South Fork Dam — the Johnstown Flood: a Plugged Dam Overtopped and Killed 2,209

On the afternoon of 31 May 1889, the South Fork Dam — an earthfill embankment some 22 metres high and 284 metres long on the Little Conemaugh River, 14 miles above Johnstown, Pennsylvania — was overtopped by floodwater and eroded away in roughly an hour, releasing Lake Conemaugh and its 14.55 million cubic metres of water down the valley. The flood reached Johnstown about an hour later as a churning mass of water and debris and killed 2,209 people. It remains the deadliest dam failure in United States history. The embankment itself did not fail by piping, sliding or foundation defect; it failed because it had no margin left to pass a flood. A succession of owners had lowered its crest, removed its low-level discharge pipes, and screened its single spillway to keep fish in the lake. When extraordinary rain raised the reservoir, the water ran over the top and cut the dam open.

The dam was not new and was not the work of careless hands at the moment it broke. It had been built by the Commonwealth of Pennsylvania between roughly 1838 and 1853 as the Western Reservoir, a feeder for the state’s canal system, and was for its day a competent embankment with a cut-stone spillway and five cast-iron discharge pipes that let an operator draw the lake down at will. The railroad made the canal obsolete; the reservoir was abandoned, partially breached in 1862, and left derelict for nearly two decades. In 1879 the site was acquired by the South Fork Fishing and Hunting Club — a private retreat for Pittsburgh’s industrial elite, including Henry Clay Frick and, later, Andrew Carnegie — which rebuilt the embankment as a resort lake without a civil engineer in charge. Every modification the club made to suit a pleasure lake subtracted from the dam’s ability to survive a flood: the discharge pipes were never replaced, the crest was cut down by about a metre for a carriage road, and iron fish screens were fixed across the spillway. None of these acts was individually unthinkable; together they converted a sound embankment into one that could not pass its design storm.

The storm came on 30–31 May 1889, when six to ten inches of rain fell on the catchment in 24 hours. The lake rose toward the lowered crest, the debris-clogged spillway could not pass the inflow, and by early afternoon water was running over the embankment. Earthfill has almost no resistance to overtopping: the overflow cut a notch in the downstream face, the notch deepened and widened by headcut erosion, and the dam unzipped. The 1891 investigation by the American Society of Civil Engineers concluded the dam would have failed regardless of the club’s changes — a finding that protected the club from liability and that modern hydraulic analysis has since overturned. South Fork is the canonical American lesson that a dam is only as safe as its spillway and its freeboard, and that an embankment with no way to release water is a flood waiting for a date.

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