Located in San Francisquito Canyon, above the present day city of Santa Clarita, it was designed and built by The Los Angeles Bureau of Water Works, now known as the Los Angeles Department of Water and Power. The collapse of the St. Francis Dam is considered one of the worst engineering disasters of the 20th century.
The St. Francis Dam was a concrete structure and of the gravity-arch design, 185 feet in height and 700 feet in length. At the time of collape it was impounding a filled reservoir 2.8 miles long which contained over 38,000 acre-feet of water, weighing almost 52 million tons.
Within the canyon, the flood wave had a depth varying from 100 to 140 feet, washing pieces of the dam, some which weighed more than ten thousand tons over one-half mile downstream. The first victims were the dam keeper, his girlfriend, and his son. Followed five minutes later, by 63 of the 65 workmen and their families of the hydroelectric pant #2, one and a half miles below the dam.
As the flood continued out of the canyon it made its way into the Santa Clara River channel. It cut a deadly path which ended 54 miles away. 5 hours and 27 minutes past the inception, it emptied into the Pacific Ocean at 5:57 am, near Montalvo, a small town in Ventura County.
Before doing so, behind it the towns of Newhall, Castaic, Piru, Fillmore, Saticoy and Santa Paula were devastated. In the flood's wake, a partial review of the records reflect over 431 dead, 1100 homes and buildings totally destroyed, 331 damaged, 10 bridges washed out and over 23,500 acres of agricultural land decimated. Unlisted among others, is the extensive amount of damage to roads, railways and utilities.
The frequently used estimate of 431 deaths is now believed to an underestimate by a factor of as much as 35%, as at the time there were many immigrants, undocumented and other transient workers and their families known to have lived in the areas affected. Making this the second worst disaster in terms of loss of life.