347 
relation to environmental factors that was found in the case 
of the Illinois. This is seen in the increased winter produc- 
tion, in the vernal rise, in the decline after the vernal pulse, 
and in the unusual autumnal development. The tributary 
stream, with but four exceptions, was acting as a diluent of 
channel plankton at each examination of its plankton content. 
These four exceptions—on Feb. 26, Noy. 2 and 30, and Dee. 28— 
are due in the first instance to channel flood, and in the last 
three cases to exceptionally low water in the tributary and less 
stable chemical conditionsin the channel. In the four years in 
which Spoon River was examined they are the only exceptions 
to the general rule that these tributary waters are dilu- 
ents of the channel plankton. The average production for 1897 
(1.257 em.*) is 180 times that recorded in the last half of 1896, 
and 43 times that for 1898—as a result of the low-water con- 
ditions discussed above. 
1898-1899. 
(Tables IV., XI.; Pl. XXIV., XLVII.) 
There are 14 collections at intervals of four or five weeks 
in the 15 months included in this period, and they fairly rep- 
resent the contributions of this tributary in a year of consider- 
able flood and repeated access of storm water. In 1898 there 
is but a trace of plankton in the January (.017) and February 
(.016) collections, while that in the March collection (.124) isthe 
maximum for the year. At this time the spring flood is nearly 
at its height (16.5 ft.), and the waters of Spoon River are in quite 
. free connection with the general overflow that spreads over 
the surrounding bottom-lands. On the day of the Spoon River 
collection there was .43 cm.’ of plankton in the Illinois and .79 
the week prior in Thompson’s Lake, three miles above Spoon 
River (Pl. II.). There is thus three and a half times as much 
plankton in the main stream and six times as much in Thomp- 
son’s Lake. With its maximum burden of plankton, the trib- 
utary is still a diluent, and its plankton content at this time is 
probably in large part derived from the run-off of the contigu- 
