Avian cholera, which poses virtually no risk to human health, surfaces in the region nearly every year in wetlands of the Lower Klamath National Wildlife Refuge, but the recent waterfowl die-off there is the worst in over a decade, said Matt Baun, a spokesman for the U.S. Fish and Wildlife Service.

"We estimate 10,000 to 15,000 birds will die, after everything is said and done," he said, adding that snow geese, American coots, American wigeon ducks, white-fronted geese and Northern pintail ducks have been the hardest hit.

The 53,600-acre (22,900-hectare) refuge encompasses a patchwork of shallow lakes, freshwater marshes and grasslands that serve as key roosting, nesting and feeding grounds for some 2 million birds that pass through the region along the Western migratory corridor called the Pacific Flyway.

The refuge lies at the heart of the larger Klamath River Basin, an area long considered the Everglades of the West and fed mainly by runoff from melting snow in the Cascade mountain range.

Water flow into the basin is controlled through dams and reservoirs operated by the federal Bureau of Reclamation, which must balance of needs of birds and other wildlife with endangered fish and the irrigation demands of farmers and Indian tribes.

For the first few months of the 2012 winter-spring migration, the refuge received only enough water to cover about half the 30,000 acres of its wetlands, according to the American Bird Conservancy, an environmental group.

The drier conditions have forced birds to congregate in smaller areas, causing crowded conditions that accelerate the spread of avian cholera.

"We anticipate this continuing to happen until there is better management of the water," said Steve Holmer, senior policy adviser for the conservancy.

The problem was aggravated by below-normal snow pack levels in the Cascades until early March, said Kevin Moore, a spokesman for the U.S. Bureau of Reclamation said.

The snow pack, however, has rebounded since then, growing from 63 percent of normal in late February to 115 percent of normal today, he said.

As a result, Baun said, federal water managers were able to flood an additional 4,000 acres of the refuge since mid-March. Moreover, many of the birds that had crowded into the area have moved on, easing congestion, said John Beckstrand, a refuge biologist.

In the meantime, wildlife workers and volunteers have been gathering and incinerating the carcasses of dead birds to try to stem the cholera outbreak.

Water remains a highly contentious issue in this area. Several species of fish are listed as endangered or threatened, and the Endangered Species Act makes them a top priority for water management, Baun said.

"We also have legal contracts with agricultural irrigators to supply them with water, when water is available," Moore said.

Still, the American Bird Conservancy says it remains concerned about maintaining adequate water flows through the basin.

"We need to see a more equitable distribution of water and ways of managing this land," Holmer said. "Right now we are not getting it done."

(Editing by Steve Gorman)

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