Field of Science

March 12, 1928: The geological induced collapse of the St. Francis Dam

Sometimes ignoring the geological structures and resulting problems of construction sites can have disastrous consequences. Following the post of the Stava dam collapse, a similar event from the New World, where ignoring the geological circumstances caused death and destruction:

At the end of the 19th century, the city of Los Angeles was lacking an important factor limiting the economic development - the water supply was insufficient and couldn't satisfy the demand. William Mulholland, an Irish immigrant, and a very ambitious man had managed to become a self-educated engineer and the superintendent of the Los Angeles Bureau of Water Works and Supply. He seemed the right man for the job to bring the needed water to the city.
So he planned and constructed a system of aqueducts that collected and redirected water from the Sierra Nevada to Los Angeles. Almost
72 kilometers north from Los Angeles the main aqueduct ended in the San Francisquito Canyon, where two hydroelectric power stations were built. Mulholland decided to amply the original project and constructed a supplementary dam to regulate the water flow.

Fig.1. The completed St. Francis Dam (figure from CHAMP 1983).

The construction site in the San Francisquito Canyon follows a tectonic lineament that separates conglomerates and sandstones with gypsum from schists with layers of talcum. In addition to these very unstable rocks, surrounding the contact between the two lithologies were lenses of serpentinites, weak rocks inadequate to support a heavy building like a dam.
In 1926 the St. Francis dam was inaugurated, 61 meters high, with a span of 210 meters and at the base 50 meters thick. Soon after the completion of the dam water leaks were observed, but it seemed normal for such a large reservoir. On the morning of May 12, 1928, the technician Tony Harnischfeger observed a leak on the eastern side of the dam. Mulholland was informed and arrived soon thereafter to inspect the damage. The water was clear, sign that no sediment was eroded. At 23.57 p.m. a power blackout in Los Angeles marked the collapse of the dam.
Fig.2. The remains of the St. Francis Dam (figure from CHAMP 1983).

The resulting waves inundated highway 126 in the Santa Clara Valley, destroying at least 50 cars and killing an estimated 300 to 600 people, the exact number of victims remains unclear to this day. The waves finally reached the Pacific, more than 70 kilometres distant from the San Francisquito Canyon.


CHAMP, C. (1983): Planet Earth - Flood. Time-Life-Books. Amsterdam: 176


  1. Nice set of pdf's about the St Francis dam failure.

  2. Great stuff. I've been studying up on this stuff since I injured my leg from some fire damage in Honolulu, so I appreciate you posting.

  3. Wonderful information. The St Francis dam story has so many aspects to it - engineering, geology, history, economic...I've been researching the resulting floods here and found it fascinating.

  4. The leaks Tony Harnischfeger saw were actually on the west abutment, right below the wing dike, near the thrust block.

    Been doing intensive study on the St. Francis Dam for 30 years, and I go out to the site a lot.

  5. The leaks Harnischfeger saw were actually on the WEST end and abutment of the dam, near the thrust block and the start of the wing dike. Mulholland and Van Norman arrived at 10:30 AM the morning of March 12, 1928, and stayed for a few hours. Van went down to the base of the thrust block near the dam's groin and used a clear water tumbler, a drinking glass, to sample the leaking water, which he declared to be "clear."

    I've been studying this story intensively for over 30 years, and have been to the dam site many times. The references you cite are poorly researched and not well-fleshed-out. The best material on the subject to date has been published by Dr. J. David Rogers, one of the world's leading experts on the disaster. Charles Outland published the remarkable volume MAN-MADE DISASTER years ago, and it's research is impeccable.
    I've also published some work on the story, Google A TEST OF INTEGRITY by Pony Horton.

  6. The leaks Tony Harnischfeger saw were actually on the west abutment, right below the wing dike, near the thrust block.

    Been doing intensive study on the St. Francis Dam for 30 years, and I go out to the site a lot.


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