St Francis Dam Catastrophe – 1928

One of the key issues affecting the development of Los Angeles was the provision of an adequate and dependable water supply. The city’s Bureau of Water Works and its chief engineer, William Mulholland, attracted national attention in the early 1900s with the bold construction of the Owens River Aqueduct which brought water a staggering 380 km (237 mi) from the mountains. By the 1920s, however, the rapidly expanding city was experiencing regular water shortages. Mulholland’s solution was to create a network of new water storage facilities closer to the city.

One of these, the St Francis Dam, was built between 1925 and 1926 across the St Francisquito Canyon, 56 km (35 mi) northeast of Los Angeles. The dam was an imposing curved structure 56 m (185 ft) high built of concrete on the gravity principle. The artificial lake behind it was 4.5 km (2.8 mi) long and took two years to fill. Just five days after the reservoir had reached full capacity the dam suffered a catastrophic breach. The concrete wall shattered, sending a terrifying wall of water raging down the valley. The torrent swept up everything in its path, abating only when it reached the Pacific Ocean near Ventura, 86 km (54 mi) away. In just 70 minutes the St Francis reservoir had been emptied of 12.4 billion gallons of water.

At the official inquest afterwards the blame for the disaster was placed on the dam’s foundations and the unpredictable nature of the rock on which it had been built. Although the existence of a minor fault line was known, the understanding of geological movements was not very advanced at the time. More recent research suggests that the real culprit was an ancient and unknown landslide that had been re-activated.

When: March 12 1928

Where: Santa Clarita, near Los Angeles, California, USA

Death toll: More than 450 people were killed in the flood waters; the bodies of many who had been swept out to sea were never recovered.

You should know: The City of Los Angeles paid out more than $7 million dollars in compensation to the victims’ families and those who had lost homes and livelihoods.