Natural and Restored Tidal Marsh Surface Accretion Trends in the Stillaguamish River Estuary

For centuries, coastal wetlands have been diked and isolated from tidal influence to generate and protect agricultural land, leading to long-term subsidence and vulnerability to sea level rise. Restoration of coastal wetlands can re-establish hydrological connectivity, leading to sediment accretion that can increase long-term resilience to rising sea levels. From 2011 to 2014, twenty-one surface elevation tables (SETs) were installed in the Stillaguamish River estuary to monitor long-term elevation changes in natural and restored marsh zones. This report, created for The Nature Conservancy, details net accretion and erosion rates over the last fifteen years at each monitoring site in the Stillaguamish River estuary using SET data. Surface elevation change rates are utilized to gauge sediment availability and the effects of restoration on the marsh’s capacity to accrete sediment at a rate faster than relative sea level rise in both natural and restored marsh. This report finds that all natural and restored zones in the Stillaguamish River estuary currently have net accretion rates outpacing the rate of relative sea level rise in the Puget Sound region.