On the night of March 12-13, 1928, the ends or wings of this 205-foot-high, arched, gravity-type of concrete dam gave way, bringing death to some 450 people in the valley below, sweeping away a power house, bursting an aqueduct, and doing millions of dollars' damage to roads, railroads, farms, etc. A central portion of the dam, about 100 feet long (the total length was 650 feet), remained standing but both wings were wrecked by the release of the 38,000 acre-feet of water held in the reservoir. The torrent, said to have been a hundred feet deep, carried huge portions of the structure almost half a mile down the valley.

The dam was located in the San Francisquito Canyon about 1½ miles above Power House No. 2 of the Los Angeles Municipal Power Bureau and roughly 45 miles north of the city of Los Angeles. Completed early in 1926 and built to provide storage for the Los Angeles aqueduct water for use in years of deficient run-off, it was practically full at the time of failure. Although some seepage had occurred around the dam, an investigation the day before the disaster showed clear water, with no indication of erosion.

In general, the reports of the many committees which investigated the failure agree in stating that the dam was built of good materials so that its failure in no way reflects on the safety of this type of structure when built on adequate foundations. On the other hand, a fault classed as dead an in which no movement took place at the time of the failure, passes through the dam site and results in the adjoining rock masses being badly shattered. These rocks consist of a mica schist of very poor quality and a rock described as red conglomerate and supposed to have been formed of gravel loosely cemented with gypsum. Both materials, particularly the latter, have been found to soften under water. It is difficult, therefore, to state positively that any type of dam would have been absolutely safe at this site, and it is even more clear that it was not a suitable foundation material for the gravity type; a type which requires a solid bed-rock base due to the high and unusual pressure exerted by this particular form of dam at its foundation.

Source: New International Year Book, 1928, page 208.

The water development of Los Angeles suffered a serious reverse in the breaking on March 15 [sic] of the Municipal water supply dam known as the San Francisquito Dam [sic], on the Santa Clara River [sic: it's a tributary]. The break released without warning twelve billion gallons of water, which swept down the valley, causing the loss of more than 400 lives and damage estimated at $10,000,000. The city met damage claims by authorizing a special issue of bonds for the purpose. An investigating committee appointed by the City Council rendered an opinion on April 10 that the collapse of the dam had been caused by its resting on a defective foundation.

Source New International Yearbook, 1928, page 126.