St. Francis Dam Failure – March 12, 1928

This dam, part of the Los Angeles aqueduct system, was constructed on a fault. The dam failure caused the deaths of five hundred and it also changed the course of the Santa Clara River.

Shortly before midnight on March 12, 1928, the St. Francis Dam, part of the Los Angeles aqueduct system, collapsed and released twelve billion gallons of water, destroying a swath of land all the way to the ocean and killing five hundred people in its path. The dam had been constructed on a fault, a common geological feature in that part of the country. Unfortunately, no geological investigation was made before the dam was built. The collapse of this dam, built by the city of Los Angeles, is a classic example of the neglect of scientific expertise. At that time engineering projects did not consult geologists prior to selecting dam sites.

The accident that followed, California’s second worst, next to the San Francisco earthquake of 1906, became a landmark in the history of dam construction. Some have called it America’s greatest civil engineering failure of the twentieth century. Never again, in the multiple-fault, earthquake- prone state of California would a major dam be built without a prior, extensive, geological assessment. The St. Francis Dam was a curved, concrete, gravity dam in San Francisquito Canyon, two hundred feet high, with a span of six hundred feet across the mouth of the canyon, and a dike along one side of the canyon. It was built to provide an additional 38,000 acre-feet of water storage for the Los Angeles aqueduct system.

Behind the dam, the second biggest in a chain of storage basins, were twelve billion gallons of water, enough to meet the needs of the city of Los Angeles for more than two months. The catastrophic failure that occurred on March 12, 1928, happened as the dam filled to full capacity for the first time. Only much later, in the light of better knowledge of the geology of the Los Angeles area, was a full explanation provided for the failure of the dam.

The architect of the dam was William Mulholland, chief engineer of the Los Angeles Bureau of Water and Power. He had a history of success in the years before 1928 with a variety of projects that were designed to provide both water and hydroelectricity for Los Angeles. It was he who planned and supervised the construction of a 233-mile-long aqueduct that came through the Sierra Nevadas and brought water and electrical power to Los Angeles. Early in 1928 he was working on a very ambitious plan to bring water from the Colorado River across 250 miles of deserts and a series of tunnels to Los Angeles.

Thus, when he proposed building the San Francisquito Dam, his plan was enthusiastically welcomed. He was a trusted man, and he was allowed to go ahead with construction of the dam even in the face of warnings from geologists. He ignored their warnings and the local laws of that time allowed him to do this. What he was told by geologists, however, proved to be the cause of the disaster that followed on March 12: they had pointed out that the type of rock under the dam was too weak to sustain the weight of the dam and, furthermore, the dam site was a major geological fault.

For about a week before the failure of the dam, farmers in the surrounding area reported leaks and their quantity and volume seemed to increase day by day. On the morning of March 12, one man observed an unusual amount of water escaping through cracks at the dam’s base. This report was immediately brought to Mulholland’s attention. He inspected the location and pronounced it safe. Rain had been falling steadily since March 6 and the level of water in the dam had reached the maximum possible, to the point of overtopping. Either the water release mechanism had failed to operate or it had never been installed.

Close to midnight, while most of the residents of the Santa Clara Valley were asleep, billions of gallons of water swept down the Santa Clara River. There was no time for local residents to escape. Houses were crushed, farms turned into seas of mud, and cars tossed around like toys. A wide swath of land, in places as wide as sixty miles, was cleared of everything as the water cascaded along a seventy-five-mile-long path to the ocean. Five hundred people had been killed and damage estimates reached $20 million.

This failure represents but one of a number of important dam failures that occurred in the 1920s and 1930s, when American civil engineers were pushing the limits of a technology that was still in its infancy. Like most major engineering failures, looking back on it one can see that considerable long-term societal benefits resulted from the public outcry that followed the disaster. One immediate action was the establishment of a dam safety agency, the first of its kind anywhere.

This new organization required geological assessments of dam sites before the design stage, including a normalization of uniform engineering criteria for testing of compacted earth. Foundation material of this kind is still in use worldwide. All the Los Angeles Department of Water and Power dams and reservoirs were assessed in the light of the St. Francis experience and one of the outcomes was an extensive retrofit of the Mulholland Dam. Mulholland’s reputation as an outstanding engineer ended suddenly in the wake of the failure of the St. Francis Dam.