All the features can be seen from the recently closed road, which should make this EarthCache available to all. The only challenge is getting by the roadblocks at either end of the road. At the given parking coordinates there is a 4-foot wide break in the concrete barricades with a locked chain between them. You are welcome to get a closer look at the features using the provided coordinates, but that will take the terrain up to at least a 3.
The St. Francis Dam was built by the City of Los Angeles in 1925-26 as storage for the Owens River Aqueduct. When completed the dam was 185 feet high and created a 2.8 mile long lake with 38,000 acre-feet of water. That much water is roughly equivalent to a lake 1 mile wide, 6 miles long, and 10 feet deep, 12 billion gallons of water. At the time it was one of the world’s larges arch-support dams. The dam had been designed by William Mulholland. The same man who was the architect of the aqueduct system that still provides water for Los Angeles by transporting water from the Sierra Nevada to Southern California.
As the dam filled, cracks appeared in the face of the dam from 1926 to 27, some of which began to leak. Mulholland inspected the cracks and deemed them normal for a dam of its size. On March 7, 1928, the dam was full for the first time. More leaks were noted, but Mullholland again did not think they were a threat to the dam. On the morning of March 12 the damkeeper discovered a leak that he thought was undermining the dam. Mulholland declared the leak safe and typical.
The dam catastrophically failed near midnight on March 12, 1928 emptying the entire contents of the dam in an estimated 90 minutes. The flood of water was 140 feet high and even 1 mile downstream flowed over 120 foot high hills. Blocks of the dam weighing as much as 10,000 tons were moved up to ¾ of a mile downstream. It is estimated that the flood was traveling around 18 miles per hour. The flood traveled 54 miles through the towns of Castaic, Camulos, Filmore, Santa Paula, and Saticoy. At about 5:30 AM the flood finally reached the ocean at Montalvo, officially killing 450 people, but likely more as the flood inundated areas with high immigrant and undocumented populations.
While a number of factors contributed to the ultimate failure of the Dam, including some engineering principles that are explained in the references, the geology of the area was ultimately unsuitable for the construction of the dam and the trigger for the failure. The following discussion will stick to the geology. See the reference for the complete analysis, it is quite good with many pictures and simple diagrams.
Initial theories suspected inactive San Francisquito fault that runs up the canyon. This fault can be seen at N34 23.850 W118 30.748 or N34 32.812 W118 30.842. On the west side of the fault red beds of the Sespe Conglomerate in the Vasquez Formation. The Sespe Conglomerate is a bunch of rounded rock pieces cemented together by a fine-grained red matrix. On the east side of the fault is the grey Pelona Schist. At either coordinate you can see the fault running up the relatively straight canyon.
Image from J David Rogers at rogersda@umr.edu
The rock that holds together the Sespe Conglomerate loose their cohesion when submersed in water. Along the fault, the Sespe would have already been weakened. With these two factors, it was thought that a tunnel formed under the dam along the fault and undercut the foundation of the dam. Once the west side of the dam failed, an eddie formed on the east side that eroded the Pelona Schist out from under the east side of the dam, leaving only the middle section standing.
This may have happened eventually, however, recent reanalysis by Rogers concludes that the east side of the dam was built into an ancient landslide and failed first. Looking across the valley to N34 32.787 W118 30.712 you can see exposed Pelonia Schist on the hillside above rocks that have slid down into the canyon. Landslides intermittently created natural dams in the exact spot of the St. Francis Dam, temporarily forming a natural lake, possibly as recently as in the last 100,000 years (Pleistocene). Sediment typical of lakes can be found up in the valley. Evidence of other ancient landslides can also be found up and down the canyon.
This ancient landslide was reactivated by the water pressure created by the water behind the dam. Water from the reservoir infiltrated into the rocks of the Pelona Schist and just like a boat, the water began buoying up rocks, reducing their weight. The water also acted as a lubricant making it easier for the rocks to slide past each other.
Rogers suggests that a landside in the Pelona Schist initiated the dam failure. The initial landside drove the left side of the dam across the front of the dam. As the waters emptied around the east side of the dam, it eroded material out from under the remaining portion of the dam until it tilted to the east allowing water to flow through a crack in the west side of the dam. Flow out the west side of the dam eroded the foundation of the west side of the dam and it crumbled. Later pieces of the center portion of the dam fell off the west side from additional undercutting of the foundation leaving just the central portion of the dam standing.
This central portion was later demolished. The pieces of dam that were transported downstream were manually broken up to try to resemble natural rock. As with each spectacular engineering failure, this dam failure stimulated additional regulation and oversight of dam building and additional research into dam building techniques and placement that have made subsequent dams safer.
Logging requirements:
Send me a note with :
- The text "GC11HZH St. Francis Dam Failure, Saugus California" on the first line
- The number of people in your group.
- At N34 32.810 W118 30.773 what kind of rock to you see directly to the east and where did it come from, how can you tell?
- Looking at the Pelonia Schist on the far side of the canyon N34 32.787 W118 30.712, how do you think the layering in the schist contributed to the slide?
- Looking up and down the canyon how many additional significant slides can you see? Look for them as you drive up the canyon.
The above information was compiled from the following sources:
- Rogers, J. David, Karl F. Hasselmann, Reassessment of the St. Francis Dam Failure, PDF of a PowerPoint Presentation available at http://web.umr.edu/~rogersda/st_francis_dam/ . This reference has many colorized photos of the dam before and after the failure. It also goes into much more detail on the engineering problems with the dam construction.
- Outland, Charles F. Man-Made Disaster: The Story of St Francis Dam. A.H. Clark Company: 1977. Outland's study of the dam and the ensuing flood, first published in 1963, is the only widely published comprehensive work about the dam, the failure, and the disaster.
- Nunis Jr., Doyce B. (Ed.). St. Francis Dam Disaster Revisited. Historical Society of South California. 2002. ISBN 0914421271. This collection of articles about the dam includes contributions from Catherine Mulholland, William Mulholland's granddaughter, and Dr. J. David Rogers. .