Mr. Gehry was recruited by the L.A. River Revitalization Corporation, 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 Mr. 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, Mr. 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 expected 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,” Mr. 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 Mr. 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.”

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Mr. Gehry said that he had 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,” he said. “They are going to have to collaborate.”

A Saturday picnic and concert last month on the Los Angeles River under the Glendale Freeway. The 51-mile river flows from the San Fernando Valley to the Pacific Ocean.
A Saturday picnic and concert last month on the Los Angeles River under the Glendale Freeway. The 51-mile river flows from the San Fernando Valley to the Pacific Ocean. - 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. © Monica Almeida / The New York Times