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. A comparison of Tables III. and IV. and 

 Plates XL 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 (ireraije 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 it is most 



