Officials hope they've finally eradicated a noxious plant choking a stretch of the Chehalis River in south Thurston County, more than 10 years after it was discovered there.

The Thurston County Noxious Weed Control Board completed the annual removal of Brazilian elodea from the salmon-bearing river at the end of September and could find no remaining trace of the plant.

It will continue to monitor the river, and the plant will be deemed eradicated if there’s no evidence of its return in the next three years, said Rick Johnson, Thurston County’s noxious weed coordinator.

“We’re thrilled we’re in front of it instead of playing catch-up,” he said.

Brazilian elodea is a pesky plant. Rooting in waters up to 20 feet deep, the dark-green, fast-growing plant forms dense mats of stems and leaves near the water’s surface.

It blocks fish passage and lowers dissolved oxygen by slowing movement in the water, which concerned officials. In 2007, in the midst of an aggressive removal project, dissolved oxygen levels in the river increased about 8 percent after the removal, according to the board.

SOURCE

Brazilian elodea, native to South America, once was a popular aquarium plant. The state has since banned its sale.

Officials suspect someone dumped the contents of an aquarium into Plummer Lake in Lewis County. Johnson said aquarium gravel was discovered in the lake near the boat launch. The plant eventually flowed into the Chehalis River and took root in the slow-moving stretch of the river near Prather Road.

The plant was discovered in 1998 during a survey in search of other problem weeds.

Thurston County began removing individual plants a year later. Individual hand removal was deemed impractical after several years because the infestation grew too large.

FIVE YEARS AGO

Dive teams were used to help remove the plant beginning in 2004. The U.S. Department of Fish and Wildlife, state Department of Natural Resources and state Department of Ecology contributed funding. Johnson estimated that removal work has cost $200,000.

The Chehalis Indian Tribe, state Salmon Recovery Funding Board and Nature Conservancy assisted in the effort.

EXTENT

At its peak, the plant infested 35 acres of the river, Johnson said. Divers removed the remaining 8 acres of the plant this season. He estimated that 300,000 pounds of the plant have been removed.

It is the only location in Thurston County infested with Brazilian elodea, officials said.

Gene Little, the weed board’s chairman, said in a news release that the removal project is a lesson about how much damage one thoughtless act can do, as well as about the need to educate people about invasive species.

“But there’s also a very positive lesson about how, when government agencies, tribes and nonprofit citizens organizations collaborate, we can make things right in our rivers and our environment,” he said.

Christian Hill: 360-754-5427

chill@theolympian.com