kcweeds

Flooding & Weeds

December 2025 was a record-breaking month for floods in King County, Washington. Floodwaters reached levels we haven’t seen since the 1990s in some places. Rivers across the county reached moderate to severe flood levels, with five out of the six major rivers in King County reaching severe flood phase 4.

Staff from across the County’s Department of Natural Resources—including five folks from the Noxious Weeds team—came together to work tireless flood patrol shifts, logging a combined 952 hours during the flood period (more on these efforts here). 

Four workers in safety vests walk along a sandbag barrier near a swollen river, with trees in the background and cloudy skies.
A person dressed in a bright orange jacket and yellow hard hat is standing by a muddy river, taking notes on a tablet. Surrounding vegetation includes bare trees and some shrubbery.
Flood patrol photos from our noxious weed staff member who went to monitor the White River after the flooding started to go down.

The perfect recipe for flooding 

Flood risk is part of life in King County—so much so that multiple programs are dedicated to assessing and mitigating it. We’re situated in one of the rainiest regions in the United States, with peaks and valleys, snowmelt-fed rivers and creeks running like veins across the landscape. 

Add in human development—roads built alongside rivers, property boundaries that edge up to waterways, dam infrastructure—and the growing influence of climate change, and it’s no surprise that King County is especially vulnerable to flooding events. 

A flooded area with muddy water covering the ground, trees in the background, and a person walking along a green path.
Emergency responders in high-visibility gear navigating a flooded road at night, surrounded by trees and power lines.
A nighttime scene showing sandbags arranged along a waterway, illuminated by a flashlight, with a tree in the background and scattered stones on the ground.
Photos taken by our staff during flood patrol shifts along the Green River (1st & 2nd) and Cedar River (3rd).

So…what does this have to do with weeds? 

From a program perspective, flooding has shaped our work for decades. A core part of our team focuses on controlling noxious weeds along riparian corridors—especially knotweed. 

That work really took off in 2004, following the major floods of 1995–96. Those events triggered a dramatic expansion of knotweed along our major rivers. Now, 30 years later, we look back on that experience to help anticipate what might come next. 

Generally, weeds aren’t great at erosion control.

This is one of the many reasons we focus efforts on weed control in floodplain areas. Many of our widespread weeds are abundant in areas where soil has been disturbed because they’re good at reproducing in the most extreme conditions. Unfortunately, many of these weeds have shallow root systems relative to some of our native plants. When strong river conditions come in contact with monocultures of plants, entire swaths can uproot and spread downstream, leaving the underlying soil vulnerable to further erosion from water and wind. This is just another reason riparian restoration and native planting is so important!

Looking back to look forward 

Beyond the botany, our team has been working with weeds in the floodplains for almost 30 years and have a lot of past experience to look back on that can inform future projections when it comes to flooding & weeds.

A changing landscape 

Historic flooding in the ’90s reshaped entire sections of land, and we can anticipate the 2025 flood will have similar effects. When large chunks of earth are displaced, the topography changes—and so does the way water moves through it. Rivers carve new paths, carry sediment, and reshape the land over time. It can take years for things to settle. 

Dorre Don Natural area along the Cedar River is a great example of how flooding has changed our project access approaches. The photo on the left is 2017, the right is 2023. You can see how the main channel moves from the right to the left side, expanding a former back channel into the main one.

Our team has followed the same survey routes for years, so when big landscape changes happen, we get a rare opportunity to rediscover familiar places. It pushes us to step back, get reoriented, and approach the land with fresh eyes—finding new, and sometimes complicated ways to access old sites and adapt to a shifting landscape.

A muddy forest floor covered with ferns and scattered sticks, with trees in the background.
A foggy forest scene featuring a sandy area with patches of grass and fallen leaves, and a person in an orange suit walking in the background among trees.
Early flood impacts from a 2026 survey on the Cedar river. Flattened vegetation and heavy silt/sediment deposits in new places.

A changing PLANTscape 

Water doesn’t just move soil—it moves seeds and plant fragments, too. Species like knotweed can spread through broken pieces that take root in new locations. 

Because we’ve tracked specific species for years, we expect to see shifts in plant distribution over the next few seasons as seeds settle and soils stabilize. 

We’re already seeing early signs. During our 2026 surveys, we’ve documented changes in both native and weedy species. After the 2020 floods, our specialists noted a large influx of monkeyflower from a protected watershed. This season, we’re finding big patches of wild basil along the Cedar River—a noxious weed we haven’t seen in these areas before. 

We’re especially curious to see how flooding will affect existing infest areas of species that spread by seed, like garlic mustard, versus those that spread through fragments, like invasive knotweed. 

A fallen tree stump covered in moss beside a small stream, surrounded by muddy ground and sparse green vegetation.
A close-up view of shallow water covered with small green plants and ripples from raindrops.
Photos from a garlic mustard survey in April 2026 along Soos Creek illustrates how seedy species can thrive in flood conditions, even surviving submergence.

How this informs our work 

If you work in natural resources in the Pacific Northwest, floods are always part of the equation. We plan around the wet season, build flood scenarios into our fieldwork, prepare emergency gear, and monitor river flow rates—especially on days when crossings are involved. Many of us also train in swiftwater safety. 

For our team, that also means thinking ahead: how will major flooding events reshape weed distribution, and how do we adapt our response? 

A scenic riverbank with a rocky shore, surrounded by trees and a clear sky.
This is the former backchannel, and current main channel for the Dorre Don site (map slider above). The debris and rocks here were scoured and dumped from upstream during the flood.

What can we do about it? 

For now, we wait and see. 

The long-term impacts of flooding take time to unfold. Water needs to find its new paths, soil needs to settle, and seeds and plant fragments need time to establish. We can make informed predictions—but ultimately, nature will show us what comes next. 

We’ll be sharing updates in the coming seasons. Read on to check out flood-relevant resources and stories. 

In the meantime, we’d love to hear from you—how have floods changed the landscape or plant life in your area? Send us your photos and stories, and we may include them in a future write-up. 

Until then, stay safe—and happy weeding! 

Relevant resources 

King County flood services – King County, Washington: King County Government’s official landing page for all things flood, including maps, flood alerts, preparation tips, contact information, and more. 

By the numbers: A look back at King County’s response to historic flooding – DNRP Field Notes: A write up on how King County responded to the 2025 flood, thank you to all of our flood patrol first responders! 

King County floodplain maps – King County, Washington: Interactive flood maps of King County where you can assess the flood risk of any location in the area, see historic flooding photos, and see live flooding statistic updates during a flood event. 

Flood recovery – King County, Washington: Resources and potential funding support for landowners impacted by the 2025 floods.

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