PAPERS ON ZOOLOGY 273 



the stream (as 200 to 1), causes little inconvenience to 

 the animal life and is doubtless of some benefit because 

 of the additional food material that is added (Forbes 

 and Eichardson, 1919, p. 146). But the streams seldom 

 remain long in this innoxious condition, the sewage be- 

 coming more and more concentrated and less diluted until 

 the whole stream may be supersaturated with noxious 

 substances, the amount of oxygen in saturation reduced, 

 and the biota finally driven out or killed. 



The Illinois River is one of the most striking examples 

 of the effect of sewage pollution on the life of a stream. 

 .Under the direction of Dr. S. A. Forbes, studies of this 

 river have been carried on for more than forty-two years 

 (since 1877) and a mass of reliable data has been gath- 

 ered. The opening of the Chicago drainage canal in 1890 

 produced most revolutionary changes in the life of the 

 nhnois Eiver by the discharge into it of the sewage of 

 Chicago, as well as commercial wastes from this city and 

 other places along the river (Forbes and Richardson, 

 1913, 1919). The effect of this sewage and other pollu- 

 tion has been to cause the animal life to be almost ex- 

 cluded from the upper parts of the river. That the pol- 

 luted condition is creeping down stream is shown by com- 

 parisons of collections made in 1911 with those made in 

 1918. In the earlier years a foul water fungus disap- 

 peared from the river near Starved Rock; in 1918 it 

 was found at Henry and Lacon, 35 and 41 miles farther 

 down the river (Forbes and Richardson, 1919, p. 145). 

 At the present time (1919) optimum conditions and a 

 normal ri^^er fauna are not encountered until Peoria is 

 reached, a distance of about 120 miles from the chief 

 source of pollution at Lockport. Sewage from the towns 

 and cities along the river also contribute to the general 

 septic condition and retard the natural purification that 

 occurs in all bodies of water. 



A striking example of the deadly effect of sewage pol- 

 lution on the mussel life of a stream is given by "Wilson 

 and Clark (1912, -p. 34) in their study of the Kankakee 

 River mussel fauna. ''The DesPlaines River, which joins 

 the Kankakee to form the Illinois River, is simply an^ 



