315 



floods of this year are responsible in large degree not only for 

 the increased silt but also for the reduced production of plank- 

 ton. The floods of this year were not only more numerous, 

 but they were also more effective as reducing factors, since they 

 rarely reached stages of considerable overflow, So long as the 

 flood does not exceed bank height its flushing action is concen- 

 trated in channel waters, and impounded backwaters do not 

 contribute so largely to channel plankton, nor are they so im- 

 mediately connected with the channel on account of the bank 

 development along the stream. The floods of 1896 were of 

 such a character that they continually flushed the channel 

 without at any time, except midwinter, forming any large body 

 of impounded water in which the plankton had time to reach 

 any marked development. Although there was a plankton in 

 the backwaters — e. g. Thompson's and Phelps lakes — which was 

 more abundant than that in the channel (see PI. XXXVII. and 

 XL.), contributions from such areas to the channel plankton 

 are relatively small owing to their slight connection with the 

 stream in this year. 



As shown in Table X., the average amount of nitrates in 

 1896 is 2.34 parts per million; in 1897, 1.66; and in 1898, only .81. 

 The smallest production of plankton observed in the years cov- 

 ered by our data has thus taken place in water richest in ni- 

 trates. Other forms of nitrogen than the nitrates vary in the 

 same general direction with these. It seems probable that ni- 

 trates, or available nitrogen generally if not, indeed, nutrition 

 as a whole, are less dominant in determining plankton produc- 

 tion in our waters than other environing factors, as, for exam- 

 ple, in this instance, hydrographic conditions, or, more specific- 

 ally, current in its relation to time for breeding. 



1897. 



(Tables III., X. ; PL XL, XLIV., LII.) 



There were 34 collections in this year, of which 6 were 

 made at intervals of about a month from February to July, and 

 the remainder at approximately weekly intervals, or less, during 



