inches, which is somewhat less than the average 

 (37.858 in.) for the whole State of Illinois; but since in Illi- 

 nois as in Indiana the rainfall in the northern part is less than 

 that in the southern part it is not improbable that the precipi- 

 tation in the Kankakee basin in Indiana is about the same as 

 that in corresponding latitudes of Illinois. Under these cir- 

 cumstances, the statistics of rainfall for the whole State of Il- 

 linois may be taken to represent with considerable accuracy 

 the conditions within the basin of the Illinois River. 



