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 Pl. XX XVII. 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 coy- 
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 specitic- 
ally, current in its relation to time for breeding. 
1897. 
(Tables III., X.; Pl. XI., XLIV., LIL.) 
There were 34 collections in this year, of which 6 were 
madeat intervals of abouta month from Februaryto July, and 
the remainder at approximately weekly intervals, or less, during 
