J. David Rogers, St. Francis Dam engineering expert, says: "This is one of the most important post-failure images in that it shows so many important details, such as: the pile of Pelona Schist lying sluiced along the depressed steps of Block 5 (lower right), a good 30-plus feet above the stream channel; the top of Block 5 has also dropped about 40 feet from where it cleaved off the steps of Block 1, attesting to its original elevation, floating on a matrix of schist slide debris; and the oakum patch exposed on the downstream face of Block 5, which likely played a pivotal role in destabilizing the dam monolith."

Rogers says the "oakum patch" is the dark line (crack) running diagonally across five steps of Block 5 at lower right. Oakum is a soluble fiber that's used to plug leaks. Trouble is, if they were going to use it, they would need to use it on the opposite side/face of the dam, where the water was pushing against it.

Compare to this photo.

A section of the St. Francis Dam, known as the "tombstone," remained standing after the dam broke on the night of March 12, 1928. To the consternation and embarrassment of Los Angeles water officials, it became a tourist attraction. In May 1929, more than a year later, it was dynamited into oblivion after an adventurous young man fell from the top of it and died.

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.