If you want to film a car chase in Los Angeles, here's the playbook. Head downtown. If you're coming from the west, you'll exit the I-10 freeway at Grand Avenue. Then, you'll turn north, maybe on Alameda, where you'll speed past warehouses and fast food restaurants and strip joints. When you get to Sixth Street, you'll maneuver around to Santa Fe Avenue, named after the old railroad line that used to run here. At this point, Sixth Street is now elevated, running above you. As soon as you're in the shadows of the overhanging structure, make a hard left. Suddenly, you're in a long, unlit tunnel. The only daylight comes out of a rectangular opening a few hundred yards ahead of you. Hit the gas (everybody does) and when you burst into the light, pull the wheel hard to the left and head north.

What you've just done is illegal—you need a permit—but you've arrived, and you know this spot. You're underneath the Sixth Street Viaduct, the most iconic and generally agreed-upon most beautiful of the 13 pre-World War II spans that traverse the city's eponymous river, separating downtown from East Los Angeles. You might recognize it from the 1978 film Grease, where it was the site of John Travolta's climactic drag race. Or, in digitized form, from Grand Theft Auto, the video game that seeks to train junior carjackers and flesh traders. Or from Them, the 1954 classic of paranoid science fiction, featuring giant irradiated ants that crawled from the very tunnel you just excited. Or from dozens upon dozens of music videos, from Kanye West to Madonna to Kid Rock.

According to Film LA, the organization that helps the film industry book municipal locations, over 80 movies, television shows, music videos, and commercials are shot on or underneath the Sixth Street Viaduct each year. That's partially because of the bridge's swooping metal arches, perched on an art-deco concrete platform; and partially because of the river underneath and that access tunnel: if you want to film something set in Los Angeles that makes reference to the city's automotive culture, or if you're just looking for a place to shoot a car chase that's cheaper and more available than a clogged freeway, the channelized, concretized bed of the Los Angeles River is your best choice.

Except that the bridge officially no longer functions that way, as of this week. It's going away completely. And the river? It's on its way to becoming a river again.

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Peter Kent spent much of the 1980s and 1990s working as Arnold Schwarzenegger's stunt double. You can see him on board a Harley-Davidson in Terminator 2—which many car chase enthusiasts believe contains the greatest vehicular pursuit ever filmed—racing down a narrow, junk-strewn concrete creek that feeds into the Los Angeles River. He's filmed underneath the Sixth Street Viaduct dozens of times, including in the former California governor's The Last Action Hero, and he was genuinely shocked when I told him earlier this fall that demolition would soon begin on the span.

"Why?" he asked, his dismay obvious.

I explained to him what many Los Angeles residents—as well as city officials and sad-but-resigned preservationists—already know: the Sixth Street Viaduct (it isn't officially classified as a bridge; the term Viaduct means that the structure crosses over multiple features, in this case two sets of train tracks, two freeways, and numerous city streets, in addition to the river itself) has cancer.

Each of the 10 bridges that cross the L.A. River in downtown Los Angeles is unique. Built through the 1920s and the early 1930s, they remain the most visible and lovely expression of then-chief city engineer Merrill Butler's belief in the "City Beautiful" movement, the late 19th/early 20th Century design philosophy that, as William H. Wilson wrote in his 1994 book about the phenomenon, saw "Americans attempt to refashion their cities into beautiful, functional entities. That effort involved a culture agenda, a middle-class environmentalism, and aesthetics expressed as beauty, order, system, and harmony."

Butler wanted each bridge to have a theme or aesthetic that represented the city and his design philosophy; his Macy Street Bridge (the street is now called Cesar Chavez Avenue, but the bridge retains its original designation) runs along the old Camino Real, the route used by Spanish conquistadors and missionaries as they explored California in the 17th Century, so the bridge includes baroque-inspired spiral columns and the city seal of Los Angeles. Further south, the Olympic Avenue Bridge features balusters separated by round forms emblazoned with an acanthus leaf pattern, mimicking a design common in the Greek and Roman eras.

The Sixth Street Viaduct's aesthetic is more modern—or at least, modern to 1932, when it was built. The lines are mostly art deco. Louis Huot, the architect who designed it, said: "The viaduct is conformable to the automobile which it carries across the chasm." In other words, it wasn't just a bridge over a river; it was a bridge between eras, ushering in Los Angeles's dedication to the automobile. Over time, the span also served as a cultural connector, acting as a gateway between downtown and points west to the Latino communities of East L.A., Boyle Heights, and beyond.

At the same time, the Sixth Street Viaduct has proven to be as dysfunctional as it is beautiful. Since the span was so large, a shortcut was taken: water from beneath the bridge—that's most likely why the access tunnel now familiar to filmmakers was built—was used to mix the structure's concrete. The result was a building material with a high alkali content; that lead to an alkali-silica reaction that cracked and crumbled the bridge's undercarriage. Over the years, the bridge was patched, often with heavy, metal plates, which further weighed the span down. By the early 2000s, the California Department of Transportation had estimated that the viaduct had a 70 percent chance of collapsing during the next major earthquake.

