333 
other month of our plankton operations. It is a decline from 
overflow exceeding bank height to the midsummer minimum 
within four weeks. This sweeping change in river levels in- 
volves great hydrographic modifications and accompanying 
disturbance in the equilibrium between the plankton and its 
movement. It brings about a reduction of the reservoir action 
of the backwaters toa minimum. Such waters as Phelps and 
Flag lakes (Pl. II.) speedily lose all connection with the river, 
and the greatly reduced contributions enter from those which 
maintain permanent connection with the stream, for example, 
from Thompson’s Lake. The inflow of water from tributary 
streams thus comes to form more and more the principal source 
of channel waters as levels decline. These tributary waters are 
mostly of recent origin, from rains or springs, and have not had 
time as yet to breed a plankton of much volume. I believe this 
growing preponderance of tributary waters to be one of the 
factors responsible for the slight amplitude of this July pulse. 
Along with this there comes also a further decline in 
nitrates (Pl. XLV.) and a slight increase in free ammonia and 
chlorine indicating a greater proportion of sewage. The heat 
pulse of the last fortnight in July is not attended by any simi- 
lar movement in plankton production. 
The August pulse has a duration of 28 days,—from July 26 
to Aug. 23,—with a maximum amplitude of 1.62 em.’ per m.* 
on the 2d. Its mean falls on the 9th, 22 days after that of the 
preceding pulse. In this month there begins a series of small 
rises in the river which flush the stream repeatedly at inter- 
vals of one to two weeks until October. Two of these fall 
within the period of this pulse, result in its suppression, and 
shift its apex and mean to the left. The total movement in 
levels in this month is 8.2 ft.—a distance not equaled in any 
other August of our records. The result is seen in the low av- 
erage of production (.91 em.*), which is but one fourth to one 
tenth that in other years save only 1896 (1.12)—also a year of 
much hydrographic disturbance. 
