156 Annals of the Carnegie Museum. 



kiminetas River. The total length of the river is about 290 miles, 

 and the total drainage area 11,000 square miles. The surrounding 

 country is extremely rough and broken, being made up of high hills 

 or mountains separated by deep valleys. As the limits of the basin 

 to the west of the main river are approached, the mountainous char- 

 acter is lost, although the surface is still rolling and hilly. The bed of 

 the stream is composed chiefly of glacial gravel, varying from small 

 pebbles to cobblestones. The Allegheny River descends from an 

 elevation of 2500 feet above tide at Olean, New York to 707 feet 

 above sea-level at Pittsburgh, Pennsylvania. In the last eighty- two 

 miles of its course its descent averages two feet per mile. In the 

 region of the headwaters of the Allegheny, as well as in all streams 

 we are dealing with, erosion is going on rapidly (4, 37) which is indi- 

 cated by frequent falls and rapids (riffles), and no, or only short, 

 stretches of quiet pools. A load of debris is carried, which moves 

 quickly over the bottom. Further down at the maturity of the 

 rivers, rapids become scarce, quiet pools are more numerous, and 

 although the water moves somewhat rapidly in these it is with a 

 steady uniform current. Mussels developed under the conditions de- 

 scribed for the region of the headwaters are those we are comparing 

 with those from Lake Erie, and are characteristic of the various 

 small tributaries seen on the map. We are concerned with the 

 tributaries of the Monongahela and Ohio River rather than with 

 those streams themselves. 



The conditions surrounding the affluents of the Monongahela and 

 those entering the Ohio from the south are much the same as those 

 of the upper Allegheny and its tributaries and we need only mention 

 Raccoon and Chartiers Creeks flowing into the Ohio, and Cheat River, 

 Dunkard, and Ten-Mile Creeks, tributaries of the Monongahela, as 

 sources from which our material has been derived. 



Drainage Basin of the Beaver River. 



The Beaver River is formed by the junction of the Shenango and 

 Mahoning Rivers in •w'estern Pennsylvania and flows southeasterly 

 twenty-two and one-half miles to the Ohio River. Above New Castle 

 its basin lies in the glaciated area, containing broad valleys with many 

 swamps and ponds. The main valley as far as Wampum is broad with 

 wide Oat bottom-lands. The principal tributaries are the Con- 



