LOS ANGELES >> At 86, Frank Gehry, the architect, seems to be everywhere these days, overseeing philanthropic and commercial projects from Watts to the Sunset Strip as he settles into the final years of his career in the city he has called home since 1947. He is the subject of a new exhibition at the Los Angeles County Museum of Art and the recipient of this year’s J. Paul Getty Medal for artistic achievement from the trust that operates the Getty Museum.

Yet none of Gehry’s farewell enterprises seem more daunting — and fraught — than his involvement in rebuilding the Los Angeles River, a bleak and dispiriting 51-mile channel that winds its way through fields, suburbs, dark city corners and industrial wastelands from the San Fernando Valley to the Pacific Ocean. Not many people even know it is there: Bone-dry most of the year, the river is hidden under a blanket of concrete poured down 80 years ago to hold back the flow that overwhelmed its banks with devastating floods during the rare rains here.

Gehry was recruited by the L.A. River Revitalization Corp., a nonprofit group founded by the city in 2009 to coordinate restoring the river, and encouraged by Mayor Eric M. Garcetti to quietly work up a plan. It would be the latest in a shelf full of restoration plans including, most promisingly, a $1.35 billion Army Corps of Engineers project to restore 11 miles of river with shrubbery, parks, bikeways and walkways.

And not only restoration: In this time of punishing droughts and mandatory water conservation, a plan could also offer a way to reclaim millions of gallons of water lost each year as it streams down the concrete-covered river channel and into the ocean.

But the news that there was a new architect on the case, which leaked out after Gehry had been working on the plan unannounced for nearly 10 months, met a storm of resentment from surprised community leaders who have made reviving the Los Angeles River their mission. By now, Gehry is accustomed to criticism and controversy — he had asked city officials not to disclose his involvement until his preliminary report was finished because he had anticipated some community concern — but he seemed particularly dispirited over criticism of his taking on what he called "such a big gash in the landscape."

"I don’t want to get into a catfight. We’re not trying to take their rights away," Gehry said in an interview in his sprawling studio in Playa Vista, where visitors must sign an agreement promising not to describe the models and sketches that are scattered across his cluttered workshops.

"When you get the kind of blowback from those people that I know and who I thought were smarter than that, you begin to question their integrity," said Gehry, who is not being paid for the project. "Going forward, do I really want to work with those guys? I’m doing something that’s going to be good and trying to be inclusive, and they are trying to cut me up before I even get out of the gate. That’s not nice. I don’t want to create a fight with them, but they should grow up."

He leaned forward in his chair. "Tell them to grow up," Gehry said.

Fairly or not, Gehry has come to embody concerns that the river will be exploited by gentrification and that this architect, with his portfolio of brash and billowing buildings, was not the right person to take on a project involving outdoor space. In envisioning a future Los Angeles River, people are more likely to think of the High Line in New York or the Georgetown Waterfront Park in Washington than the Walt Disney Concert Hall in Los Angeles or the Guggenheim Bilbao in Spain, two of Gehry’s most acclaimed buildings.

"I don’t think it would be too much or too little to say that Frank Gehry has had nothing to do with the L.A. River," said Lewis MacAdams, a poet who is the co-founder of Friends of the Los Angeles River and one of the people who recoiled upon learning of Gehry’s involvement. "I don’t think Frank Gehry has been to the L.A. River. His name has never been attached to the L.A. River. And then here comes the Gehry plan essentially out of nowhere."

MacAdams helped set up the Friends group nearly 30 years ago and has been active in it ever since; the organization stages annual river cleanups and other events. With the news that Gehry had been pulled in, "people feel that the city has betrayed the trust of the people who cared about the river," MacAdams said. "Frank Gehry is a great architect, no question, but nature is an even better architect."

Steve Appleton, an artist who has attended dozens of meetings devoted to the waterway’s future, said that "river folks," as he calls them, are wary of outsiders. "The river has brought people together and gotten them to step back from their egos," he said. "And now we have got one of the biggest egos in town dropping on it."

Garcetti said in an interview that he had encouraged Gehry to work on the project, both because of the technical expertise he could bring and also for the message it would send about an undertaking that had once been seen as a pipe dream.

"He would elevate this so the civic elite of L.A. realizes this is not a hobby of the activists but one of the grand projects of our time," Garcetti said. "Los Angeles is what made him who he is, and I think in the twilight of his life, he wants to leave a number of things that connect him to his home."

Gehry, the mayor added, was "always somebody who has embodied L.A.’s irreverence — and what’s more irreverent than trying to take a concrete river and return it to nature?"

The Los Angeles River has long been viewed as a squandered resource in an era when cities from Baltimore to San Francisco have reclaimed waterfronts. "Virtually every major city in the country that had forgotten about their river, even places like Detroit, have turned back to the water because they see this as the single greatest asset in the city," said Ed McMahon, senior resident fellow at the Urban Land Institute in Washington.

For a variety of reasons — political, cultural and hydrological — that has proved particularly challenging in the Los Angeles area. The river runs through 15 different cities. Thirty-two miles of it is in Los Angeles, although Gehry has been asked to look at all 51 miles. The work is estimated to cost $100 million a mile, and it is not clear where the funding would come from.

The initial plans presented by Gehry were technical, dealing first with how to balance two priorities: protecting the city during rare if catastrophic floods — the problem that prompted the original concrete encasement in the late 1930s — and accommodating development. "Everybody has hopes and dreams about what we can do with the river," said Omar Brownson, the executive director of the L.A. River Revitalization Corp. "But it is all predicated on addressing that 1 percent event."

Gehry said most of his work so far has been on hydrological issues. But in the interview, he floated a few design concepts, such as an inflatable dam to create a lake. He suggested that much of the concrete, no matter how cold, desolate and forbidding it appears, was architecturally interesting and could be left in place.

"There’s a toughness about the river with the walls that is an asset," he said.

The river now bears little resemblance to the Potomac. There are patches of green and enough water in one stretch for kayakers. "That right there is an osprey, which is a pretty rare bird," Appleton said as he walked the river in Frogtown, the neighborhood named for the red-legged frogs that once lived along the riverbank. "I’ve seen them land next to me and pull out a fish. To be five, 10 minutes from downtown and have that experience?"

Still, most of the river is covered bank to bank with concrete. In downtown Los Angeles, by the Sixth Street Bridge, one stretch is so bleak that it is a favorite spot for filming car chase scenes. Farther down, in the poorer communities south of Los Angeles, the river cuts through industrial areas, with wires hanging overhead.

Mia Lehrer, a Los Angeles landscape architect who helped prepare a master plan for the river in 2007, said Gehry’s involvement had distressed people wary of top-down directives, and raised fears that he would derail the plan by the Army Corps of Engineers just as it was gaining momentum.

"If what the River Corporation was trying to do was to bring attention to this, they’ve done it," she said. "Maybe it could have been done more elegantly."

Still, she said Gehry was welcome to join the fray. "He’s a creative dude," Lehrer said. "So the answer is, ‘Why not?’"

Gehry said that he has treasured his years in Los Angeles — from his early career when he could grow "under the radar" as an architect to now, when working near home keeps him off airplanes — and that he was intent on making sure this signature endeavor would succeed.

"For this thing to work out, all these people who are complaining, we are going to need them to be worker bees, not complaining bees," Gehry said. "They are going to have to collaborate."

© 2015 The New York Times Company