Unidentified persons inspect the north side (reservoir side) of the concrete St. Francis Dam in San Franciscquito Canyon after the break. The object in the foreground apears to be a backpack.

St. Francis Dam engineering expert J. David Rogers writes:

The horizontal lines across the lower 17 feet of the upstream faces of Blocks 2 and 3 are silt-water lines of the draining St. Francis Reservoir. These testify to fact that the reservoir was nearly empty when Block 2/3/4 tumbled backward, breaking into three large pieces seen here. The more faint "titled lines" seen on the upstream face are from the five-foot high forms used for the mass concrete in the dam, which were originally horizontal. These show that Block 2/3/4 fell backward, and ended up tilting about 45 degrees from its original horizontal position.

The more dramatic "stains" on the concrete blocks are from effervescence salts, precipitated from the dam's mass concrete. These were a surprise to many engineers after the dam's failure, because they testified to the mass permeability and saturation of the dam's concrete, which countered most engineer's assumptions in those days. Up to that time most civil engineers erroneously assumed that mass concrete was more or less impermeable, and that dense bedrock materials, like granite, were also impermeable. In fact, these arguments really erupted in the wake of the St. Francis failure (although there were a few engineers who had it right for many years previous, like John R. Freeman). The naysayers were proven incorrect in the 1940s when instrumentation from Hoover Dam was released, which showed that the dam's mass concrete (of much higher density and overall quality than that in St. Francis) exhibited more-or-less instantaneous pore water pressure response to the fluctuating water levels of Lake Mead, through several hundred feet of dense and seemingly impervious mass concrete. The era's greatest engineers were never able to explain the scientific reasons for this response, but they were able to verify it on other instrumented mass concrete dams, such as Grand Coulee and Shasta (built after Hoover Dam). By 1945 they opined that the near-instantaneous pore pressure response must be ascribable to microfissuring, or something similar, which remains the prevailing view.

Photograph shot on or shorly after March 13, 1928. The dam collapsed at 23:57:30 on March 12 and it took about 72 minutes for the 12.5-billion-gallon reservoir to empty (Outland 1977).

About the image: One of seventeen 5x7-inch glossy photographs found in a box in the Photography section of the San Francisco Public Utilities Commission's Engineering Archives in 2013.^[1] There was no record of the origin of the photographs and no writing on the back to identify subjects or dates. Current SFPUC staff members did not know the reason for their inclusion in the archive.

Ownership transferred via email communications (on file) from SFPUC to SCVTV-SCVHistory.com. Prints received July 11, 2014; subsequently transferred to the Santa Clarita Valley Historical Society.

1. A hand-written notation by current San Francisco PUC staff indicates the images were emailed Aug. 8, 2013, to someone who identified them as the St. Francis Dam. SFPUC staff alerted us to their existence in the Spring of 2014.

Construction on the 600-foot-long, 185-foot-high St. Francis Dam started in August 1924. With a 12.5-billion-gallon capacity, the reservoir began to fill with water on March 1, 1926. It was completed two months later.

At 11:57:30 p.m. on March 12, 1928, the dam failed, sending a 180-foot-high wall of water crashing down San Francisquito Canyon. An estimated 411 people lay dead by the time the floodwaters reached the Pacific Ocean south of Ventura 5½ hours later.

It was the second-worst disaster in California history, after the great San Francisco earthquake and fire of 1906, in terms of lives lost — and America's worst civil engineering failure of the 20th Century.