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 
