The Nooksack and Sumas Watershed Transboundary Flood Initiative

During the December 2025 atmospheric river event, emergency managers used improved cross-border flood forecasting and warning systems, including real-time data from new flood gauges and cameras, to issue earlier alerts and warnings and refine evacuation areas. Flood engineers and planners found the improved hydraulic models were helpful in predicting water depths and timing of flood waters — critical information for both managing emergencies and designing flood management projects.

TFI partners continue to meet regularly to coordinate local work plans and share information. These strong, collaborative relationships remain a priority and have continued to grow.

In 2025, the TFI streamlined coordination and review of several projects, including:

• A joint habitat assessment project in the Sumas Watershed
• Replacing a state highway culvert near Everson
• Sediment modeling in Whatcom County

Joint habitat assessment in the Sumas Watershed

The TFI partners, led by Semá:th First Nation and Nooksack Tribe, are working together on a project to understand and improve aquatic health in the Sumas Watershed. The project looks at water quality, fish habitat, streamside areas, fish passage, cultural site access, land connections and species at risk to set priorities for restoration in the watershed. Partners are also collecting DNA samples from Sumas River salmon, as part of the first assessment of salmon populations.

The team is starting with a pilot study in the Saar and Arnold sub-watersheds, which cross the U.S.-Canada border. They are using data and aerial photos to map streamside conditions and find areas for restoration. This study will help guide plans to include nature-based flood solutions in local projects.

To move this work forward, Canadian TFI partners hosted a tour of key sites on the north side of the watershed, including Barrowtown Pump Station, Sumas River Floodboxes, the former Sumas Lake (Semá:th X̱ó:tsa), and the Semá:th fish weir.

The event gave First Nation partners a chance to share knowledge about their traditional lands and let all partners see the area in person. Seeing these issues and challenges helps everyone build a shared understanding and adds real-world context to the technical work. This will lead to better flood and habitat solutions for everyone.

Replacement of the State Route 544 culvert

Protecting critical infrastructure is a key flood planning priority. An example of critical infrastructure is State Route (SR) 544, which crosses the Nooksack River at the city of Everson. On the south side of the main bridge, there is an arched culvert (tunnel) that carries extra water during a flood. The culvert was damaged in the November 2021 floods and received an emergency repair.

The Washington State Department of Transportation (WSDOT), which manages SR 544 and its crossings, plans to construct a permanent fix in 2026. WSDOT will replace the culvert with a new bridge to allow more floodwaters to flow through the area. This is intended to help protect the existing bridge and reduce. The TFI Technical Table reviewed and provided feedback on design plans in June 2025.

Whatcom County sediment modeling work

Understanding how and where sediment accumulates in the Nooksack River is a major factor in effective flood management. According to the U.S. Geological Survey, the Nooksack River carries nearly a million tons of sediment every year.

Sediment in the Nooksack does not behave like rocks placed in a bucket of water. It is much more complex. The river bottom is constantly changing with floods and long-term climate patterns, which makes modeling and predicting sediment dynamics challenging.

Sediment is not deposited uniformly along the river’s length. As the river flows to the ocean, more gravel moves along the riverbed at Everson, Washington (WA) than it does at Ferndale, WA. Coarser sediment tends to settle out of the river between Deming and Everson, where the land becomes flatter and the river slows down. Sediment can also build up faster where levees and revetments (structures to protect against bank erosion) confine the river. These changes in the riverbed can make flood impacts difficult to predict.

Whatcom County developed models to better understand how sediment moves along the river at different water levels and how it affects flooding. The models looked at current and future conditions to provide important information for project planning. A summary of this work is in the “Everson Corridor Sediment Modeling Investigation Report” (2025), on Whatcom County’s website.

One of the TFI’s goals is to share data about flood conditions, land use, priorities and risks. This creates a common technical understanding on both sides of the border. This shared understanding will:

• Create a framework to help local decision-making
• Guide projects that combine flood management with habitat restoration

Technical milestones reached in the first year include:

• Alignment of the United States and Canada hydraulic models
• Improved flood warning

Alignment of the U.S. and Canadian hydraulic models

Hydraulic models predict where floodwaters will go and their impacts. The United States and Canadian models are consistent with each other and can share data and projections.

Improved flood warning

The Nooksack River changes depth due to sediment. This makes it challenging to forecast floods. The National Weather Service worked with TFI partners to improve the flood forecasting protocols for the Nooksack River. This resulted in new flood impact statements and flood warning bulletins for the Everson Overflow Corridor. 

Meeting together and spending time on the land is an opportunity for First Nations to share knowledge, and it’s also an opportunity for the respective partners to have their technical and policy folks, and decision makers to exchange ideas. Sharing that time together helps us to articulate the challenges, issues and opportunities for managing floods and restoring habitat.

 — Murray Ned, Semá:th First Nation Advisor

There are many agencies involved in transboundary flood issues and things become difficult when you start bringing a lot of people into the room, especially with varying levels of government. TFI has helped us collectively establish working relationships and begin complicated work.

— Justin Laslo, Leq’á:mel First Nation

TFI provides an opportunity for governments on each side of the border to understand others’ priorities and objectives. The members of the TFI are coordinating complex data across the international border, improving flood warning systems, and eventually will design projects to reduce risks while protecting the environment. We have made tremendous progress over the first two years of the initiative, and I look forward to building on that success to benefit the communities we serve.

— Tom Buroker, Washington State Department of Ecology