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 em.*, 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.; Pl. 1., IL, XIV., XXII., XXIV., XLVI., XLVIL.) 
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 (Pl. 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 ofthe tributa- 
ry watersreceived 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 (Pl. 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 
