Rivers have influenced civilizations since the dawn of time. For instance, without the 4,130-mile-long Nile River, perhaps the Egyptian Civilisation, with its celebrated pyramids, would not have arisen. In the United States, few rivers have shaped the country's history as the Allegheny River, the third longest river in Pennsylvania. This 321 miles long river is one of the country's national wild and scenic rivers, flowing through Pennsylvania and New York.
The Course Of The Allegheny River

The source of the Ohio River (formed at the confluence of the Allegheny and Monongahela Rivers at “The Point” in Point State Park, Pittsburgh, Pennsylvania. Turning southwest, Pennsylvania's third-longest river zigzags for 120 miles, a journey involving a southeast turn around Kennerdel in Rockland Township, another southwest turn around Templeton in Pine Township, before eventually joining the Monongahela River at Pittsburgh to form the largest river in the Keystone State: The Ohio River. By the time it pours its waters into "La Belle Riviere," as the French called the Ohio River, Allegheny Rivers has traveled a distance of about 321 miles, a distance that places it an impressive third in the state after the Susquehanna River and the Ohio River.
Role In America's Colonial History
While the Mississippi River, the second-longest river in the United States, is bigger than the Allegheny, it is too far to the west to have played an impactful role in the early history of the United States. And while the Hudson River and the Potomac were crucial, their influence never stretched to America's heartland. On the other hand, the Ohio River would be nothing without Allegheny feeding into it. Indeed, the Allegheny River may not be among the ten longest rivers in the United States, but its history is more intertwined with that of the United States than many of its celebrated peers.After the Revolution, for instance, the Allegheny River would quickly emerge as the primary passage of adventuring immigrants heading to the Wild West. No wonder Pittsburg, where the Allegheny and Monongahela Rivers unite to form the Ohio River, would earn the title "Gateway to the West."
Formation

Geologists believe that what brought the three Alleghenys together was the Ice Age that took place during the Illinoian Period between 350,000 and 550,000 years ago. Ostensibly, huge walls of ice up to a mile thick descended on the western sides of New York, blocked the northward flow of Lower Allegheny, and created huge lakes that would then press for outlets. The glaciers forced Lower Allegheny and Middle Allegheny to reverse course and join each other, while the Upper Allegheny would bend and join the Middle Allegheny near Salamanca, New York. According to geologists, the glacier theory explains the glacial sediments on the islands found throughout Allegheny.
Plant And Animal Life

Then again, French Creek, one of Allegheny River's tributaries, is among the most biologically diverse watersheds in the state. For instance, the creek is home to four federally endangered freshwater mussel species in addition to more than 80 species of fish. Lastly, Erie National Wildlife Refuge and a number of Audubon Society Important Bird Areas constitute part of the French Creek watershed.