COEUR d'ALENE - On the northward swing of a long migration route, thousands of white tundra swans land in the Coeur d'Alene River Basin every spring, congregating in wetlands near Cataldo, Rose Lake and Harrison.

The long-distance travelers, on their way to Alaska for the summer season, munch on roots and tubers. They build strength for the coming journey.

By mid-May, most of the swans have already left the Panhandle. Some, however, cannot leave. They remain behind, disabled and dying.

"Once they're here for awhile, they actually get so much contamination they're disoriented - they can't fly very far," said Julie McKarley, an Idaho Fish and Game senior conservation officer based in St. Maries. "It's kind of a death sentence for them."

According to Idaho Fish and Game and the U.S. Fish and Wildlife Service, about 150 swans die every year in North Idaho waters. An estimated 92 percent of those deaths are caused by lead poisoning.

"Generally, all the ones I've seen (recently) around here in the Coeur d'Alene Basin, they're goners," McKarley said.

Because tundra swans dig into the ground while foraging for food, they ingest large amounts of contaminated sediment, McKarley wrote in an IDFG article.

For decades, upstream mining operations dumped waste into the Coeur d'Alene River. Mine waste disposal is now strictly regulated, but the damage was done: Lead and other toxic materials have contaminated most of the river basin.

"Approximately 80 percent of the wetland habitats (in the basin) have enough lead concentrations to be lethal to swans," the article states.

The lead shuts down the swans' digestive systems. Food presses against their windpipes, the article says, and the birds gasp for air. Eventually they become emaciated and starve to death.

Swans die every year, but 2011 is looking like an especially bad season. Many of the birds stayed in North Idaho longer than normal, McKarley said, because of a late-arriving spring in the far northern latitudes.

And the longer they're here, the greater the risk.

"We have had a bad mortality year this year," said Brian Spears, a USFWS resource contaminants specialist.

In conjunction with the Environmental Protection Agency, USFWS conducts waterfowl surveys along the Coeur d'Alene River. This year, working from riverside roads, surveyors counted more than 100 dead swans.

"And that's a high number for us," Spears said.

Had the counters trekked deeper into the wetlands, he added, more dead birds would have been located.

Altogether, 11 species of waterfowl suffer the ill effects of lead poisoning, along with several riparian songbirds, Spears said. The unique ecosystem harbors a wide variety of bird life.

"The 'problem' with Coeur d'Alene is the Lower Basin is really a world-class riverine wetland complex," Spears said.

IDFG, the Fish and Wildlife Service, EPA and conservation groups are working to improve the health of the basin. The Schlepp Wetland Project near Medimont is nearly finished, said EPA Environmental Scientist Anne Dailey.

A cooperative effort between the EPA, USFWS, Ducks Unlimited and private parties, the 400-acre project will transform contaminated wetland into healthy, lead-free waterfowl habitat.

"We're very close to completion of the cleanup action," Dailey said. "We're using a couple of different techniques."

The Schlepp Project is funded by mining settlement monies, she added. Once complete, the Schlepp wetlands will be a safe haven for both migratory and resident birds.

Given the vast size of the Coeur d'Alene Basin, the ongoing projects are fairly small. But USFWS is already seeing some positive signs, Spears said.

"We're actually looking for other places to conduct more work. It's going to take (more projects), though, to really see a basin-wide impact."