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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 to a minimum. Such waters as Phelps and 

 Flag lakes (PI. 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 (PI. 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 cm. 3 per m. 3 

 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 cm. 3 ), which is but one fourth to one 

 tenth that in other years save only 1896 (1.12) — also a year of 

 much hydrographic disturbance. 



