The Johnstown Flood National Memorial in Pennsylvania commemorates the most devastating flood of the 19th century in the United States and the greatest national catastrophe in the post-Civil War era. The Johnstown Flood was caused by the giving way of the South Fork Dam and is an example of what can happen when people disregard principles of engineering and hydrology. The flood has provided a vast literature with important lessons for environmental management today. At present, all that remains of the historic earthen dam (originally about 900 feet long and 75 feet high) are the north and south abutments, the spillway cut around the north abutment to carry off excess water, and a few remnants of wood and culvert foundation stones representing the location of the control mechanism.

The story of the break of the South Fork Dam begins innocently enough. The dam had been built between 1838 and 1853 by the Commonwealth of Pennsylvania to provide water for the operation of the Western Division of the Pennsylvania Mainline Canal between Johnstown and Pittsburgh. Located some 12 miles east of Johnstown at a point where the South Fork branch of the Little Conemaugh River and several mountain streams converged, the dam created what was, at the time, one of the largest artificial lakes in the nation, more than two miles long and nearly a mile wide in some places. The Pennsylvania Railroad Company purchased the entire Mainline works in 1857 and left the dam and the reservoir virtually unattended.

In 1879 a group of wealthy industrialists formed the South Fork Fishing and Hunting Club and bought the dam and the reservoir for a private summer resort. By 1881 the dam had been repaired–without benefit of an engineer–and the reservoir filled to capacity to form the now nearly three-mile-long Lake Conemaugh. A clubhouse with 47 rooms fronted the lake. From its large porch, members could watch the club's two steam yachts setting off on excursion trips. A number of club members built large cottages nearby. For the next eight years the summer resort offered fishing, hunting, boating, and other recreational opportunities for club members, until, in 1889, the dam broke and sent some 20 million tons of water crashing down the valley towards Johnstown. When it was over, the flood had claimed more than 2,000 lives.