465 



the channel — is more prompt and attains a greater amplitude. 

 A comparison of after effects of the June flood in 1896 in the 

 river (PI. X.) and in Quiver (PL XXVII.), Dogfish (PL XXXI.), 

 Flag (PL XXXIII.), and Thompson's (PL XXXVII.) lakes will 

 conclusively demonstrate these conditions. While the recov- 

 ery from flood effects in the river is delayed till about June 10, 

 in the various backwaters it occurs from a week to 10 days 

 earlier. 



It is evident from the pulse-like character of the movement 

 in plankton production, to which attention has been called re- 

 peatedly in the detailed discussion, that the recovery from flood 

 conditions is complicated with this phenomenon. Flood con- 

 ditions in backwaters, by influx of waters laden with organic 

 detritus, and by destruction, submergence, and decay of vegeta- 

 tion, tend to accelerate the appearance and, it may be, increase 

 the amplitude, of the pulses. When, however, the invading 

 waters have little plankton and largely replace the previous 

 contents, they tend to delay and depress the pulses, and, as in 

 the case of the May- June and August pulses, the flood may even 

 obliterate the apex of a normally developing plankton pulse. 

 Flood conditions thus affect these pulses profoundly, but they 

 do not seem to be fundamentally their cause. 



Periods of declining river stages stand in sharp contrast 

 with rising levels. It is for the plankton a period in which re- 

 construction and growth are possible. The decreased propor- 

 tion of recent water and the prolongation, of impounding in 

 backwaters, characterize this as a time of relatively stable con- 

 ditions as contrasted with the chaos caused by the rising flood. 



In a general way there is some correlation between the to- 

 tal annual movement in river levels and the average plankton 

 content per cubic meter of water. In the table on page 466 

 the available data are tabulated for the channel plankton. 



The arbitrary division by years results in one misleading 

 presentation of the data. The December flood of 1895 (12.6 ft.) 

 occurred in the last days of the year, while its effects continue 

 beyond that limit. If we subtract the 9.8 ft. of this rise from 



