463 



tained within the lake and augmented by the synthetic activi- 

 ties of the phytoplankton and other vegetation. 



The fluctuations in hydrograjphic 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. Illustra- 

 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 suffice 

 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- 



