Great cities tend to grow from great rivers. Paris has the Seine, London has the Thames, New York has the Hudson. Los Angeles, meanwhile, has a 51-mile long cement gutter, a linear dumping ground so bleak that it’s best known for providing a post-apocalyptic backdrop in Hollywood movies. In the land of film-lot fakery and pasteboard fantasies, it’s no surprise that the vein of blue that snakes its way across the map of LA, optimistically labelled with the word “river”, is nothing of the sort.

“Landscapes tell stories,” director Wim Wenders once said, “and the Los Angeles river tells a story of violence and danger.” The desolate concrete channel, which looks more like an abandoned freeway than a watercourse, has starred in everything from Grease and Terminator 2 to Transformers and The Italian Job, providing a relentless gulley for nail-biting chase scenes, a raw gauntlet of urban infrastructure at its most brutal. Remember a massive truck exploding in a gritty urban wasteland? That was probably filmed in the LA river. Lewis MacAdams, the 70-year-old poet and activist who founded Friends of the Los Angeles River(Folar) in 1985, once commissioned a montage of film scenes shot at the river. The result, he says, was “an unremitting catalogue of urban isolation and despair”.

But the sprawling city that’s capable of eternal reinvention on screen is now rewriting the script in real life. When you land at LAX airport, one of the first things you see is a billboard of the glowing mayor, Eric Garcetti, not sitting at his desk or posing with the Hollywood sign, but perched on a kayak surrounded by greenery – paddling down the LA river.

“Welcome to Los Angeles,” reads the poster, “where nature catches you by surprise.”

This ‘desolate concrete channel’ has starred in everything from Grease and Terminator 2 to Transformers and The Italian Job. Photograph: Allstar/Paramount

MacAdams and his river-loving chums were similarly caught by surprise this summer when it was announced that the kayaking mayor had invited the city’s premier designer of concert halls and feted international crumpler of titanium,Frank Gehry, to master-plan the entire length of the river. Not only that: he had been working on it in secret for almost a year.

The news, uncovered by the LA Times, came just two weeks after confirmation that $1.4bn of federal and city funding would be allotted to restore a central 11-mile stretch of the river, an achievement due in no small part to MacAdams and others’ decades of tireless grassroots campaigning. The Gehry announcement, hastily confirmed at a press conference at the end of August at which no actual detail about the project was disclosed, came as a bolt from the blue. It threatened to throw Folar’s carefully laid plans, developed with other local architects since 2007, entirely up in the air.

“No matter how they disguise it, Frank Gehry knows nothing about the LA river,” says MacAdams, when we meet in downtown Los Angeles, a few blocks from the gaping concrete gash that carves its way through the eastern edge of the city. “He’s never shown any interest in it. To us, this is the epitome of wrong-ended, top-down planning. Everything we’ve ever done with the river has been bottom-up, driven by the community that lives around it.”

MacAdams arrived in LA from Dallas in 1980 and was immediately transfixed by the dystopian spectre of the river, shocked by what had become a dried-out dumping ground in the middle of the city. “It was totally fucked up in every way you could imagine,” he says, in a gravelly Texan drawl. One night, armed with a pair of wire cutters, he and a couple of friends snipped a hole in the fence near the First Street Bridge, slipped beneath a sign that read “No Trespassing: $500 fine or 6 months in jail”, and ventured down the concrete slope to the flat-bottomed riverbed.

“We felt like we were exploring the moon,” he later wrote in the Whole Earth Review. “The air around us was an unholy din … The odour was industrial. The scene was a latter-day urban hell. We asked the river if we could speak for it in the human realm. We didn’t hear it say no, and that was how Friends of the Los Angeles river began.”

The concrete never took hold on the soft-bottomed riverbed of the Glendale Narrows, so nature has since taken its course. Photograph: Oliver Wainwright

So started what he describes as a 40-year performance art work, aimed at raising awareness of the plight of the river. It was once the cradle of LA, the very reason that native tribes first settled here, but it was canalised by the US Army Corps of Engineers in the 1930s, in a spectacular piece of depression-era job creation, after a series of major floods had devastated the city. Since then it has operated as an efficient infrastructural machine with one central purpose: to whisk storm-water run-off out to sea as fast as possible. Most of the time, the downtown stretch stands dry, a gargantuan gutter containing a pitiful trickle.