Today a river of concrete passes through Sepulveda Canyon, one of the three main portals between the San Fernando Valley and the Los Angeles Basin. But as recently as 1934 that mighty river – the 405 freeway – was only a modest stream, a winding, unpaved road that snaked through the Santa Monica Mountains.

For centuries the region’s native Tongva people had hiked a faint footpath through Sepulveda Canyon, and in 1769 the soldiers and clergy of Spain’s Portola expedition followed that ancient trade route on their way to Monterey. Trail became road in 1875, when the two wheat barons of the San Fernando Valley, Isaac Lankershim and Isaac Newton Van Nuys, widened the footpath to allow for the passage of sturdy wagons laden with grain and bound for ships docked at the Santa Monica Pier. But when the Southern Pacific soon lowered its freight rates, the wheat ranchers instead sent their harvest by train to San Pedro. The new, neglected road eroded into the hillsides.

Eventually, booming development in the San Fernando Valley during the 1920s persuaded the city and county to rebuild the road for automobiles. Traffic was overwhelming the two established routes between the Valley and the Basin, Cahuenga Pass and San Fernando Road, both of which were out of the way for residents of faraway Van Nuys and Owensmouth. New Sepulveda Boulevard – a 50-mile highway stretching between San Fernando and Long Beach – would provide the Valley with a more direct link to the Basin and harbor beyond at San Pedro Bay.

Sepulveda Canyon before the 405. A 1934 view of Sepulveda Boulevard near Mulholland Highway, courtesy of the California Historical Society Collection, USC Libraries.

Groundbreaking for Sepulveda Boulevard through Sepulveda Pass, 1929. Photo courtesy of the Security Pacific National Bank Collection – Los Angeles Public Library.

The Sepulveda Tunnel in 1930, when Sepulveda Boulevard was still an unpaved road. Photo courtesy of the Los Angeles Examiner Collection, USC Libraries.

Construction lasted several years and culminated with the opening of a 650-foot tunnel beneath the summit at Mulholland Drive, an event the city celebrated with a grand Spanish-style fiesta. Despite the festivities, by the time traffic started flowing in September 1930, the new Sepulveda Canyon Road was already inadequate. Five years later the state spent $275,000 to pave it, and by the late 1950s traffic engineers had envisioned an audacious construction project that just might keep traffic flowing freely over Sepulveda Pass forever.

The engineers’ plan? Tear Sepulveda Canyon apart and then rebuild it to allow a superhighway to pass through. Beginning in August 1960, earthmovers carved a gorge 1,800 feet wide and 260 feet deep through the mountains, accomplishing in two years what might take natural erosional forces two million. The bulldozers' total haul: 13 million cubic yards of slate, shale, and dirt. Workers then built massive retaining walls to keep the unnaturally steep slopes from slipping and reconfigured the area's natural drainage through a series of culverts. By 1962, an eight-lane concrete freeway with a maximum grade of 5½% sliced through the mountains.

Though traffic did flow freely at first, the San Diego Freeway (originally signed as California 7 and later redesignated Intestate 405) eventually became one of the Southland’s most hated stretches of pavement. And so the work Lankershim and Van Nuys began in 1875 to improve an ancient trail continues to this day. Regional planners are now considering a menu of options – including an underground toll road, a subway, even a monorail – to relieve congestion, and in 2015 Metro completed a five-year, $1.1 billion project to widen the canyon’s concrete river and its artificial gorge.

1957 aerial view of the San Diego (I-405) Freeway under construction through Sepulveda Canyon, courtesy of the Los Angeles Examiner Collection, USC Libraries.

1957 aerial view of the San Diego (I-405) Freeway under construction through Sepulveda Canyon, courtesy of the Los Angeles Examiner Collection, USC Libraries.

1961 photo of grading work on the San Diego (I-405) Freeway, courtesy of the Los Angeles Times Photographic Archive. Department of Special Collections, Charles E. Young Research Library, UCLA.

1962 aerial view of the San Diego (I-405) Freeway emerging from the Sepulveda Pass, courtesy of the Dick Whittington Photography Collection, USC Libraries.

A version of this story first appeared on Los Angeles Magazine's website on March 12, 2014. It has been updated here with additional images and information.