Six Rivers, Six Stories

Salmon, orcas, and people all rely on our region’s rivers for clean, healthy water. Invasive plants put these river ecosystems at risk.

Rivers are the lifeblood of Washington. They morphed and changed over millennia as native plants evolved with them to support a thriving, ever-changing river ecosystem. When intact, native plants protect our rivers. They balance erosion, filter polluted stormwater, provide food and shelter to aquatic and terrestrial wildlife, and cast shade—keeping waters cool and rich with oxygen. Deforestation, development, and invasive plants, however, have disrupted these riparian ecosystems—impairing the health of rivers, endangering salmon—and impelling us to act.

Forterra is working to re-establish riparian buffers—the wetlands and plants adjacent to rivers—on six waterways: Bear Creek, Cedar River, Green-Duwamish River, Sammamish River, Skykomish River, and Thornton Creek. Each has its own story, from the wild Skykomish spilling down from the Cascades to the resilient Green-Duwamish, a polluted yet recovering Superfund site in Seattle beloved by the community and championed by conservationists and public agencies. No matter the river, however, invasive knotweed—a once-popular ornamental plant that grows in towering thickets and shoots roots that defy excavation—has proven our greatest restoration challenge as it has escaped from private gardens and entered our river systems.

Knotweed (Polygonum spp.)

Knotweed (Polygonum spp.) is a bamboo-like, non-native plant that wreaks havoc on properties and ecosystems. Once established, knotweed forms dense stands that crowd out native vegetation and clog small waterways, which erode the river’s banks and reduce the suitable habitat for fish and wildlife. It spreads rapidly and is doggedly difficult to remove. Winter floods and beaver activity can spread knotweed downstream even after it has been removed. Without control, knotweed takes over the banks of whole river systems. Controlling knotweed from upstream to downstream is our first step in restoring rivers.

Working together

Removing knotweed in one area won’t improve overall river health. These six waterways each run a long course through public lands and private property, linking strangers, wildlife, forest companies, and distant towns. Forterra works with private landowners, municipalities, and nature itself to address the health of the whole river, not just one part.

We develop and manage restoration projects involving a variety of partners, with concerns about safe drinking water, flood hazards, forest protection, salmon habitat, and property values. We also work in the field with a designated contractor to remove knotweed and re-establish native plant communities. We use grant money to plant native trees and shrubs along private streambanks, at no cost to the landowner. These trees and shrubs help stabilize the banks, store and filter stormwater, and keep temperatures in the creek cool for fish. Our planting projects are designed to fit the context they are installed in—from the urban Thornton Creek to the rural Cedar River.

The future of our rivers

After 10 years, Forterra has removed 98% of the knotweed along the Cedar River. But even there, our work is not done. Left alone, knotweed, which can grow 10 feet in 10 weeks, would quickly reclaim its ground. Invasive weeds can grow at an exponential rate, meaning just a few plants can turn to thousands the following year, requiring constant monitoring and maintenance even in areas with success. The other five rivers still require extensive work, and all six require sustained monitoring and maintenance.

With continued restoration, cedar and Douglas-fir saplings can grow in areas once dominated by knotweed. Native gardens of sword fern and Oregon grape can return to their homelands for healthy riverside habitat and the enjoyment of community members. Water quality and flood control will improve, and salmon populations will become more resilient.

Support riparian restoration

Knotweed spreads easily, regenerating from mere fragments of root or stalk. Successful intervention therefore depends on the participation of all landowners living along these rivers. Learn more about the rivers Forterra works with and how you can be involved.

How can I join?


Forterra’s Riparian Restoration team works with dozens of partners, from city and county governments, other nonprofits, community groups, and individual landowners. Working at a landscape scale is a huge challenge that requires collaboration.

We complete our work to restore riparian ecosystems through funds provided by private donors and: