The most momentous disaster to afflict the people of the Alleghenies was the Johnstown Flood — locally known as the " Great Flood of 1889 " — which occurred on May 31 of that year after the catastrophic failure of the South Fork Dam on the Little Conemaugh River upstream of the town of Johnstown , Pennsylvania . The dam broke after several days of extremely heavy rainfall , unleashing 20 million tons of water ( 18 million cubic meters ) from the reservoir known as Lake Conemaugh . ( This body of water had been built as part of the Main Line of Public Works , then abandoned . ) With a flow rate that temporarily equalled that of the Mississippi River , the flood killed 2,209 people and caused US$ 17 million of damage ( about $ 425 million in 2012 dollars ) . The American Red Cross , led by Clara Barton and with 50 volunteers , undertook a major disaster relief effort . Support for victims came from all over the United States and 18 foreign countries . After the flood , survivors suffered a series of legal defeats in their attempts to recover damages from the dam 's owners . Public indignation at that failure prompted the development in American law changing a fault-based regime to strict liability
Johnstown Flood
Johnstown Flood
Allegheny Mountains