463 
tained within the lake and augmented by the synthetic activi- 
ties of the phytoplankton and other vegetation. 
The fluctuations in hydrographic conditions consequent upon 
changes to river levels constitute the one pre-eminent factor 
peculiar in the fluviatile environment in the extent to which it 
is developed and to which it influences and controls the course 
of plankton production. These fluctuations operate by bring- 
ing about changes in area, depth, and volume, in current, in age 
of the water, in rate of renewal and period of impounding, in 
relative proportions of tributary and impounded water, in 
chemical contents and sewage contamination, in relative dom- 
inance of vegetation, and in the interrelations of channel with 
backwaters and of the backwaters with each other. I[llustra- 
tions of each and all of these results have been cited in the de- 
tailed discussion of the course of plankton production in the 
channel and the various backwaters. It will therefore suftice 
in the present connection to cite briefly certain prominent fea- 
tures, and to deal particularly only with some of the general 
phases of the problem not readily followed through the maze of 
details of the previous discussion. 
The rise in levels results usually from access of tributa- 
ry flood waters in local or up-stream territory, and rarely from 
backwater due to entrance of storm water in lower reaches of 
the river only. These flood rises occur in both high and low 
river stages, though they are more frequent and of shorter du- 
ration at lower levels, since in the much contracted volume of 
the stream at such stages slight increases in the run-off which 
would scarcely cause a ripple in the hydrograph in overflow 
stages, now cause considerable change. 
The river channel itself is most immediately affected by 
this access of flood waters of recent origin, since tributaries, 
with few exceptions, discharge directly into the channel, and 
even when these courses across the bottom-lands are submerged 
in general overflow the tributary current is always maintained, 
in part at least, along its old path. 
The result of this invasion is always a dilution of the plank- 
