St. Francis Dam
In the early twentieth century the majority of the water used by citizens of Los Angeles came to the city via the Los Angeles Aqueduct, which had been designed and built under the authority of William Mulholland (and is still used today). Mulholland envisioned a city reservoir created by a dam, which would ensure adequate water for the city in the event of drought, or of damage to the aqueduct. The dam would also be used as a source for electricity for the growing city. The dam was built on a site which held geological obstacles well known to Mulholland; he had personally discovered some of them. Construction of the dam began in 1924.
The dam was built in the San Francisquito Canyon, of curved concrete stepped face design, and by 1926 was of sufficient completion to begin filling the reservoir. In March of that year filling began and almost immediately cracks appeared in several locations on the dam, with water seepage evident. In April seepage pipes were installed to collect the water which gathered near the base of the dam and route it to the area occupied by the dam keeper. Meanwhile, as the water continued to rise in the reservoir, cracks and additional seepage appeared along the dam face and where it was anchored in the geologic deficiencies which Mulholland had identified before construction began.
The dam itself was mostly dry, most of the seepage was along the seam of where poured concrete was joined to the natural rock of the canyon walls on which the dam was anchored. Water oozed through and around the natural rock and the concrete, steadily eroding it away. Rather than drain the reservoir and address the issue, problems with the aqueduct were threatening the water supply of the city, and increasing the amount of the water contained in the reservoir was Mulholland’s primary concern. More and more seepage was detected, but efforts to fill the reservoir to capacity continued.
In March 1928 significant leaks were known around the abutments of concrete and rock. Essentially the entire structure was being sawed from its moorings by the erosive flow of the water. On March 12, 1928 the dam suffered a catastrophic failure. The breach was such that the entire reservoir was drained in just over an hour as a cascade of water raced down the canyon. The wall of water was over fifty feet high as it swept down on the towns of Fillmore and Bardsdale, destroyed much of Santa Paula and then reached the sea. Many bodies of victims were never found, likely washed out to sea after drowning in the flood. Officially there were 417 fatalities.
The cause of the failure was the defective manner in which the foundations had been attached to the natural rock, which allowed the water, under great pressure from the amount of water in the reservoir behind it, to cut through the rock. Essentially the entire dam was simply shouldered aside as cracks yielded to the water pressure. The center section remained more or less in place after the abutments on both sides were swept away, acting like a thumb on the end of a hose, increasing the velocity of the water being directed around it. It was eventually demolished. The St. Francis dam was not rebuilt, another dam and reservoir system was built elsewhere.