High in the mountains, near the small town of South Fork, the South Fork Dam was originally built between 1838 and 1853 by the Commonwealth of Pennsylvania as part of the canal system to be used as a reservoir for the canal basin in Johnstown. It was abandoned by the commonwealth, sold to the Pennsylvania Railroad, and sold again to private interests.

On May 31, 1889, the dam broke in causing a flood in which 2,209 people died.

When the South Fork Dam (elevation 1,650 feet) was breached, the lake waters followed their natural course downhill along the river, growing stronger and more destructive as the flood waters picked up and carried along everything in their path. The first town struck was South Fork, two miles downstream. The flood claimed its first four victims and 20 to 30 homes were destroyed.

The wave headed toward East Conemaugh. A witness said the water by now was almost obscured by the debris, resembling "a huge hill rolling over and over,"tossing up logs high above its surface. Before the flood hit East Conemaugh, train engineer John Hess tried to warn the residents by tying his train whistle down and racing toward town ahead of the wave. His warning saved many, but 50 people died, including about 25 passengers on trains that had been stranded in the town by earlier flooding caused by the rain.

The Stone Bridge, which was a substantial arched structure, carried the Pennsylvania Railroad across the Conemaugh River. The debris that was carried by the flood formed a temporary dam, stopping further progress of the water. The flood surge bounced upstream along the Stoney Creek River.

Eventually, gravity caused the surge to return to the dam, causing a second wave to hit the city, but from a different direction. Some people who had been washed downstream became trapped in an inferno as debris that had piled up against the Stone Bridge caught fire, killing at least 80 people.