156 



This lake receives but little water from a few springs and 

 creeks along the bluffs, and like many others in the bottom- 

 lands serves only as a reservoir from which the water is slowly 

 drawn off as the river falls, but when once the lower stages are 

 reached its contributions cease. Still others, like Quiver and 

 Matanzas, maintain direct and open connection with the 

 river, and since they receive tributary streams they continue 

 to feed the river, but in reduced volume. Though the number 

 of tributary areas is thus much reduced at low-water stages, 

 the individual peculiarities of the tributary waters in the bot- 

 tom-lands become more pronounced. As each one loses its 

 connection with the general flood it becomes a separate unit of 

 environment, with its local differences in those factors which 

 determine the character of the plankton developing in its 

 waters. The resulting contributions may thus differ greatly in 

 amount and component organisms, and accordingly tend to 

 diversify the river plankton of low water to a degree even more 

 marked than that of high water. 



With the confinement of the river waters to the channel 

 goes a marked condensation of the sewage, which, under condi- 

 tions of uninterrupted low water, leads at times to an excessive 

 development of the plankton, or, if the river is closed by ice, to 

 stagnation conditions. But few years, however, offer such op- 

 portunities; for, as a rule, in most low- water periods sudden 

 and heavy rains are wont to occur which flush the stream, 

 wash away the sewage and plankton-laden waters, and store 

 anew the reservoir lakes without causing any considerable 

 overflow. After each catastrophe of this sort the decline of the 

 flood affords a new and favorable opportunity for the develop- 

 ment of the plankton. 



In this instability lies the great distinction between the 

 river and the lake as a unit of environment — for I believe we 

 are justified in applying this phrase to the conditions of fluvia- 

 tile life, though it must be admitted that the "fluviatile unit" 

 is an exceedingly complex one. As the discussion of the river 

 fluctuations indicated, there is a seasonal routine which the 



