At 4:07 PM on May 31, 1889, more than 20 million tons of water, carrying virtually everything that had once stood in the Conemaugh Valley, struck Johnstown, Pennsylvania. More than 2,200 people died in the flood or, even more horribly, trapped in the fire that flashed through the wreckage of the town that had collected at a stone railroad bridge.
On May 28, 1889, a storm formed over Nebraska and Kansas, moving east. When the storm struck the Johnstown-South Fork area two days later it was the worst downpour that had ever been recorded in that section of the country. The U.S. Army Signal Corps estimated that 6 to 10 inches (150 to 250 mm) of rain fell in 24 hours over the entire section. During the night small creeks became roaring torrents, ripping out trees and debris. Telegraph lines were downed and rail lines were washed out. Before daybreak the Conemaugh River that ran through Johnstown was about to leave its banks.
During the day, the situation worsened as water rose in the streets of Johnstown. Then, in the middle of the afternoon of May 31st, the South Fork Dam, 14 miles (23 km) upstream, burst, allowing the 20 million tons of Lake Conemaugh to cascade down the Little Conemaugh River. On its way downstream towards Johnstown, the crest picked up debris, such as trees, houses, and animals. Occasionally this debris formed a temporary dam at narrow parts of the canyon, which caused water to build up behind this dam before breaking through. Because of this, the force of the surge would gain strength periodically, resulting in a stronger force hitting Johnstown than otherwise would be expected. Just before hitting the main part of the city, the flood surge hit the Cambria Iron Works, taking with it railroad cars and barbed wire.
The inhabitants of the town of Johnstown were caught by surprise as the wall of water and debris bore down on the village, traveling at 40 miles per hour (64 km/h) and reaching a height of 60 feet (18 m) in places. Some, realizing the danger, tried to escape, but most people were hit by the surging floodwater. Many people were crushed by pieces of debris, and others became caught in barbed wire from the wire factory upstream. Those who sought safety in attics, or managed to stay aloft of the flood water on pieces of floating debris, waited hours for help to arrive.
At Johnstown, 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 80 people. The fire at the Stone Bridge burned for three days. Afterwards, the pile of debris there covered 30 acres (120,000 m²). As of 2007, the Stone Bridge is still standing, and is often portrayed as one of the images of the flood.
The National Park Service has a teacher's guide that has a number of maps and photos as well as some eyewitness accounts.
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 aftermath of the Johnstown Flood was the first time the fledgling American Red Cross ever performed their now-familiar disaster relief efforts.