No. 631] ANIMAL LIFE AND SEWAGE 



157 



Few data are at hand indicating how far the polluted 

 water must flow before it can purify itself enough to be- 

 come favorable for animal life. In the Illinois River, life 

 is being affected by the Chicago sewage at Peoria, a dis- 

 tance of 110 miles from the source of infection. A recent 

 study of the Salt Fork of the Vermilion River, into which 

 the sewage of Champaign and TJrbana is discharged, in- 

 dicated that the polluted water was inimical to molluscan 

 life for a distance of 14 miles in which no living mollusks 

 were found, and one must pass down the stream for a 

 distance of nearly twenty miles before a normal mussel 

 fauna can be found. 



In the Allegheny River, Ortmann found that whole 

 stretches of the stream and some of its tributaries had 

 been made into a desert by pollution, principally in the 

 form of chemicals from the numerous mines situated in 

 this part of the State. Ortmann remarks that ' 'with re- 

 gard to the animal life in our rivers, sewage does not 

 seem to be harmful; on the contrary, certain forms 

 (fishes, crawfishes, mussels) seem to thrive on it" (p. 97). 

 This is probably true in a case where water is but slightly 

 contaminated ; but in streams where pollution by sewage 

 is greatly concentrated (a condition reached sooner or 

 later in all streams used for sewage disposal) it is cer- 

 tainly inimical to the forms of life mentioned. It is very 

 true that a stream polluted by chemicals soon becomes 

 x destitute of the larger forms of animal life (if, indeed, not 

 all life) and in such waters the return of life will be very 

 slow and in many cases it may be impossible for life to 

 return on account of the chemicals which cover the bot- 

 tom and shores. 



In the case of the Genesee River, we have a striking 

 example of the history of a polluted stream and its effect 

 on the animal life. Previous to the discharge of sewage 

 into the stream there was a varied molluscan fauna very 

 numerous in individuals. In the course of eleven years 

 the gill-bearing mollusks were forced out and after a 

 lapse of fourteen years all molluscan life ceased to live 



