401 



rise is 26.9 feet — sufficient to fill the channel for 81 miles. 

 When we add to this the consideration that at levels above 6 

 ft. water is continually passing through the lake with brief im- 

 pounding, the length of channel filled by the run-off of this 

 area must be considerably extended. 



The relationship of plankton production in this lake to the 

 plankton content in channel waters in this year may be in- 

 ferred from the yearly averages. Thompson's Lake contained 

 9 cm. 3 per m. s to 1.16 cm. 3 in the river. The net result would 

 therefore be an enrichment of the channel plankton in a 

 ratio dependent upon the relative volumes of the mingling 

 waters. No quantitative statement of this ratio is possible in 

 the absence of data as to the run-off of Thompson's Lake. Not 

 only is the net result an increase in the channel plankton, but 

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

 the coincident or approximate individual collections (Tables 

 III. and. ; VIII.) in every instance exhibit a higher plankton con- 

 tent in this lake than in channel waters. The monthly aver- 

 ages range from 2 to 251 times greater in Thompson's Lake 

 than in the river — ratios within which most, if not all, of those 

 of individual collections fall. The data all indicate that this 

 impounded water of the lake breeds a plankton whose run-off, 

 without exception throughout this year, enriched channel waters. 



The effect of invading and plankton-poor river waters upon 

 the plankton content of the lake is not conclusively apparent 

 in the data, since we have also to deal with the phenomenon 

 of pulse-like changes in plankton content which are combined 

 with other factors in affecting the movements in production. 

 It may be significant of the diluent action of invading river 

 water that plankton content falls in the lake with the first en- 

 trance of the May- June, the July- August, the October, and the 

 November floods (PI. XXXVII.). The recovery in production 

 follows promptly in each case with the impounding of the en- 

 tering waters. Since, however, declines in content, as in June, 

 July, and August, occur also when flood waters are not enter- 

 ing, we cannot conclude that the decline upon this entrance is 



