Marshes to Mudflats: Climate Change Effects Along a Latitudinal Gradient in the Pacific Northwest

Karen M. Thorne, Bruce Dugger, John Y. Takekawa
Posted on: 7/18/2022 - Updated on: 7/11/2023

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In the Pacific Northwest, coastal wetlands support a wealth of ecosystem services including habitat provision for wildlife and fisheries and flood protection. The tidal marshes, mudflats, and shallow bays of coastal estuaries link marine, freshwater, and terrestrial habitats and provide economic and recreational benefits to local communities. Climate change effects such as sea-level rise are currently altering these habitats, but we know little about how these areas will change over the next 50-100 years.

Our study examined the effects of sea-level rise on nine tidal marshes in Washington and Oregon, with the goal of providing scientific data to support future coastal planning and conservation. We compiled physical and biological data, including coastal topography, tidal inundation, vegetation structure, and current and historic sediment accretion rates to assess and model how sea-level rise may alter these ecosystems in the future. Multiple factors, including initial elevation, marsh productivity, sediment availability, and rates of sea-level rise affected marsh persistence. Under a low sea-level rise scenario, all marshes remained vegetated with little change in the present configuration of marsh plant communities or gradually increased proportions of mid, high, or transition marsh vegetation zones. However, at most sites mid sea-level rise projections led to the loss of middle and high marsh and a gain in low marsh habitat. Under a high sea-level rise scenario, marshes at most sites eventually converted to intertidal mudflats. Two sites (Grays Harbor and Willapa) appeared to have the most resilience to a high sea-level rise rate, persisting as low marsh until at least 2110.

Our main model finding is that most tidal marsh study sites have resiliency to sea-level rise over the next 50-70 years, but that sea-level rise will eventually outpace marsh accretion and drown most high and mid marsh habitats by 2110. 


Thorne, K.M., Dugger, B., Takekawa, J.Y. (2015). Marshes to Mudflats: Climate Change Effects Along a Latitudinal Gradient in the Pacific Northwest. U.S. Geological Survey.

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