On March 17 and 18, 1936, the city of Pittsburgh, Pennsylvania, witnessed the worst flood in its history when flood levels peaked at 46 feet (14 m). This flood became known as The Great St. Patricks Day flood, and also affected other areas of the Mid-Atlantic on both sides of the Eastern Continental Divide.

The Allegheny River ( AL-ə-GAE-nee) is a 325-mile (523 km) long headwater stream of the Ohio River in western Pennsylvania and New York, United States. The Allegheny River runs from its headwaters just below the middle of Pennsylvania's northern border northwesterly into New York then in a zigzag southwesterly across the border and through Western Pennsylvania to join the Monongahela River at the Forks of the Ohio on the "Point" of Point State Park in Downtown Pittsburgh, Pennsylvania. The Allegheny River is, by volume, the main headstream of both the Ohio and Mississippi Rivers. Historically, the Allegheny was considered to be the upper Ohio River by both Native Americans and European settlers.

The shallow river has been made navigable upstream from Pittsburgh to East Brady by a series of locks and dams constructed in the early 20th century. A 24-mile long portion of the upper river in Warren and McKean counties of Pennsylvania and Cattaraugus County in New York is the Allegheny Reservoir, created by the erection of the Kinzua Dam in 1965 for flood control.

The name of the river comes from one of a number of Delaware/Unami phrases which are homophones of the English name, with varying translations.