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, Nov. 2 and 30, and Dec. 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 conditions in 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 cm. 3 ) 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. ; PI. 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, whilethat 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. 3 of plankton in the Illinois and .79 

 the week prior in Thompson's Lake, three miles above Spoon 

 River (PI. 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- 



