1909.] FRESH-WATER FAUNA IN PENNSYLVANIA. 109 



sion by way of restocking our rivers with game and food fishes, this 

 is a useless undertaking in all those streams which are polluted. 

 Any fishes set free in such waters will not live, or will not stay 

 there, if they can. The other suggestion is furnished by the fact, 

 repeatedly mentioned above, that a river, badly polluted at a certain 

 point, improves in its further course, provided no additional pollu- 

 tion in great quantities is reaching it.^* This is seen first of all in 

 the Ohio itself in Beaver County, and further in the Allegheny in 

 Armstrong County. Additional examples are Slipperyrock Creek, 

 Mahoning River (Lawrence County), Raccoon Creek, Brokenstraw 

 Creek, Cheat River. This improvement of the waters, of course, is 

 partly due to the dilution of the injurious substances by the addition 

 of clear water from tributaries. But it seems as if this is not the 

 only source of the improvement. In the case of the Allegheny in 

 Armstrong County, the main tributaries (Clarion, Red Bank, Ma- 

 honing) themselves are polluted, and the other tributaries are very 

 insignificant in comparison with the size of the Allegheny. This is 

 also seen in the Mahoning River in Lawrence County, which hardly 

 has any tributaries along its course, where the improvement takes 

 place. I think the precipitation of the injurious substances to the 

 bottom plays an important part here. We always have, in polluted 

 streams, some sort of precipitate upon the bottom, most noticeable 

 in streams charged with mine water, where it consists of sulphate 

 of iron,^^ and, consequently, the injurious element must be elimi- 

 nated, at least to some degree, from the water. This observation 

 suggests a natural remedy — if we could prevent the water charged 

 with polluting substances from reaching our streams directly, that 

 is to say, if we could arrange it that this water is kept in basins or 

 reservoirs for some time, till it has gone through this natural clearing 

 process, and if we allowed only the overflow of these clearing basins 

 to reach our rivers, that is to say, the most superficial strata, which 

 contain the smallest amount of polluting substances," I think this 



"See Stabler, Water Supply and Irrigation Paper no. i86, 1906, p. 28. 



" See Leighton, /. c, p. 24. 



" Of course, the oil from the oil wells floats on the surface, but this 

 floating oil does not do much damage. It is well known that before the 

 discovery of oil in these parts, the Allegheny was famous for the oil floating 

 upon its surface. 



