On May 31, 1889, the greatest disaster in Pennsylvania history occurred when the South Fork Dam on the Little Conemaugh River burst. The waters of Lake Conemaugh went roaring through the valley, causing significant damage to the communities downstream and resulting in the death of over 2,200 people, making it, at the time, the largest loss of life in a single event in US history.
Source: Explore Johnstown’s legacy and the 1889 flood that changed Pennsylvania forever | PennLive.com