Dean Wilson arrived in Louisiana from San Sebastian, Spain, with a bow and arrows, a spear and several hooks. His ultimate destination was the Amazon. But he needed to get used to humidity, so he stopped in the Atchafalaya Basin, the largest wetland forest in North America. His plan was to stay a few months.

Wilson, then 24, never left. He became a commercial fisherman and created Atchafalaya Basinkeeper, an organization dedicated to protecting the land. Thirty-five years later, it’s a place he knows as well as he knows himself. And as he steers his aluminum bateau through a flooded put-in south of Baton Rouge, he’s already grieving it.

The basin is like a bathtub, he said, collecting water that would otherwise flood an area stretching from southwestern Louisiana to Mississippi. But the tub is filling up with dirt, sand and silt. Along with threats from logging and oil and gas drilling, the sedimentation is likely to lead to the basin’s destruction.

“If nothing changes, 50 years from now, this will all be hardwood forest,” he said. “The sea level is rising, and the coast is going to be gone.”

Atchafalaya, a bastion of Cajun culture, is the largest river swamp in the country and a vital habitat for migratory birds. The land is so damply verdant that it seems not for this world, which it soon won’t be given the rate at which acres of wetlands are disappearing.

Wilson recently cruised down unnaturally straight waterways dredged to accommodate barges. His engine cut through birdsong, and a great blue heron took flight. He pointed out an oil pumping station, all metal and sharp angles, rising out of the water in the shadow of tupelo trees.

He turned off the canal and plunged into the swamp. He navigated under branches strung with necklaces of Spanish moss, through carpets of water lilies and wild hyacinth as tall as the boat, and around cypress trees, some a century old, discolored by lichen. Salvinia was sprinkled like chartreuse confetti on the water’s surface. The water was the color of long-steeped green tea, brightened by the reflection of the cypress’ feathery emerald leaves. The occasional trumpet flower provided a dash of bright coral.

He paused by a tree marked with a pink tag identifying crawfish traps and, in a voice still accented by the language of his youth, explained what is at stake. A barred owl perched above.

When the basin goes, so goes the crawfish. So goes the barred owl, and the region’s natural defenses from storm surges, and a rare and fragile ecosystem. In the distance, bullfrogs croaked in a deep-bellied chorus.

There is a reason that magic — the Cajun werewolf rougarou, the banished princes-turned-frogs — is thought to lurk in these waters. The swamp seems endless and enchanted. But Wilson fears it is mortally wounded.

“Look at this,” he said, gesturing all around him. “There’s no other place on Earth you can make something like this. This is a treasure that can be created nowhere else on the planet.”