528 
also of almost all of the single catches when comparison is 
made of those taken in the two streams on the same day, or 
on the nearest dates when they were not coincident. The ex- 
ceptions are four, one in February, 1897, when flood conditions 
prevailed in both streams and the amount of plankton in each 
was small, and three in the autumn of the same year during 
the period of prolonged low water. During this period the dis- 
charge from Spoon River was very slight, the current scarcely 
perceptible, and the temperatures somewhat higher than usual 
at this season of the year, so that the tortuous channel of the 
stream through the bottom-lands became practically an im- 
pounding lagoon. comparison of Tables III. and IV. and 
Plates XI. and XVII. shows a general correspondence during 
this period between the two streams in that the autumn maxi- 
mum and winter decline are present in both, the maximum 
being somewhat greater in the Illinois and the decline some- 
what less pronounced in Spoon River. It was during the period 
of this winter decline that Spoon River attained a somewhat 
greater productiveness than the main stream. The autumn 
rains of that year were long delayed and were largely absorbed 
by the parched earth, so that the run-off was but slight, and 
Spoon River was not subject to the repeated and often almost 
continuous flushing which characterizes many autumnal seasons. 
By reason of these unusual conditions Spoon River exhibited 
at this time more of the environmental features of an im- 
pounding backwater than those of a tributary stream, and in 
these conditions lies the occasion of the unusual plankton pro- 
duction manifested by the tributary at this season. As before 
stated, the average for this low-water period was greater in the 
main stream than in the tributary, though, as the data in the 
tables referred to show, there was a period of about two months 
in which the tributary directly increased the relative amount 
of plankton in the main stream. 
In other years than’ 1897, and especially in other parts 
of the year, the contrast between the amounts of plankton in 
the two streams is much more marked. In general itis most. 
