68 ILLINOIS BIOLOGICAL MONOGRAPHS [166 



tries, and chemical and other factories that border these rivers. In the 

 Susquehanna River the same condition prevails in many places (Leighton, 

 1904). Such pollution causes a complete extermination of the fauna 

 (and largely of the flora) and leaves the streams in such condition that 

 restocking by either natural or artificial means is practically impossible. 



Pollution by sewage, when the polluting material is of small percentage 

 as compared with the pure water of 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 Richardson, 1919: 

 146). But the streams seldom remain long in this innoxious condition, the 

 sewage becoming more and more concentrated 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 gathered. The open- 

 ing of the Chicago Drainage Canal in 1890 produced most revolutionary 

 changes in the life of the Illinois river, 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 pollution has been to cause the animal life to be almost 

 excluded from the upper parts of the river. That the polluted condition 

 is creeping down stream is shown by comparisons of collections made in 

 1911 with those made in 1918. In the earlier years a foul-water fungus 

 disappeared from the river near Starved Rock; in 1918 it v/as found at 

 Henry and Lacon, 35 and 41 miles farther down the river (Forbes and 

 Richardson, 1919:145). At the present time (1919) optimum conditions 

 and a normal river 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. 



A striking example of the deadly effect of sewage pollution on the 

 mussel life of a stream is given by Wilson and Clark (1912:34) in their 

 study of the Kankakee River mussel fauna. "The DesPlaines River, 

 which joins the Kankakee to form the Illinois River, is simply an immense 

 sewer bringing down the Chicago sewage. Both rivers, but especially the 

 DesPlaines, are full of the characteristic algae and other vegetation whicli 

 grow in such waters^ and the combination of a copious vegetation with the 

 sewage has effectually killed off all the mussels in the vicinity. Not a 

 single living specimen could be found in either river; but there were hun- 

 dreds of dead shells along the banks, most of these old and well bleached, 

 but still capable of identification." This statement, of course, applies only 



