Destroyed highway and railroad bridges | Click to enlarge.

According to St. Francis Dam engeering expert J. David Rogers, this is a rail-mounted, steam-powered pile driver, owned by the Southern Pacific Railroad. It's being used here to install timber piles for a new trestle bridge to replace one that washed away when the floodwaters hit Castaic Junction (near the present-day Magic Mountain amusement park) during the early morning hours of March 13, 1928.

Source: Photograph from the Edward C. Lamb collection, believed to have been created by his brother Henry C. Lamb. Henry's name alone is associated with this set of 30 St Francis Dam Disaster photos.

Henry C. Lamb was a graduate of Stanford University who worked for the Edison company. His descendants report (2014) that they do not believe he was present at the Edison camp at Kemp at the time of the disaster. Although they suspect Henry shot the photgraphs himself, it is possible he received them from a third party.

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.