414 
also years of higher water, of greater uniformity of environment 
—hecause of greater extent of open water, of greater interchange 
of water in overflow stages, and therefore of greater agreement 
in the course of plankton production. The similarity in the 
course of plankton production in different bodies of water is in 
a large measure a function of the similarity of their environ- 
ment and the resemblance of their planktons in the matter of 
constituent organisms. 
STATION F, PHELPS LAKE. 
(Table 1X.; Pl. XXI., XL.-XLIL.) 
ENVIRONMENTAL CONDITIONS. 
This body of water lies on the western side of the river about 
a mile below the city of Havana, in the elevated bottom-lands 
below the mouth of Spoon River. It trends northeast and 
southwest for a distance of seven eighths of a mile, has a width 
of 400-690 feet, and a total area of 50-60 acres. Its bottom lies 
about 6.5 feet above low-water mark, and the greatest depth re- 
corded in it at high water at the point of collection was only 
10 ft. It is but slightly deeper toward the lower end. Its out- 
let is by a tortuous slough choked with driftwood, which runs 
for two fifths of a mile ina southerly direction to the river. 
The elevation of the bottom of this slough at its entrance to 
the river is 8-9 ft. above low-water mark, so that all run-off 
from the river drained hy this slough ceases when it drains to 
this level, and it is not reinvaded by floods below this elevation. 
When the river falls below the level of the outlet and the lake 
drains as fully as the outlet permits, there still remain about 
1.5 ft. of water from whick no further run-off occurs. The vol- 
ume is then slowly reduced by evaporation or increased by sum- 
mer rains. 
The lake is not fed by springs or tributaries of any sort be- 
yond seepage from the level alluvial bottoms in which it lies, 
and which nowhere in the vicinity rise more than 10 feet above 
its bottom and generally very much less than this distance. At 
river stages of 11 ft. and above, backwater from Spoon River 
