324 
current and discharge reduced, and plankton content below the 
normal. The effect of prolonged low water, with its attendant 
stability, is shown in increased plankton content, and in the 
increased contamination by sewage, which under the ice results 
ultimately in stagnation and great reduction of production. 
1898. 
(Tables III., X.; Pl. XLV., LII.) 
As shown on page 167, this is a year of normally located 
spring floods of considerable amplitude followed by a disturbed 
summer and a considerable autumnal rise. It accordingly af- 
fords our best opportunity for tracing the vernal movement of 
production in flood conditions, and also another chance to note 
the effect of floods at times of reduced flow of the stream. 
Both vernal and autumnal changes in temperature came on 
gradually, and chemical conditions were free from catastrophic 
fluctuations. The collections of this year number 52, all at 
weekly intervals excepting only in January, when the regular 
interval is slightly varied. They are without exception pump 
collections. The interval between collections is so brief that 
the cyclic movement can be traced, as a rule, and this point 
of view will continue to control the discussion, though in this 
year the suppression of production by floods increases the ele- 
ment of conjecture in this method of treatment. 
The January pulse has a duration of 28 days,—from Dec. 
28 to Jan. 25,—with a maximum amplitude of .81 em.’ per m.° 
on the 21st. Its mean falls on the 22d, 37 days after that of 
the preceding pulse, This isa period of stagnation followed 
by a small flood which carried off the ice-sheet. A fall in tem- 
perature immediately closed the river again, and levels fell only 
to continue again an interrupted rise during the last fortnight, 
carrying the imperfect ice-sheet with it, but not breaking it up 
and carrying it out (Pl. XI. and XII.). This second rise removed 
the stagnation conditions in the third week, producing a sharp 
decline in chlorine and free ammonia (Pl. XLV.) toward a nor- 
mal status. There is also a fall in the several forms of organic 
