City News

Willamette Slough restoration project reaches final year

The restorationists dragged their canoe onto the north shore of Minto-Brown Island and embarked on a slow walk around the shoreline, wearing waders and backpacks sloshing with herbicide. They left blue in their wake.

It’s the fourth, and hopefully final, year of herbicide treatment in an ongoing battle against Uruguayan water primrose, also called Ludwigia, in the Willamette Slough. The invasive plant spreads across the river and its long, impassable roots leech oxygen from it, altering the temperature and balance of the water.

Monday launched a week’s worth of treatment by the city of Salem and Willamette Riverkeeper, which participants said will be the least strenuous yet. An area that once used to be blanketed in the plant now has limited growth to address.

Boaters, swimmers and paddlers are advised to stay clear of the area during treatment, and for 24 hours after the last treatment on Friday, July 28.

They’re on track to finish on time, said Matt Mellenthin, director of habitat restoration at Integrated Resource Management, a restoration consulting firm.

“The first season was the heavy lift. That’s where the bulk of the time was spent. And we had really good success the first year, major die-off,” he said. “The person-hours that were needed after the first year was significantly less. We went from spending two days up in this front area to just a morning in the front. And same with the back channel back there.”

Ludwigia is a green, aquatic plant with yellow flowers originally from South America. Though it looks pretty, it spreads out and chokes up clear waterways, reducing the space for fish to swim, dropping oxygen levels that fish need to thrive, and blocking sunlight.

It first became an issue in the Salem area about a decade ago. Though its exact source is unknown, it’s become a popular plant in home aquariums and ponds and spreads fast when introduced to a new space.

Willamette Riverkeeper began to address the issue several years ago, and in 2018 received a grant from Meyer Memorial Trust and Bonneville Power Administration to treat the plant in the Willamette Slough. The total budget was $229,334 for management costs, treatment, materials and milage, according to the group.

The restoration group is based in Oregon City, but monitors over 130 miles of the Willamette, including in Salem. They were able to stretch three years’ worth of funding into four.

Crews spray a mixture of a contact herbicide and a blue dye that only impacts the plants it’s sprayed directly on, Mellenthin said. It gets diluted in the water so it doesn’t impact native plants.

“There’s been very little collateral damage. Actually, in some of the areas, we’re seeing natives rebound pretty well,” he said. Areas that were once covered in Ludwigia now have wapato growing, and there’s more open water and passable channels for wildlife and kayakers.

Overhead drone photos show the progress, with bright green plants around the periphery of the slough giving way to clearer shores and water.

Drone photos comparing the Ludwigia infestation in the Willamette Slough between 2020 and 2022 (Courtesy/ City of Salem)

“You would probably get 10 feet of water along the edge, at least, was just this pure mat of green,” said Jennifer Mongolo, senior natural resources planner at the city of Salem. She said herbicide is needed when it’s in big clusters.

“If you try to pull it, a lot of times pieces break off. So you have to be really careful to get all the way down to the root,” she said. “That’s why when there’s a big infestation, it’s better to do herbicide treatment because all of those little fragments that you leave behind can create new plants.”

This week, as treatment is underway, visitors to Riverfront Park can get more information from Willamette Riverkeeper, which has a tent set up with pamphlets and staff.

Vanessa Youngblood, restoration manager, manned the booth along with intern Alex Salazar Monday morning.

The weed will never be fully eradicated from the river system, Youngblood said, just reduced and controlled along with other invasive species.

“There’s not as much funding for it anymore,” she said. Now that it’s largely controlled, “this is our last year of treatment here.”

Future years will include volunteer paddle-and-pull sessions to reduce growth.

The group considers the effort a success. Channels have been opened up, allowing for a better habitat for native creatures and plants, and clearer channels for canoes and other recreation.

“We’re really grateful we were able to do a fourth year. I think that really is huge,” she said.

Along with the clearing, the groups also planted 12,500 native trees and shrubs in March, which largely survived high water levels earlier this year. Those will help create shade so the Ludwigia has less fuel to grow.

Nearby, a blanket of Ludwigia grows untreated at Oxbow Slough, visible from near the third parking lot at Minto-Island Brown park. It’s the groups’ next target.

“It’s in the same situation that this slough was,” said Mongolo. “It is definitely a source of potential reinfestation for this slough that we’ve invested so much in. So that’s our next project, is trying to find funding to do the same kind of project.”

Crews spray blue-tinted herbicide to combat the invasive Ludwigia plant along Minto Island on July 24, 2023 (Abbey McDonald/ Salem Reporter)

UPDATE: This story has been updated with a more precise project budget.

Contact reporter Abbey McDonald: [email protected] or 503-704-0355.

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Abbey McDonald joined the Salem Reporter in 2022. She previously worked as the business reporter at The Astorian, where she covered labor issues, health care and social services. A University of Oregon grad, she has also reported for the Malheur Enterprise, The News-Review and Willamette Week.