345 



in the Illinois River channel (turbidity, 2 cm., silt, 26.33 cm.') 

 contains but .27 cm.'' per m."' of plankton, but the vernal pro- 

 duction in the tributary does not suffer so marked a decline, 

 remaining at .25 cm.^ so that its diluent action on this occasion 

 was slight. As in the main stream, so also in the tributary 

 there is a drop in production in August (.056) to about a fourth 

 that in June-July. 



From this time throughout the remainder of the year the 

 production in Spoon River is considerable, exceeding that in 

 the main stream, however, only in the last two months of the 

 year. The production rises on Aug. 26 to 1.248 and on Sept. 11 

 to the unprecedented record of 7.296 cm.^ while that in the 

 main stream is only 2.77 on Aug. 24, and on Sept. 7 and 14 is 

 8.47 and 19.80 respectively, On the 21st and 29th of September 

 production in the Illinois falls again to 3.00 and 4.04 cm.^ re- 

 spectively, while in Spoon River on the 30th it is only 2.96, 

 production in the tributary thus remaining below that in the 

 channel throughout this period. The low chlorine in Spoon 

 River at this time (3 to 4 parts per million. See Table XI.) 

 as compared with the main stream (21-50 parts) indicates 

 that the Spoon River water is not contaminated by channel 

 water, and that we are dealing with an indigenous plankton. 

 None of our collections falls in the period in October (PI. 

 XLVI.) in which chlorine in Spoon River rises temporarily. 

 The three collections of November-December, 1.351, 1.99, and 

 .599 cm.^, are respectively 22.5, 1.6, and 20 times as great as the 

 production in the main stream at the same time. The month- 

 ly averages for this period are 1. and .56 for the Illinois, and 

 1.671 and .599 cm.^ for Spoon River, so that the excess in the 

 latter is apparently not more than 50 per cent. Under these 

 conditions the tributary stream enriches the plankton of the 

 channel instead of diluting it, but its discharge is slight. 



Hydrographic, thermal, and ice conditions are similar in 

 the two streams, and we find the main difference in the chem- 

 ical conditions> Aside from evidences of sewage contamina- 

 tion in the last weeks of October, the Spoon River records 



