396 
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 Ceratophyllum and other aquatic plants, which 
choked the lake from shore to shore and from Quiver Creek far 
down the chute towards its mouth (Pl. XV.). The very slight 
plankton production in its waters during the summer is due, I 
believe, to this predominance of vegetation. The rise in pro- 
duction when river levels rose in July and September (PI. 
XXVI.) attends, among other factors favorable to production 
already discussed, a reduction in the relative abundance of 
vegetation. 
1896. 
(Table V., Pl. XXVII.) 
There are 31 collections in this year, with an average of 
2.59 em.’ 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 
