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 

 river and 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- 



