200 



waters a volume of sewage equal to the pumpage, though their 

 imperfectly developed systems of drainage, combined with the 

 surface run-off, carry some sewage to the stream. 



Population and Pumpage in Cities without Sewage Systems. 



The principal sources of the sewage contributed to the 

 Illinois River above Havana are Chicago, Peoria, and the 

 smaller cities within the drainage basin. The amounts 

 contributed by each are approximately 520,275,109, 5,000,000, 

 and 16,007,378 gallons respectively per day. The total amount 

 of 542,282,487 gallons per day or 838.7 cubic feet per second is 

 about 8 per cent, of the average flow of the river at Havana and 

 exceeds by 40 per cent, the estimated low-water flow at Cop- 

 peras Creek dam, eighteen miles above our plankton station. 

 In 1890 the population of the two larger cities and the total of 

 the remaining smaller ones was respectively 849,850, 41,024, 

 and 188,817, a total of 1,079,691, 250,000 having been deducted 



