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South Fork Dam
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Vital Statistics:
Location: Johnstown, Pennsylvania, USA
Completion Date: 1852
Cost: $166,647
Reservoir Capacity: 2.7 million cubic feet
Type: Embankment
Purpose: Water supply for canal, recreation
Reservoir: Lake Conemaugh
Misc: collapsed 1889
Materials: Rock, clay
Engineer(s): Sylvester Welsh

On the afternoon of May 31, 1889, a private dam in western Pennsylvania burst, sending 20 million gallons of water and debris into the unsuspecting town of Johnstown with the force of a tidal wave. The catastrophe killed 2,209 people, left thousands homeless, and transformed the prospering city of Johnstown into a virtual wasteland.

South Fork Dam
Click photo
for larger image.

Before it burst, South Fork Dam held back Lake Conemaugh, the pleasure lake of the South Fork Fishing and Hunting Club. It was an embankment dam made of clay, boulders, and dirt. Through the years, the spillway became clogged with trees and other floating debris. When it started raining on Memorial Day in 1889, the lake swelled and seeped over the top of the structure. The earth-and-rock structure collapsed, releasing a thunderous wave 40 feet high and half a mile wide into the valley. Water slammed into Johnstown with the force of Niagara Falls. It carried huge amounts of debris, including houses, barns, animals, and people. The wave destroyed the city in 10 minutes.

The South Fork Dam collapsed because the spillway was poorly maintained. Today, large dams and their spillways are inspected frequently by qualified engineers.

Here's how this dam stacks up against some of the biggest dams in the world.
(reservoir capacity, in cubic feet)

Chart showing the relative size of the biggest dams in the world South Fork Dam
2.7 million cubic feet

Fast Facts:
  • The average speed of the wave on its trip to Johnstown was 40 miles per hour.
  • The wave was 35 to 40 feet high at its crest as it hit Johnstown.
  • The volume of water that destroyed Johnstown was equal to the volume that goes over Niagara Falls in 36 minutes.

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