Warmer-than-normal temperatures rapidly melt snow and ice on the upper Allegheny and Monongahela rivers, leading to a major flood in Pittsburgh.

The Allegheny River is a 325-mile-long (523 km) tributary of the Ohio River that is located in western Pennsylvania and New York in the United States. It 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 at Point State Park in Downtown Pittsburgh, Pennsylvania.

The Monongahela River, sometimes referred to locally as the Mon, is a 130-mile-long (210 km) river on the Allegheny Plateau in north-central West Virginia and Southwestern Pennsylvania. The river flows from the confluence of its west and east forks in north-central West Virginia northeasterly into southwestern Pennsylvania, then northerly to Pittsburgh and its confluence with the Allegheny River to form the Ohio River. The river includes a series of locks and dams that makes it navigable.

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. Patrick's Day flood, and also affected other areas of the Mid-Atlantic on both sides of the Eastern Continental Divide.