The river joined by the Allegheny and Monongahela rivers at Pittsburgh is the Ohio River.
The Allegheny flows southwest from northern Pennsylvania, while the Monongahela flows northward from West Virginia and southwestern Pennsylvania. They meet at Pittsburgh’s celebrated “Point,” now Point State Park, where their combined flow becomes the Ohio River.
From Pittsburgh, the Ohio travels generally southwest through the central United States before joining the Mississippi River at Cairo, Illinois. Its watershed covers a large portion of the eastern Mississippi drainage basin and has been an important route for settlement, commerce, industry, and navigation.
The confluence also explains Pittsburgh’s nickname, the “City of Three Rivers”: the Allegheny and Monongahela are the two upstream rivers, while the Ohio begins at their meeting point. A related hydrological curiosity is that the Ohio carries more water than the Mississippi immediately above their confluence, even though the Mississippi is treated as the main river by convention.