514 
received at Utica, the upper end of the basin under considera- 
tion in this paper, the drainage of 10,365 square miles of catch- 
ment-basin, over one third of the total basin of the stream, to 
which was added at La Salle, a few miles below, the water from 
the Illinois and Michigan Canal containing the sewage from 
the Chicago River. <A large volume of water, richly fertilized, 
is thus provided for the reception of tributary streams, no one 
of which, with the exception of the Sangamon, has more than 
one ninth of the drainage basin above the point of its union 
with the river. The Sangamon has at its mouth a basin 
one fourth as large as that of the river above. This presence 
of a considerable initial volume and the distribution of tribu- 
tary waters in relatively small streams at intervals along the 
course are conditions which favor the mingling of the constit- 
uent waters of the stream and tend to maintain the uniformity 
of the plankton in the main channel. 
The tributary waters, with a few minor exceptions, such as 
Quiver Creek, enter the main channel directly. The elevated 
deposits built up across the bottoms by their agency confine 
their floods within their banks except during the maximum 
stages of overflow, when they contribute directly to the back- 
waters, but even under these conditions a strong current is still 
maintained along their customary channels directly to the 
main channel of the river. Their contributions are thus, as a 
rule, carried directly to the river and mingle with it without 
any period of impounding, and their effect upon the plankton 
is direct and immediate. 
Two tributaries, Spoon River and Quiver Creek, were availa- 
ble for examination at Havana, in both of which collections 
were made by us which throw considerable light upon the 
character and quantity of their plankton contributions. 
QUIVER CREEK. 
In Quiver Creek we have a small tributary with a basin of 
only 220 square miles, largely of alluvial second bottom with 
more sand and less heavy loam and clay than the adjacent 
