340 



late spring and high levels with rapid current, result in but 

 slight plankton production in channel waters. The pulse is 

 barely perceptible, and its amplitude is very slight. The 

 monthly average is but .28 cm.^, a production somewhat less 

 than that in 1898 (.33) and 1897 (.38), though exceeding that 

 of 1896 (.07). 



In comparison with other seasons these three months of 

 1899 exhibit a greater production, reaching even tenfold, and 

 this result is correlated with the freedom from stagnation and 

 the gradual change in river levels in the first two months. 



COMPARISON WITH TRIBUTARIES AND BACKWATERS. 



STATION M, SPOON RIVER. 



(Tables IV., XI., XIV. ; PI. I., II., XIV., XXII., XXIV., XLVI.. XLVII.) 



ENVIRONMENTAL CONDITIONS. 



This is a tributary on the right bank, draining 1,870 square 

 miles of fertile prairie, and entering the Illinois about a mile 

 and a half below our plankton station (PI. I. and II.). No 

 large cities lie in its water-shed, so that its pollution by sewage 

 is not excessive. Its waters therefore represent the normal 

 run-off of the central water-shed, and are typical of the tributa- 

 ry waters received by the Illinois below La Salle. A study of 

 plankton content and chemical conditions will accordingly 

 throw light upon the relations existing between channel and 

 normal tributary streams in general in the matter of plankton 

 production. 



Our station at which collections were made in Spoon 

 River was located immediately below the abandoned trestle of 

 the Chicago, Peoria, and St. Louis Railroad (PI. XIV. ), less than 

 forty rods from the mouth of the stream. A blockade formed 

 by a raft of driftwood prevented further progress up stream 

 during a part of the time, and in the winter the ice which 

 formed and continued in the tributary when at times the main 

 stream was open, made approach, even to the bridge, difficult. 



Owing, however, to the current, our collections, with one 



