191 
five times as great as that in Spoon River (3.8), while that in 
Thompson’s Lake is more than three times the amount in 
Quiver Lake. The large amount of chlorine in the Illinois 
and in Thompson’s Lake—which draws its water supply mainly 
from the river—is due to contamination by the sewage of Chi- 
cago, Peoria, and other cities within the drainage basin. Quiver 
Lake receives water from the river only during flood periods, 
when the sewage is diluted, and at other seasons it contains 
more nearly the chlorine of the uncontaminated prairie stream. 
Its chlorine thus averages low (3.8). That of Spoon River runs 
higher (4.8), in part because of backwater from the main stream 
to the point of collection. The sewage systems discharging 
into this stream are few and but slightly developed, and its 
chlorine is correspondingly low. While it is true that the 
chlorine is not a precise measure of the amount of sewage or 
of the adventitious fertilizing material received by a stream, it 
is nevertheless significant that ratios of chlorine and plankton 
production not only trend in the same direction but are quanti- 
tatively somewhat similar when lake iscompared with lake and 
stream with stream. Thus in Quiver and Thompson’s lakes 
the ratio of their chlorine content is 1 to 3.4 while that of 
the plankton production is 1 to 4.2. The corresponding ratios 
in Spoon and Illinois rivers are 1 to 5.7 and 1 to 5. An increase 
in chlorine due to sewage or animal wastes seems thus to be 
accompanied by a proportionate increase in the plankton pro- 
duced. It is safe to infer that it is one of the factors producing 
the increase, but,as shown elsewhere in this paper, other fac- 
tors, such as vegetation and current, are also potent in produe- 
ing the contrasts in plankton production above noted. 
The oxygex consumed in oxidizing the organic matters af- 
fords an additional index of the quantity of these substances 
present in the water, but since all kinds of organic matter are 
not oxidized in the analysis it does not yield a criterion of 
the total quantity of organic matter. A comparison of the 
oxygen consumed in the four localities yields results very sim- 
ilar to those obtained by a comparison of loss on ignition, ex- 
