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) anda 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 was 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 which 
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 
