211 
graphs on Pl. XLVIII. and XLIX.) the lake receives in addition 
to the drainage of its own catchment-basin some access of 
flood waters from the bottom-lands above and from the adja- 
cent river. The water along the eastern shore, even in flood 
conditions, is “lake” rather than river water, as a comparison 
of the plottings of the analyses of water from the two sources 
clearly demonstrates. Our collections of plankton and water 
samples were taken within or near this belt of lake water, in 
which contamination by flood waters was not usually noticea- 
ble. Compare in this connection the chlorine curve of the 
river and lake (Pl. XLV. and XLIX.). To some slight extent, 
then, the analyses pertain to two sources: to the waters of 
overflow, largely belonging to the colder months and period of 
the maximum of nitrogenous substances ; and to the waters of 
a spring-fed lake, delimited during the period of low water and 
of the minimum of nitrogenous substances. The data at hand 
do not cover low-water conditions during a “maximum ”’ period, 
which might give evidence of a seasonal cycle in chemical 
conditions in this lake independent of the river overflow. From 
conditions elsewhere it seems probable that such a cycle does 
occur here also, though the overflow and probable contamina- 
tion may serve here to heighten somewhat the contrast be- 
tween the maximum and minimum periods of the seasonal 
cycle. 
In the autumn months of 1896 and 1897 covered by the 
analyses, the rise in nitrates only is indicated (Pl. XLVIII.), the 
summer minimum continuing through the low-water period 
of autumn. 
In 1898 and the first three months of 1899 (Pl. XLIX.) the 
period of maximum, November to May, is well distinguished 
from that of the minimum, May to November, and not only in 
the nitrates but to some extent also in all of the other substan- 
ces, appearing most clearly in the free and albuminoid ammonia 
and the organic nitrogen. 
As in the Illinois River, so here also the spring maximum 
of the plankton (Pl. XLIX.) comes at the close of the period 
