229 
further factor of uncertainty not present, to alike degree at 
least, in the reservoir waters of the lake. 
That natural waters in this locality are not subject to the 
presence of chlorine (sewage) in such excess or in such fluctu- 
ating amounts appears on contrast of the chlorine curves of 
Spoon River (Pl. XLVI. and XLVII.) with those of the Illinois 
(Pl. XLITI.-XLV.). In the former, barring a few instances of 
apparent contamination by river water (October, 1897, January 
and February, 1899), the chlorine runs uniformly low through- 
out the year, dropping but a trifle with rising floods. 
In Quiver Lake (Pl. XLVIII. and XLIX.) the chlorine 
similarly runs low during the period of individuality of the 
lake, that is, of low water. The increase in chlorine comes 
only at times of invasion of flood water from the river or the 
bottom-lands above, as, for example, in November and Decemes 
ber, 1898 (Pl. XLIX.). The periods of fertilization of this lake 
by sewage thus depend upon floods, and occur at times of 
greatest dilution. 
In Thompson’s Lake (PI. L.) the chlorine (sewage) content 
exhibits the same general tendencies found in the river, from 
which its water supply is derived. The chlorine content runs 
high during low water and drops with the rise of the flood. 
The abrupt and numerous fluctuations of the chlorine of the 
river do not, however, appear in the lake, being diminished and 
equalized by its greater permanency. Even under these favor- 
able conditions it is difficult to find any constant or well- 
defined correlation between the chlorine pulses and those of the 
plankton. It may be that the fertilizing elements of the sew- 
age which the chlorine is regarded as representing have already 
been exhausted, so that the chlorine curve no longer represents 
a commensurate fluctuation in the fertility. In a few cases, as, 
for example, in December, 1897, and in January and September, 
1898 (Pl. L.), a slight correlation in the chlorine and plankton 
curves appears, though the only relation between the two may 
lie in the effect of changing river levels upon both, a declining 
