348 



ous impounding' backwaters. No Spoon River collection falls 

 in the week of the vernal maximum (see PI. XII.), but the col- 

 lections of May (.023), June (.096), and July (.036), all exhibit 

 a considerable rise above the usual level of production, and all, 

 moreover, were made daring the run-off of the spring flood and 

 receive slight contributions from impounded waters. It is in 

 the direction, of the movement in production that the tributary 

 and mainstream are alike at this season. In the amplitude of the 

 curve of production the difference is very great, production be- 

 ing respectively 491-, 41-, and 16-fold greater in the latter in 

 the three months named. 



Throughout the remainder of the year 1898 plankton pro- 

 duction in Spoon River is at a minimum, there being but the 

 merest trace of living organisms in the catch. None of these 

 catches was taken in rising flood water (PL XXIV.), though 

 they all show the results of the flushing action of the frequent 

 floods which wash out with rapid current whatever plankton 

 may have developed in the tributary, and at the same time af- 

 ford little opportunity for its replenishment. The relative ab- 

 sence of backwater feeders in the tributary stream at this 

 stage of river levels serves also to emphasize the poverty in 

 production of the tributary. 



The average for 1898 (.029 cm. 3 per m. 3 ) is exceeded over 

 73-fold by the yearly average of the Illinois (2.13). The tribu- 

 tary waters are at all times — at least in so far as the data go — 

 diluents of the channel plankton, reaching their lowest ratio, 

 .124 to .38, in March, when they share most in impounded back- 

 waters of the main stream, and at the same time are at the max- 

 imum of their own reservoir action. 



This meager production occurs in waters almost as rich in 

 nitrates (av., .67 parts per million) as the main stream (.809), 

 and, save on rising floods (PI. XLVII.), in normal chemical con- 

 ditions. The potent environmental factor is rather to be found 

 in the recent origin of the tributary waters than in any availa- 

 ble chemical data. 



In the three months' collections of 1899 we find that the low 



