On March 12, 1928, the St. Francis Dam collapsed. The floodwaters destroyed thousands of acres of arable land, killed more than 400 people, and ended Mulholland’s professional career.
It remains one of the worst American civil engineering disasters of the 20th century and the second-greatest public health and safety hazard in California history.
In fact, the leakage started as soon as the reservoir filled with water in March 1926.
That was about two months before construction was completed.
They filled the cracks in the concrete with hemp and oakum.
However, the oakum was inadequate.
A significant leak appeared in the wing walls, so a drainage pipe was installed.
Then, the dam functioned normally until March 1928.
Heavy rains throughout early 1928 filled the reservoir.
Increasingly severe leaks appeared.
On the morning of March 12, 1928, the damkeeper reported numerous leaks, including one muddy leak that could have resulted from seepage under the foundation.
The dam’s chief architect, William Mulholland, and his deputy engineer examined the site and noticed multiple leaks at the east and west abutments and a five-foot sag in the roadway along the east abutment.
He pronounced it safe because the muddy water was clear at its source.
And no action was taken.
At around 8 p.m., water-level readings taken atop the dam began indicating a perceptible drop in the reservoir.
But there were no signs that increasingly severe leakage was causing the drop.
In fact, as the reservoir sat full for weeks, water seeped through mica schist, which formed the layers of the canyon’s east side.
As the water continued to seep through the mica schist, the uplift pressure increased, slowly pushing the dam up from the canyon floor.
Three minutes before midnight, the east abutment broke.
12.5 billion gallons of water from the reservoir flowed onto thousands of sleeping residents in the valleys below, creating a 180-foot tidal wave.
The flood swept up to 53 miles down the Santa Clara River Valley to the Pacific Ocean.
It swept down the narrow canyon at a speed of 70 mph.
The damkeeper and his family were the first to die.
The floodwaters destroyed two powerhouses: Powerhouse No. 2 at 12:02 a.m. and the Saugus substation at 12:40 a.m.
Electrical lines were destroyed, but phone lines remained functional. Calling was the first and fastest way to warn towns downstream of the impending flood. Santa Paula telephone operators called as many people as possible in the surrounding communities.
Another immediate safety measure was taken that night. Two motorcycle policemen knocked on doors to wake sleeping occupants in Santa Paula, then rode up and down residential areas, making as much noise as possible to awaken people.
Kemp was destroyed at 1:20 a.m., and Fillmore Bridge at 2:25 a.m.
At 3:05 a.m., the flood reached Santa Paula. The tidal wave had been reduced to 20 feet. However, the accumulation of debris destroyed the Santa Paula Bridge.
At 4:05 a.m., the flood destroyed Saticoy Bridge.
At 5:25 a.m., the floodwaters reached the Pacific Ocean.
As they raced to the ocean, they left behind a great deal of debris and devastation: more than 450 deaths, 909 destroyed structures, 331 damaged structures, and 42,000 acres of land affected.
Those who survived were traumatized, some for a couple of years, but many for the rest of their lives.
But why did the St. Francis Dam fail?
An unsuitable foundation, an old landslide, inadequate design, and uplift pressure contributed to the failure.
The canyon consisted of layers of mica schist interspersed with talc on the east side, and a highly unstable red sepiolite conglomerate on the west side.
Mica schist was exceedingly rough but could split easily into thin plates after prolonged soaking. The red conglomerate tended to disintegrate in water.
The area was unstable, with numerous landslides and an earthquake fault.
The reservoir’s capacity increased from 30,000 acre-feet in July 1923 to 32,000 acre-feet in July 1924, then to 38,000 acre-feet in March 1925. Mulholland raised the dam’s height without increasing its base width.
This inadequate height-to-width ratio reduced the dam’s stability and made it more vulnerable to uplift.
California’s 1917 dam-safety law exempted municipalities from State Engineer supervision when building dams. When Mulholland was appointed to build the dam, he was the chief engineer and general manager of the Los Angeles Bureau of Water and Power.
Mulholland had a clear escalation path above the expert.
He got a promotion from a laborer to foreman in recognition of his dedication, which led to advanced positions, including superintendent and chief engineer.
He was self-taught, had a quick mind and a remarkable memory, and developed his knowledge of hydraulic engineering through books and articles. He built a successful career, supervising many projects (e.g., the Los Angeles Aqueduct)
Mulholland chose the site, designed the dam, and inspected the work without any external review.
This catastrophe fundamentally transformed engineering, geological, and regulatory standards for infrastructure safety.
Investigations revealed that the St. Francis Dam failed because of unstable, landslide-prone geology at the site and unrecognized hydraulic uplift pressures.
Geological reports, foundation evaluations, and Proctor compaction testing became mandatory before large structures could be built.
The dam’s failure immediately altered the construction of the Hoover Dam, delaying the project and forcing a relocation to ensure safer geological footings and stricter stress criteria. Hoover Dam was designed as a curved-gravity concrete dam, like the St. Francis Dam.
William Mulholland’s lack of formal education and sole authority over the project concerned the judge, many investigators, and critics.
On August 14, 1929, California enacted its first legislation for civil engineers, known as the Civil Engineers Act. This legislation required all civil engineers to have formal education and legal registration. Its intent was to protect life, health, and property. Thus, engineering shifted from an unregulated trade into a strictly licensed profession designed to protect public safety.
Mulholland had sole authority over the construction of the St. Francis Dam. This authority was reinforced by California's 1917 Dam-Safety Law, which exempted municipalities from State Engineer supervision when building dams.
After the dam’s collapse, the law eliminated these exemptions, bringing public utilities under mandatory state regulation.
Attention was also drawn to the state of existing dams. In 1929, California established a dedicated Dam Safety Program to enforce strict state supervision over dam design, construction, maintenance, and operation.
The program required the examination, approval, or repair of all nonfederal dams. It required state approval of plans and specifications, as well as continuous construction supervision, for new dams and for the enlargement, repair, alteration, or removal of existing structures. It also established ongoing state supervision over the maintenance and operation of all dams meeting jurisdictional size thresholds.
The disaster prompted broader federal action, leading to safety inspections of dams across the United States and spurring federal legislative efforts to address critical infrastructure risks.
Although the area’s geology was the primary cause of the catastrophe, it highlighted the need for lifelong learning and peer review, as well as the limits of professional autonomy.
A journey filled with success doesn’t guarantee further success. Each project rests on a new foundation, whose hidden faults may or may not fall within the scope of prior experience.