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. 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.

On March 16, 1936, warmer-than-normal temperatures led to the melting of snow and ice on the upper Allegheny and Monongahela rivers. These rivers and their tributaries were already over their banks and were threatening the city of Pittsburgh. On March 17, 1936 the waters reached flood stage of 25 feet. Heavy rains overnight caused the waters to rise quickly, and on March 18, the water peaked at about 46 feet, 21 feet above flood stage. Five days later, on March 21, the water finally receded to 24 feet.

The aftermath to the city was devastating. About 100,000 buildings were destroyed and the damage was estimated at about $250 million (or, roughly, $3 billion in 2006 dollars). Steel mills that were located around the three rivers suffered devastating damage and 60,000 steel workers within a thirty mile radius were out of work due to the damage that the mills suffered. Sixty five percent of the downtown business district had been under water from the Point all the way up to Grant Street.

http://en.wikipedia.org/wiki/Pittsburgh_Flood_of_1936

View video of the clean up of from the Flood from archive.org

Read articles about the flood (below the video)