404 



slough at the northern end, impounded it for some time, and 

 maintained a run-off at the southern end (PI. 11.) of its plank- 

 ton-rich waters. There are in addition 35 days in June, July, 

 and August in which falling levels below 6 ft. afforded an op- 

 portunity for a run-off through the slough at the northern end. 

 Of the remaining 155 days, 10 are of rising levels below 6 ft., 

 when the lake receives water from the river but does not dis- 

 charge any into it, and 145 belong to the low-water period of 

 the last 5 months, in which there was little interchange be- 

 tween lake and river though the run-off continued in diminish- 

 ing volume for a few days after stable levels were reached, 

 early in August. About August 16 the channel discharge was 

 so slight as not to float a rowboat in the narrow channel at the 

 northern end of the lake, and connection with the river was 

 not reestablished as the river rose in October-November until 

 the level of 2.8 ft. was reached, Nov. 10. The slight fluctuations 

 during the remainder of the year practically amount only to 

 the reception of .4 ft. of water by the lake. For the last live 

 months of the year — months of heavy plankton production m 

 lake waters — there was no run-off to the river. 



On the average the lake produced this year 10.43 cm.-' per 

 m.', about 3-fold that in channel waters (3.69 cm.^) and the net 

 result of the run-off would be, it seems, an enrichment of chan- 

 nel waters. The actual enrichment is, however, much less 

 than these averages indicate. An examination of the monthly 

 averages (see table between pp. 342 and 343) reveals the fact 

 that the excessive production in the lake, when the plankton 

 content rises to 5- to 16-fold that in the channel, appears in 

 the low-water period when no run-off occurs. During the first 

 7 months, in which there is an almost continual run-off, the 

 production in lake waters is but 1.5- to 2-fold that in the chan- 

 nel except in February and June, when flood waters in the 

 latter increase the ratio to 1 to 7 and 13 respectively. The in- 

 sufficiency of the collections in this period leaves in doubt 

 the amplitude of the vernal pulse. The April and May collec- 

 tions indicate only a low level of production as compared with 