A preliminary design for the new Sixth Street Viaduct is displayed in Los Angeles on Wednesday, Jan. 27, 2016. (AP Photo/Damian Dovarganes)

In the end, the bridge had to be replaced. Even preservationists agreed: "We are very sad to acknowledge that the 1932 Sixth Street Viaduct cannot be saved. Despite years of research and consultation with experts worldwide, the Conservancy and others could not find a way to halt or reverse the alkali-silica reaction that is slowly destroying the bridge," the Los Angeles Conservancy said in a news release.

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On a Saturday night in early October, Los Angeles Mayor Eric Garcetti threw a sort of farewell party for the viaduct. The idea was to close the bridge, put up a gigantic movie screen, and show Grease. One last hurrah. I understand the choice of that particular film, but there were other choices that probably show the bridge in a more realistic light. Those include Repo Man, in which the bridge's gritty glory is captured in an encounter between Emilio Estevez, Harry Dean Stanton, and the "Rodriguez Brothers," the film's "gypsy dildo punks."

There's no official listing of every scene ever shot on or under the viaduct. Tim Kirk, a local film producer, suggests that 1952's Without Warning, about a serial killer who hunts down and murders women who resemble his estranged wife, dumping their bodies beneath the bridge, was one of the earliest. There's also Roadblock, a 1951 noir featuring a corrupt insurance agent and a seductive blonde; the former meets his demise under the span.

History helps explain the 1932 structure's first two decades of relative anonymity—or at least, non-notoriety. In the earliest pictures of the bridge—during and right after its construction—what's most notable is that the Los Angeles River, beneath, is free-flowing, devoid of concrete. The river was often dry even in that era, but when winter rains hit, it was capable of great destruction, leaving its banks and devastating communities.

As the city grew and industrialized, such floods became more and more deadly. Finally, after a 1939 flood that swept away homes and houses along the river's banks, the U.S. Army Corps of Engineers transformed the river into what Joe Linton, author of Down By the Los Angeles River, describes as a "freeway for water." Today, nearly all of the waterway's 51 miles are lined with concrete. When it rains, that water rushes toward the ocean—wasting millions of gallons of fresh water, but also effectively ending the floods that so devastated the city.

The channels that came along with this transformation—so essential to flood control, but also to the image of Los Angeles as a cold, impersonal, over-paved place—didn't appear under the Sixth Street Viaduct until the bridge was seven years old, right around the beginning of World War II. At that time, Hollywood's film industry was preoccupied with uplifting dramas and cornball, patriotic musicals. It wasn't until after the war, as the Film Noir Foundation notes, that audiences began responding a more "adult-oriented type of film"—with the darker plot points and longer shadows that made the viaduct an ideal filming location.

Los Angeles began building freeways the same year the river was covered over in concrete; by the early 1950s, mass transit was disappearing from the city, in favor of modern highways. It was no accident that during this time the car chase morphed from something wacky—imagine the bumbling Keystone Kops of the silent era, in their creaky, overcrowded clown wagons—to a cinematic form that literally turned vehicles and roadways into characters. Perhaps the most influential example of this is H.B. Halicki's all-chase-all-the-time Gone in 60 Seconds, released in 1974. Though that film's scenes are all shot near the beach, in Los Angeles' South Bay, the remake of the film, starring Nicholas Cage and released in 2000, includes extensive river chases and scenes shot at Sixth Street.

And now, it is going away. Not just the viaduct, but the river itself, in many ways. Over the past two decades, a grassroots movement to reclaim the Los Angeles River has gradually gathered steam. Two years ago, a portion of the river just north of downtown re-opened to kayakers. Bike paths have been extended toward the river's origin point, in the San Fernando Valley. Pop-up coffee shops and restaurants began to appear in former industrial areas adjacent to the banks. Still, the ten-bridge stretch through downtown—with the Sixth Street Viaduct at its center—remained off limits. The extensive and stacked infrastructure throughout the area made it difficult to determine how the concrete walls could be removed or modified.

That changed about a year ago, when federal officials finally approved a $1.3 billion plan to, according to Mayor Garcetti, "reestablish scarce riparian strand, freshwater marsh, and aquatic habitat, while maintaining existing levels of flood risk management." Though it won't be gone in sixty seconds, the entire aesthetic—a concrete "river," and a bridge that lent it a lost-world appeal—is in the process of vanishing.

It's easy to see why many L.A. filmmakers spent 2015 rushing to grab their last shots on the viaduct. A few hours before the mayor's showing of Grease, the city suddenly announced the event would be moved to the next bridge to the north, the less-splendid Fourth Street Bridge. The result was a confused and sparse crowd, and my wife and I wondered why the last-minute change had been necessary. After the screening ended—and after Olivia Newton John remade herself as a tight-leather-pants-wearing, cigarette-smoking fast-girl for Travolta—we headed toward Sixth Street. As we crossed into East Los Angeles, we noticed that one of the lanes ahead was closed.

They were filming a car commercial.