448 



its own influx of tributary water from Quiver Creek, is the most 

 liable of all the backwaters examined by us to hydrographic 

 disturbance. It is therefore not surprising that in this year of 

 extreme disturbance we should find marked depression for a 

 long period in this lake. The total movement in levels in 1898 

 is 67.2 ft. (see table p. 163), 44 per cent, above the average. Of 

 this, 50.8 ft. fall in the 8 months of depressed production, that 

 is, 76 per cent, of the movement occurs in 67 per cent, of the 

 time. To this relative excess of fluctuation in levels, and proba- 

 bly to large access of local flood and spring water, we must at- 

 tribute the low production in Quiver Lake in these months. 



Thompson's Lake has an average production of 5.06 cm. 3 , 

 or, if all collections are averaged, 5.71 cm. 3 , 39 and 28 per cent, 

 below the respective averages for all years (see table, p. 429). 

 Not only is the general average below normal, but all of the 

 monthly averages, save only those of January (7.22) and De- 

 cember (3.58), are likewise deficient by from 2 to 88 per cent. 

 The large January production is the largest plankton content 

 in this month in any year or locality, and accompanies an 

 invasion and impounding of sewage-laden river waters in the 

 lake (cf. PI. XLV. and L.). 



The cause of the low production throughout the remainder 

 of the year is again to be found in the hydrographic conditions. 

 During 8 months of the year (at levels above 6 ft., see PI. XXXIX.) 

 the lake is swept by a gentle current entering at the northern 

 end and discharging to the channel at the lower. There is, 

 thus, in this year more than the usual run-off, not only of or- 

 ganic matters in solution and suspension; received with the 

 waters of ingress, but also those developed in its impounded 

 waters or about its shores. This tends to impoverish the 

 waters, and interferes with the accumulation and flux of or- 

 ganic matter in the plankton which manifested itself in such 

 amplitude in the low water of the preceding year (PL XXX- 

 VLTL). To a much less extent than in Quiver Lake is the de- 

 pressing effect of flood waters seen in the broader expanse of 

 this body of water. While in the former the production in 



