Floods may occur during any month of the year in Pennsylvania, although they occur with greater frequency in the spring months of March and April, as a result of heavy rains that can combine with snowmelt. Serious local flooding sometimes results from ice jams during the spring thaw. Heavy local thunderstorm rains cause severe flash flooding in many areas. Storms of tropical origin sometimes deposit flood-producing rains, especially in the eastern portion of the state.
On May 31, 1889, a 450-acre man-made lake, detained by a fifty-year-old earthen dam and owned by the South Fork Fishing and Hunting Club (the exclusive reserve of a select group of Pittsburgh's wealthiest elites), ruptured its barrier and its liberated waters raced down the South Fork Creek, into the Little Conemaugh River, on its way to Johnstown, Pennsylvania, some 15 miles downstream. It took about 40 minutes for the lake to empty completely, but it did so with the force of the Niagara River. An estimated 20 million tons of water roared through the narrow confines of the mountain valleys at speeds sometimes in excess of 40 miles an hour and with a roiling wall of water and debris at times more than 70 feet high. The water scoured the valleys and hillsides to the bare bedrock, uprooting massive trees, shattering and pushing along all man-made structures: houses, stores, railroad beds and equipment, telegraph and telephone poles, stone and wooden bridges, plus uncountable tons of soil, loose rocks and huge boulders, and livestock and people and whatever else was in the path of its irresistible plunge downward as it descended some 500 feet in the 15-mile race to Johnstown.
The juggernaut of water and wreckage crashed into Johnstown and swept unstoppably over the whole town and over its several sister towns. Whole houses and businesses, and whole blocks of houses and businesses were torn loose and shattered by the impact. The wave collided with the hillside at the far side of town and returned as a massive wave of backwash surging through the ruins in the opposite direction, leveling most of what little had survived the first impact. From start to finish, the devastation took a mere ten minutes.
The official death toll ultimately was fixed at 2,209. One third of the corpses were never identified and hundreds of missing were never recovered. Human remains from the flood were found as late as 1906. Ninety-nine whole families perished; 396 children age 10 or less died; 98 children lost both parents; 124 women were left widows; 198 men were made widowers. It took five years to rebuild the town.
One area of the state, Johnstown (Cambria County), has endured numerous historic floods. Deadly floods impacted the steel town in 1889, 1936 and 1977.