443 



ters very largely independent of backwaters and tributaries by 

 reason of cutting off of communication in some cases and ces- 

 sation of run-off in others. Along with this independence goes 

 increased fertility by virtue of greater relative contamination 

 by sewage and longer time for breeding by reason of the slack- 

 ened current. As a result, production in channel waters reach- 

 es in July-November, 1897, a level unsurpassed in our records, 

 rising above the monthly means of all years (see table fol- 

 lowing page 342) 11 per cent, in July, 245 per cent, in Sep- 

 tember, 250 per cent, in October, and 14 per cent, in Novem- 

 ber, but falling behind by 6 per cent, in August. As a result of 

 this increased development of plankton in the channel the pro- 

 duction in backwaters becomes relatively less with respect to 

 the channel production than in times of high water. Thus in 

 July-December, backwater plankton exceeds that in the chan- 

 nel in only 10 out of 19 monthly averages, or, omitting Spoon 

 River, in 8 out of 15 — a marked change from the excess in the 

 preceding six months, 21 out of 25. 



This is the only season in all our records in which the plank- 

 ton content of Spoon River rises above the barren level of .1 

 cm. 3 per m. 3 , or less. The production now rises to a level ap- 

 proximating, and in November-December exceeding, that in 

 the channel as a result of the practical absence of current and 

 consequent increase in the reservoir function of the stream. 



The course of production this year in the various backwa- 

 ters is in most instances strikingly similar to that in the chan- 

 nel in its major outlines. Thus in all of them (PL XXVIIL, 

 XXXII., XXXIV., XXXVIIL, and XLI.) production rises grad- 

 ually from the midwinter minimum to an unusually low ver- 

 nal pulse in April-May and declines again in June-July. At 

 this point collections were suspended in Dogfish and Flag lakes. 

 Production in channel waters rises again in August and con- 

 tinues at high levels till November, and in like manner and 

 with even greater amplitude in Thompson's Lake (PI. XXX- 

 VIIL), while a similar movement is initiated in Phelps Lake 

 (PI. XLI.), only to be stopped by the drying up of the lake in 



