The St. Francis Dam Floods California

About the destruction of the St. Francis dam which led to a terrible flood in California in 1926, history of the disaster.


In the morning hours of a March day, Dan Mathews, a maintenance man at Powerhouse 2, about a mile below the "fail-safe" St. Francis Dam, reported muddy water flowing past his station. It had rained during the night, upstream from the dam, but the muddy water seemed to be coming from beneath the massive structure itself, not over it or through the sluice gates. Chief of the Los Angeles Department of Water and Power, engineer William Mulholland, rushed from his office to investigate. After his inspection it was decided that though the situation didn't look good, there was no real cause for alarm. Twelve hours later 137,000 cubic yards of concrete tore from anchorage points and 12 billion gallons of water rushed down the valley toward 20,000 people.

When: March 13, 1928.

Where: 40 mi. north of Los Angeles, Calif.

The Lost: 450 known deaths. 700 homes destroyed. The dam had cost $1,300,000 and there were claim damages of $30 million.

The Cause: The dam, completed in May, 1926, rested on a foundation imbedded 30' in the earth. It was 175' thick at its base and tapered upward 175' above the stream bed. Had geologists been consulted, the damsite across San Francisquito Canyon would not have been chosen. But design engineers could see no problems arising from the anchoring of one side of the dam in unstable laminated rock, and the other side in rock composed of gravel and pebbles. In between these anchorage points the dam sat not upon bedrock, but upon compressed powdered rock, a substance semisoluble in water. Though built to the highest standards of the day, the dam actually relied on its sheer weight to withstand the elements and the pressures of impounded waters. In less than 2 years from its completion the modern, fail-safe St. Francis Dam collapsed.

The Disaster: The trickle of muddy water found near Powerhouse 2 on the morning of March 13 was ignored. No one in the valley was warned of impending disaster. Then 12 hours later, 1st one side of the dam, then the other, was torn from its anchorage. A wave 80' high shot from the reservoir. Pushed by the weight of 12 billion gallons of water, the wave tore huge chunks of concrete from the solid mass and propelled some pieces a mile down the valley.

It happened suddenly. And almost as suddenly, where there had been homes and people, there was a 12-mi. swath of death and destruction. Cars traveling along Highway 126 were picked up by the raging torrent and carried toward the sea. Months after the tragedy, divers along the coast were still finding the grizzly remains.

Aftermath: Since the St. Francis Dam was built to supply water to the city of Los Angeles, the city assumed full responsibility for the disaster and paid death benefits and crop destruction claims totaling $30 million. Subsequent investigation into the cause of the tragedy spearheaded a movement to utilize the knowledge of geologists in the construction of dams, buildings, and bridges.

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