The lake seemed to leap into the valley like a living thing

From The Johnstown Flood by David McCullough. Page 101.

There were men on the hillsides near the dam who had seen what the force of water could accomplish in mining operations, how a narrow sluice could scour and dig with the strength of a hundred men. Actually anyone who had lived in the area long enough to have seen even the spindliest of the local creeks in April had a fair idea of hydraulics at work. But no one who was on hand that afternoon was prepared for what happened when Lake Conemaugh started for South Fork.

“Oh, it seemed to me as if all the destructive elements of the Creator had been turned loose at once in that awful current of water,” Colonel Unger said.

When the dam let go, the lake seemed to leap into the valley like a living thing, “roaring like a mighty battle,” one eyewitness would say. The water struck the valley treetop high and rushed out through the breach in the dam so fast that, as John Parke noted, “there was a depression of at least ten feet in the surface of the water flowing out, on a line with the inner face of the breast and sloping back to the level of the lake about 150 feet from the breast.”

Parke estimated that it took forty-five minutes for the entire lake to empty, but others said it took less, more in the neighborhood of thirty-six or thirty-seven minutes. In any case, later studies by civil engineers indicated that the water charged into the valley at a velocity and depth comparable to that of the Niagara River as it reaches Niagara Falls. Or to put it another way, the bursting of the South Fork dam was about like turning Niagara Falls into the valley for thirty-six minutes.

A short distance below the dam stood a farmhouse belonging to George Fisher. Fisher, who had been warned that the dam was about to go and had managed to escape from the house with his family only minutes before, saw everything he owned vanish in an instant.

Huge trees were snapped off or uprooted one after another and went plunging off in the torrent. When the flood had passed and the hollow was still again, the hill opposite the dam had been scraped bare for fifty feet up. Every bush, vine, every tree, every blade of grass, had been torn out. All that remained was bare rock and mud.

The water advanced like a tremendous wall. Giant chunks of the dam, fence posts, logs, boulders, whole trees, and the wreckage of the Fisher place were swept before it, driven along like an ugly grinder that kept building higher and higher.