856 



increases its impounding function. Whatever run-off from lake 

 to channel occurs under such conditions will result in a slight 

 enrichment of the plankton content of the channel waters 

 with which the tributary mingles. At all other seasons of this 

 year our collections indicate that the immediate result of the 

 access of Quiver Lake waters to the river is a reduction in 

 plankton content of the main stream, on an average for the 

 year for equal volumes of tributary and channel waters, of 38 

 per cent.; or if the relative volumes of each based on areas of 

 drainage basins are considered, the plankton content of the 

 channel is reduced to 3.19 cm.' per m.' — a decline of about 1 

 per cent. 



This was a year of maximum development of vegetation 

 in Quiver Lake. The low water of this and the preceding year 

 and the absence of floods adequate to flush the lake of its 

 loosely attached vegetation permitted an unusual and enor- 

 mous growth of Ceratophijllum and other aquatic plants, which 

 choked the lake from shore to shore and from Quiver Creek far 

 down the chute towards its mouth (PI. XV.). The very slight 

 plankton production in its waters during the summer is due, I 

 laelieve, to this predominance of vegetation. The rise in pro- 

 duction when river levels rose in July and September (PI. 

 XXVL) attends, among other factors favorable to production 

 already discussed, a reduction in the relative abundance of 

 vegetation. 



1896. 



(Table V., PI. XXVII.) 



There are 31 collections in this year, with an average of 

 2.59 cm.' per m.' as compared with 1.16 in the Illinois. The 

 maximum of 16.76 cm.' occurs on April 24, exceeding by 7.37 

 cm.^ or 78 per cent., the production in the adjacent river on 

 that day. 



The similarity in the movement of plankton production in 

 Quiver Lake and the Illinois noted as generally present in 1894 

 and but slightly so in 1895 is quite apparent throughout this 



