435 
by the low levels, the cutting off of breeding backwaters, and 
the relative occupancy by vegetation, and consequently a ver- 
nal pulse of unusual dimensions reaches a culmination in its 
area. 
The June-July pulse of the river, abnormal in its location 
and relative size and apparently without equivalent elsewhere, 
is due to the unusual development of a stagnation plankton in 
the sewage-laden river in a period when rising temperatures 
hasten the decay of its unusual load of organic matter. 
The causes above enumerated render this the year of great- 
est fertility, in so far as our records reveal production, in the 
river and in Thompson’s and Flag lakes, the next tothe greatest 
in Dogfish Lake, and the least in Quiver Lake. 
The low levels preclude any extensive impounding of flood 
waters and, moreover, the period of run-off is of shght extent. 
The rise in plankton content (PI. IX.) in the river following 
the April, July, and September floods is suggestive of the effect 
of impounding, the plankton content (cf. Pl XXVI., XXX., and 
XXXVI.) being generally greater in the discharging backwaters 
examined than in the recipient channel during these run-offs. 
At all other seasons in our records for this year there is scant 
opportunity for enrichment by tributary backwaters, and but 
little suggestion of it. 
1896. 
(PL Mee, SEVIS REXEL KK, RRXVM., XE) 
This is the most fully represented year in our series in the 
number of stations examined. There are 76 collections in the 
Illinois, 9 in Spoon River. and 31, 50, 27, 27, and 29 respectively 
in Quiver, Dogtish, Flag, Thompson’s, and Phelps lakes—a total 
of 229. It was a year of higher levels, averaging 6.98 ft. above 
low water,—almost twice the record of the preceding year—and 
witnessed a series of recurrent floods approaching or surpassing 
bank height of the stream. 
The accompanying table gives a summary of the data of 
production, 
