388 
of silt on March 30—with increase in temperatures in January— 
April, culminating in a vernal pulse in April-May, which in 
Flag Lake reaches a much higher level (203.52) than elsewhere, 
culminates later by 7 to 14 days and is not divided into two 
apices as in the other three localities, but in duration covers 
the period of two pulses elsewhere. It is further seen in the 
May-June and August pulses and in the fairly well sustained 
correspondence in direction of the changes in the September-— 
December period. The most marked disagreement appears 
with the declines in stage of the river in May and July, when 
local environmental factors are most potent, and when, also, 
vegetation is at the height of its relative occupancy of the 
lakes in question. 
One of the most striking features in the production of this 
lake, and one not without parallels elsewhere in our records 
(Pl. XXIX., XXXI.), is the very sudden decline in plankton 
content after the vernal pulse, namely, from 203.52 em.* per m. 
on May 2 to 47.7 on the 9th—a decline of 77 per cent. in 7 days. 
On the 15th it reached the low level of .72, a decline of 98 per 
cent. in 6 days or of 99.6 in 13 days. The attendant hydro- 
graphic conditions are not without significance. This pulse 
(Pl. XX XIII.) attains its growth between March 30 (1.02) and 
May 2 (203.52), in which period the net drop in levels in 
channel waters is only from 8.1 to 6.9 ft. and the total move- 
ment only 1.7 ft., while in this protected backwater the fluctu- 
ations are probably somewhat lessened, as will be seen in the 
fact that the depth in the lake changes only .5 ft. to 1.2 ft. in 
the channel. The pulse thus rises in stable conditions. 
The decline of the pulse takes place between May 2 and 23 
from 203.52 em.* per m.* to .12. In this time levels fall from 7.1 
to 4.9 (see p. 159) on the 17th and rise again to 7.2 on the 23d. 
The decline in production from the 15th (.72) to the 23d (.12) 
is so small a part of the total that its significance in the present 
connection is slight, and the rise in levels has probably not had 
time to materially affect the lake. The hydrographic influences 
potent in the decline in production have been operative prior 
