Stream Pollution 27 



pollution area has been measured in the Illinois River and reaches 

 seven miles or more annually. Such a figure indicates only the 

 significant and serious character of the change, since its precise 

 value depends on factors so numerous and variable that the record 

 obtained in one place cannot be used to measure the change in 

 another. 



The discussion just closed has shown that waters polluted by 

 domestic sewage purify themselves naturally and that under favor- 

 able conditions the process is carried out with remarkable rapidity, 

 but that when the amount of sewage added becomes very large in 

 proportion to the flow of the stream, the changes are greatly delayed 

 so that the pollution persists a long distance. It indicates that, with 

 increase in the volume of domestic wastes, a condition will be 

 reached where it is no longer possible to discharge the material 

 untreated and at the same time to maintain an aquatic environment 

 suitable for fish life and the propagation thereof. 



