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 is compared 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 produc- 

 ing the contrasts in plankton production above noted. 



The oxygen 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- 



