529 
pronounced during the spring and early summer at the time 
of the plankton maximum. Save in 1897 the contrast between 
the two streams is also well marked during the autumn maxi- 
mum. The phenomenon of these maxima is thus a function of 
the main stream rather than of the tributary. 
The great irregularities in the Spoon River plankton as 
contrasted with the main stream are due to the more profound 
effect of floods upon its life. In the main stream, floods less 
frequently and less thoroughly replace its waters owing to its 
greater volume and, especially, to its reserve of densely popu- 
lated impounded backwaters. The small tributary, on the other 
hand, is scoured very completely by every considerable rain, 
and has no reserve waters to maintain its equilibrium. The 
oscillations in the quantity of life in its waters are thus more 
frequent and more extreme than in the main river. 
The quantitative effect of the plankton ofa tributary upon 
that of the main stream may be expressed in the following 
simple formula : 
DP + dp™ 
Dae (tl 
in which D and d represent the discharge from the catchment- 
basins of the main stream and tributary, P and p, the plankton 
per cubic meter of their respective waters, and P’ that of the 
main stream below the junction with the tributary, In the 
case of Illinois and Spoon rivers the average discharge is ap- 
proximately in the ratio of the areas of the catchment-basins. 
Both lie in the same storm belt, and the slightly increased rain- 
fall in Spoon River basin, which is the more southerly, is more 
than counterbalanced by the accession of water from the Illi- 
nois and Michigan Canal due to the pumpage from Chicago 
tiver at Bridgeport. Accepting these areas as a basis for the 
ratio of discharge, we find that the average amount of plank- 
ton in the Illinois is reduced from 2.19 eubie centimeters per 
cubic meter to 2.00 if the low-water period of 1897 is retained— 
a decline of 9 per cent. If the low-water period is rejected 
from both averages the amount falls to 1.96 cubic centimeters— 
ashe ay 
