Formerly an intersection, a bridge was built to separate the two roadways. This was necessary to facilitate traffic flow into Los Angeles from the north via the soon-to-be-built Arroyo Seco Parkway.
Image and information from November 1939 California Highways and Public Works magazine via http://metroprimaryresources.info/this-date-in-los-angeles-transportation-history/december/december-17/ .
Images from July 6, 1939 Engineering News-Record.
13 million cubic yards of dry dirt were moved to make Hansen Dam in the northern part of Los Angeles in the San Fernando Valley. Two new types of earthmovers were in use for the first time in building the 122 foot high and 9.050 foot long dam. The Gar-Bro Dragveyor (top photo) loaded 16 cubic yard dump trucks faster than previous methods. LeTourneau Tournapull tractor-scrapers were used to pick up hard clay from borrow pits then transport it to the dam location. Because of the difficult materials, the 25 cubic yard scrapers are assisted by two Caterpillar crawler tractors during the pick-up process.
Hansen dam was constructed primarily as a flood control dam — made necessary after severe flooding in southern California in 1938. The area around the lake became a park.