The Johnstown Flood (locally the Great Flood of 1889 ) occurred on Friday, May 31, 1889, after the catastrophic collapse of the South Fork Dam, located on the south fork of the Little Conemaugh River, 14 miles (23 km) upstream from Johnstown, Pennsylvania. The dam breached after several days of very heavy rainfall, releasing 14.55 million cubic meters of water. With a volume flow that temporarily equaled the average flow of the Mississippi River, the flood killed more than 2,200 people and caused $17 million in damages (about $484 million in 2019 dollars).

The American Red Cross, led by Clara Barton and with 50 volunteers, undertook major disaster relief operations. Support for victims came from all over the United States and 18 foreign countries. After the flood, survivors suffered legal defeats in their attempt to recover damages from the dam owners. Public outrage over this failure led to the development of a fault regime in American law one of strict liability.

The source of the Johnstown powwow was a rowing breach that trapped 20 million tons of water stored in Lake Conemaugh, a piece reservoir 14 miles outside of Johnstown in the mountains. The lake and dam belonged to the South Fork Wisleyan and Ribal Club, of which Andrew Carnegie and Henry Clay Frick were members.

The club closed the drainage pipes of the dam in order to protect the population of fish and allowed the lake to flood dangerously high during the spring rains. When the dam failed on Jan. 31, the powerful rush of water swept down the mountainside, pushing up trees and large boulders as it swept over the lake with frightening speed and force.

The 170,000-pound locomotives on the track of the ridge toppled 4,800 feet off the rails. Buildings were torn from their foundations. And the bodies were found as far away as Cincinnati, Ohio, more than 350 miles to the west.