1909.] FRESH-WATER FAUNA IN PENNSYLVANIA. 99 



III. Sketch of the Present Condition of Our Rivers. 



(See map, plate VI.) 



I. The Ohio River Below Pittsburgh. 



At Pittsburgh, the two main rivers, Allegheny and Monongahela, 

 unite to form the Ohio. As we shall see below, both the Allegheny 

 and Monongahela are as badly polluted as they possibly could be, and, 

 consequently, it is not astonishing that the Ohio immediately below 

 Pittsburgh is also in a deplorable condition. In addition, it is 

 dammed up, this " improvement " extending down to dam No. 6 

 at Vanport (below Beaver) in Beaver County. Generally, there is 

 not much life in this part of the Ohio. Fishes are found occasion- 

 ally, during high water, due to some migration, probably from 

 farther down the river, but even this has been rendered difficult or 

 even impossible in consequence of the perfection of the dams (dam 

 No. 6 was finished and put in operation toward the end of 1907). 

 There are crawfishes in this part of the river, but they are disap- 

 pearing fast. Unionidse have disappeared long ago. There was a 

 colony of them in the left branch of the Ohio at Neville Island, 

 Allegheny County, up to 1904 ; during that year, however, they died 

 out, and in 1905 the last living one was found there. 



Farther down, below dam No. 6, conditions improve. This is a 

 very interesting and important fact. Although the Ohio collects 

 most of the polluted water of the western section of the state, and 

 although it is in a very bad condition below Pittsburgh, it loses its 

 bad qualities, at least in part, about thirty miles farther down. 

 Since there are only two important tributaries along this part of its 

 course, Chartiers Creek and Beaver River, both of them also badly 

 polluted, this improvement of the water cannot be due to dilution 

 alone, but it is evident that some of the injurious substances in the 

 water must be removed from it, and very probably by precipitation 

 upon the bottom of the river. We shall observe indications of this 

 process elsewhere, and shall discuss its significance below. Here it 

 is sufficient to point out, that at present (1908) the condition of the 

 Ohio below dam No. 6 is good or fair, life being not only possible, 

 but abundant in it, all the way down to the state line at Smith's 



