The downpour of the great storm continued with an undiminished strength during the afternoon and evening of May 31. The rain over the watershed of the South Fork Dam fell at the rate of two-thirds of an inch per hour. This record fall put 20,909 cubic feet of water per second on the area drained by the Conemaugh.
By three o’clock, 10,000 cubic feet of water were pouring into the reservoir every second. The spillway, impeded by log floats, fish guards and bridge piers, possibly carried off 6000 feet per second. The other 4000 cubic flowed over the dam; much of it over the depression in the center of the wall. Pouring over the earth in the wall, which had already been plowed, was 123 tons of water per second whose weight and speed acted as an irresistible abrasive which not even the heavy stone facing of the outer wall could long resist. 51
The small streams of water which ran from the crevices of the outer wall became large cores of water after a few stones had fallen from the top of the embankment. With a V-shaped notch cut in the outer wall, the force of the water soon cut a triangular trough to the inner wall. In a few moments this notch was ten feet deep,–deep enough to form a draining furrow 150 feet back into the lake. 52
The eroded spot in the center of the dam grew larger and larger. The roar of the escaping water and the spray at the breach frightened the speechless observers away. Roaring louder and louder, grinding deeper and deeper, the waters of the Conemaugh finally pushed a 420 foot section of the wall into the valley below. Ninety thousand cubic yards of the embankment were deposited in a gravel bar at the foot of the dam. All the fill of 1880-1881 was torn out. The force of the water even laid bare the sluice culvert of 1851.
In forty-five minutes on May 31, 640 million cubic feet of water—20 million tons—were freed from the old Western Reservoir—freed to race through the Conemaugh Valley, freed to drop from a height of four hundred feet upon the defenseless towns of the valley. Even after the dam had been drained, the waters which constantly drained into the reservoir still made a stream five feet deep through the ruined wall. 53
Excerpt from Nathan Daniel Shappee, History of Johnstown, Unpublished doctoral dissertation, University of Pittsburgh, 1940.
