424 
and averages 7.3. In three instances, March 3 and 1, (.01 and 
02), March 29 (.20 and .48), and April 26 (10.72 and 15.81) the 
lake contains less than the river. All of these instances fall at 
times of high levels, exceeding 11 ft., when Spoon River floods 
invade this territory, and this deficiency in Phelps Lake is doubt- 
less due to their diluent effect. Since our station for collections 
was located in the upper end of the lake (PI. II.), the full effect 
of the flood would be detected at this point, but would be di- 
minished by mingling with the lake waters and the adjacent 
impounded backwaters before it joined the channel. The first 
of these exceptions, on March 8, isnot accompanied by increased 
turbidity (.45) in the lake (Table IX.), but the other two, March 
29 and April 26, are attended by a marked rise in turbidity (.05 
and .16). During this period of maximum spring flood in Mareh 
and April, owing doubtless to this diluent action of Spoon River, 
the run-off from this area, as indicated by plankton content at 
the upper end of the lake, dilutes, or but slightly enriches, the 
channel plankton. This appears in the monthy averages (table 
following p. 342), which for March are .33 cm.’ for the river and 
but .25 for the lake. In April they are 4.4 and 5.6 respectively. 
In later months, during the declines of the spring flood, and 
owing to absence of the flushing action of Spoon River floods 
and to the rise in impounding function with decline in levels 
and delimitation of the lake, we find a rapid rise in the relative 
plankton content in lake waters. The production in coincident 
collections is greater in the lake than in the river by 3- to 15- 
fold, and the monthly averages for lake and river respectively 
rise to 40.44 cm.’ per m.’ and 11.80 in May; to 27.67 and 3.96 in 
June; and to 6.97 and .58 in July; that is, the production is 
from 3+-- to 12- fold greater at this season in the lake than in 
the river. During the run-off in these months this lake and its 
contributing adjacent bottom-lands serve to increase, in 
some unknown ratio dependent on their relative volumes, 
the plankton content of the channel waters with which they 
mingle. 
Although the frequency of the plankton collections is in- 
