398 
hydrographic conditions favor a run-off of this richer plankton 
of the lake into channel waters. There are two instances in 
which run-off occurs when lake waters are poorer than the 
channel, but they are both at low levels and during slow de- 
cline, so that the discharge and resulting diluent effect is but 
slight. Considering the average production, the times when 
run-off occurred, and the hydrographic conditions when the lake 
waters contained less than the channel, it is probable that even 
in this year Thompson’s Lake, owing to its reservoir function, 
served predominantly to enrich the channel plankton. Though 
this relation predominated, the total contribution of the lake 
to the stream in this year was but slight owing to the hydro- 
graphic conditions. In the April-December period covered by 
our collections, the stage of river never exceeded 6 ft. until the 
December flood. There was, therefore, never any general cur- 
rent of overflow passing through the lake and carrying the im- 
pounded waters out from the southern end (PI. II.) into the 
riverand thus discharging a considerable volume of plankton-rich 
water into the channel—a condition possible in both rising and 
falling levels above 6 ft. At the levels below this point which 
prevailed throughout this period, influx and efflux both can take 
place only through the slough at the northern end, so that con- 
tributions to the stream from the lake occur only during falling 
levels, and, moreover, owing to the tortuous course and clogged 
condition of the outlet, the volume discharged at these lower 
levels is very much less than at higher ones, across the broad 
outlet at the other end of the lake. Falling levels occurred in 
less than one half of the time in April-December, so that the 
contributions of the lake to the river were not only slight in 
volume but limited in duration and discontinuous. 
Collections were too infrequent to trace the movement in 
production with fullness or certainty. There are, however, a 
few suggestions of a similarity in the course of production here 
and elsewhere. The direction of the changes in the course of 
production in this lake and in the river in coincident or ap- 
proximate collections is the same in 9 out of the possible 13 in- 
