Reported in "GIGANTIC BLAST AIDS DAM WORK: Ninety Tons of Powder Used at San Gabriel Side of Canyon Seems Molten Before Explosive Engineering Feat Figured to Exact Result," Los Angeles Times, 27 June 1929: A10.
Wooden construction tower attached to a wooden building in the mountainside at the Pacoima Dam construction site. Workers stand on a wooden bridge near the entrance of the building.
Cloud of dust and debris cascading down San Gabriel Canyon wall after explosion during construction of San Gabriel Dam. Reported in "GIGANTIC BLAST AIDS DAM WORK: Ninety Tons of Powder Used at San Gabriel Side of Canyon Seems Molten Before Explosive Engineering Feat Figured to Exact Result," Los Angeles Times, 27 June 1929: A10.
Photo appears with article "Flood Control Project Inspected: FIRST CONCRETE POURED IN PACOIMA DAM County Supervisors Pull Lever Sending Initial Batch of Mixture to Base of Structure That Will Rise in Year to 395-Foot Height," Los Angeles Times, 06 Aug 1926: A1.
ANSWER: Yep, it's the spillway of Hansen Dam. (Didn't you notice those foothill peaks peeking through the opening?) And sure, it's racy. They have outboard motorboat races on the lake behind it and sports car races on a track directly below it.