Large river deltas in the Pacific Northwest—low-lying and already flood-prone—face a future marked by increasingly frequent and intense flooding as climate change accelerates. Nature-based restoration can substantially reduce these flood risks.
New Models Show Restoration Can Benefit Salmon While Reducing Flood Risks to Pacific Northwest River Deltas
A new report sheds light on how climate change will exacerbate flood risks to the Nooksack River and delta, located about 100 miles north of Seattle, and how those flood risks can be reduced through nature-based adaptation strategies that also benefit native salmon and trout.
The vulnerable floodplain is home to intensive agricultural production, the city of Ferndale (population 16,000), the Lummi Tribe Reservation (population 6,500), and the primary transportation corridor (Interstate 5) between western Washington and British Columbia; the river itself hosts all five Pacific salmon species and two anadromous trout species. Recent closures of the I-5 corridor due to higher flood risk during high tides is a growing concern given projections of sea-level rise.
Compound Modeling of River and Coastal Flooding
In the study, a flood model was constructed for the lower Nooksack River and delta to evaluate the extent that flood exposure is anticipated to increase with higher sea level and stream runoff, and may be mitigated by habitat restoration strategies.
A key finding of the study—which accounts for the combined effects of higher stream flows forecasted by the University of Washington Climate Impacts Group and higher sea level in the future—is that a "ten-year flood” in the 2080s will be comparable to recent flood events such as the 75-year flood that occurred in November 2021.
The study also highlights the intricate relationship between climate change and coastal dynamics. Rising seas mean that high tides and storm surges will reach further inland. These coastal changes will slow the flow of water downstream and prolong floodwater drainage to the ocean.
The compound-flood model shows the likelihood of increased sedimentation, as higher runoff carries more riverine sediment to the coast. Simultaneously, the more frequent and higher coastal water levels are expected to hinder the outflow of riverine sediment and cause increased sedimentation in coastal and estuarine systems. As more sediment fills river channels, the river conveys less water, and will overtop its banks more frequently. This compound effect is poised to intensify inland flood risks substantially.
Modeling Risk-Reduction Benefits of Restoration Scenarios
The report also emphasizes the flood-risk-reduction benefits of nature-based adaptation planning. Two habitat restoration alternatives, identified to benefit salmon recovery by reconnecting stream flow and expanding rearing habitat, are modeled and shown to substantially reduce flood exposure today (2021) and into the 2040s and 2080s.
As the Pacific Northwest grapples with challenges presented by climate change, the report underscores the critical importance of proactive measures and community-wide efforts to mitigate escalating flood risks in the region’s low-lying river deltas. This information has already been integrated into the Whatcom County Compound Flood Vulnerability Assessment to identify lower Nooksack River areas that may be most vulnerable to flooding from the combination of rising seas, increased rainfall, and increased sedimentation. Similar modeling efforts in the Nisqually and Snohomish estuaries are being integrated into vulnerability and adaptation strategy assessments to address resilience of major transportation corridors and restoration capital investments.
The report builds off of the USGS Coastal Storm Modeling System (CoSMoS), which provided the coastal water level inputs to the model.
The report, “Compound Flood Model for the Lower Nooksack River and Delta, Western Washington—Assessment of Vulnerability and Nature-Based Adaptation Opportunities to Mitigate Higher Sea Level and Stream Flooding”, was prepared by USGS in cooperation with the U.S. Environmental Protection Agency, Washington State Department of Fish and Wildlife, Whatcom County Flood Control Zone District, Nooksack Indian Tribe, and Lummi Tribe.
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