Shortly before midnight on March 12, 1928, the ground north of Los Angeles started to rumble. Houses trembled. Windows rattled. Stirred awake by a barking dog, Chester Smith heard trees and utility poles snapping in the distance. Although a minor tremor had shaken California’s San Francisquito Canyon two days earlier, the rancher knew this was no earthquake—but a man-made catastrophe. Having survived a flood in the past, Smith raced barefoot to higher ground while shouting to his family, “The dam is broke!”

Three miles upriver from Smith’s ranch, the colossal St. Francis Dam had unexpectedly collapsed, unleashing a 10-story-high wall of water down the canyon. As residents slept, nearly 52 million tons of water bulldozed through the Santa Clara River Valley, killing hundreds in what has been called the 20th century’s deadliest civil engineering disaster.

As the California sun lured Easterners to Los Angeles in the early 1900s, the city boomed—and its drinking water supply dwindled. With an unquenchable thirst for growth, civic leaders authorized William Mulholland, chief engineer and general manager of the Los Angeles water system, to construct the Los Angeles Aqueduct, which opened in 1913 and diverted water from the Owens Valley over 230 miles of desert.

Even the aqueduct, however, couldn’t slake the flourishing city, which blossomed in population from just over 100,000 in 1900 to more than 1 million in the 1920s. A 7-year stretch of near record-low rainfall and the city’s exponential growth imperiled its water supply.

To keep Los Angeles from becoming parched, the self-taught Mulholland constructed a storage system to stockpile water. The most ambitious part of the project was the St. Francis Dam, built 47 miles northwest of downtown Los Angeles. When completed in May 1926, the 208-foot-high concrete gravity-arch dam held back more than 12 billion gallons of water, enough to supply the city for an entire year.

In March 1928, winter rains led Mulholland to announce that all irrigation would be stopped for several days, a move that would save millions of dollars. The rain brought the reservoir behind the St. Francis Dam to capacity, and during a daily inspection of the concrete barrier on the morning of March 12, 1928, keeper Tony Harnischfeger grew alarmed when he noticed a large leak on the western edge spilling “dirty water,” which could signal that foundation material was being washed from beneath the dam.

After receiving Harnischfeger’s report, Mulholland inspected the dam for two hours and determined that a nearby road construction project caused the muddy water. “Seepage was observed, but it was dismissed as unremarkable,” says William Deverell, professor of history and co-director of the Huntington-USC Institute on California and the West. “It was, however, a sign that the dam was in trouble.”

While Mulholland had full confidence in the dam’s integrity, rancher Chester Smith harbored doubts. While driving his cattle that day, he noticed the big leak on the barrier’s western side and water splashing over its top. Downriver from the dam, an uneasy Smith slept that night in his barn with the doors open in case a quick escape was necessary.

At two minutes before midnight, the dam crumbled. Through the dark of night, a 140-foot-high wave of water roared down the San Francisquito Canyon at 18 miles per hour. The torrent devoured everything in its path, smashing homes, uprooting entire orange groves and twisting railroad tracks. After journeying more than 50 miles, the toxic brew finally reached the Pacific Ocean around 5:30 A.M.

Living a quarter-mile downstream, Harnischfeger, his girlfriend and 6-year-old son were likely the first victims—although the bodies of father and son were never found. The floodwaters killed 65 employees and family members at a nearby power house, 84 Southern California Edison workers sleeping in tents at a construction camp and scores of Mexican migrants working as fruit pickers.

The gigantic wave had mauled its victims, stripping away their clothes and leaving them bruised and lacerated. It took months to harvest the dead from debris piles and excavate them from the mud. Bodies washed ashore as far away as San Diego. It is believed that the flood killed more than 400 people, although an exact death toll will never be known since victims included migrant workers and those washed out to sea.

Just nine days after the catastrophe, a Los Angeles County coroner’s inquest began. A somber Mulholland took the stand. “This inquiry is a very painful thing for me to have to attend, but it is the occasion of it that is painful,” he said. “The only ones I envy about this thing are the ones who are dead.” The chief engineer testified that he had no indication that the dam would fail.

The inquest cleared Mulholland of criminal wrongdoing, but the verdict declared that “the construction and operation of a great dam should never be left to the sole judgment of one man, no matter how eminent, without check by independent expert authority.”

A commission of engineers and geologists appointed by California Governor C.C. Young concluded that the conglomerate underlying the dam’s western abutment was of insufficient strength for the mammoth structure and that water had infiltrated the St. Francis Dam’s foundation, lifting it upwards. Investigators discovered other design deficiencies, including the raising of the dam from its original 180-foot proposed height without a corresponding increase in the width of the base, and geological studies have found that the eastern abutment was constructed unknowingly on an ancient landslide that reactivated.

“The geologic structures on either side of the dam were different, and that created the tragic potential for failure,” Deverell says. “Mulholland was at the apex of his fame, power and authority. There were not nearly enough checks and balances in place to ensure the quality and safety of the dam.”

Memories of the St. Francis Dam catastrophe have faded with time, but the San Francisquito Canyon still bears the scars. The raging floodwaters carried massive fragments of the concrete dam, weighing as much as 10,000 tons, a half-mile downstream. Some of those eroded pieces of concrete and rubble can still be seen along with makeshift memorials and historical markers.

The St. Francis Dam disaster does not occupy the same place in the national memory as similar tragedies such as the Johnstown Flood, but hopes for greater recognition were raised after the federal designation of the St. Francis Dam Disaster National Memorial and Monument in 2019. The monument occupies 353 acres of federal land that is managed by the U.S. Forest Service. A design for a 50-foot-tall memorial sculpture has been selected, and project boosters are hoping to raise money to erect a visitor center, museum, memorial garden and wall with the victims’ names.