A flash flood hits Johnstown, Pennsylvania, on this day in 1977, killing 84 people and causing millions of dollars in damages. This flood came 88 years after the infamous Great Flood of 1889 that killed more than 2,000 people in Johnstown. As they had in the first flood, the dams in the Conemaugh Valley failed, bringing disaster to the town.

The flood occurred when an extraordinary amount of rain came down in the Conemaugh Valley in a short period of time. Nearly 12 inches were measured in 10 hours. The National Weather Service later estimated that this amount of rain in that location should happen less than once every 1,000 years.

The largest dam that burst was at Laurel Run. This 10-year-old earthen dam held back 100 million gallons of water. Despite having a 42-foot-high spillway, the dam failed and the resulting flood devastated the town of Tanneryville. Five other dams in the area also burst, releasing another 30 million gallons of water over the landscape.

The failure of the dams came as a big surprise. Johnstown had constructed an entire system designed to completely eliminate the flood risk. In addition, regular inspections had turned up no defects. Still, the dams were no match for the thunderstorm that stalled over the area on July 20.

In addition to the 84 people who lost their lives to the flood, $300 million in damages were suffered and hundreds of people lost their homes. President Carter declared the region a federal disaster area and the National Guard was sent to assist in the relief efforts. Despite millions spent to rehabilitate the Johnstown area, the economy never recovered. The city’s population decreased nearly 15 percent in the aftermath of the flood, as people moved away to rebuild their lives elsewhere.