Pacific Northwest tidal marshes provide valuable ecosystem services and support diverse communities of fish and wildlife. Tidal marshes have complex biogeomorphological feedbacks and can respond to sea-level rise (SLR) by increasing elevation through organic matter inputs and sediment accretion. Sediment supply can vary by watershed, however, making assessments of local accretion important for determining future vulnerability. Additionally, accurate characterization of baseline elevation is important for assessment of current and future conditions. We measured historic accretion rates across eight Pacific Northwest (PNW) tidal marshes using radioisotope dating, and evaluated several watershed-scale metrics as potential explanatory variables. We developed and employed a calibration technique for lidar (Lidar Elevation Adjustment with NDVI, LEAN) to improve baseline elevation accuracy using elevation survey data and multispectral airborne imagery. We then used the Wetland Accretion Rate Model for Ecosystem Resilience (WARMER) to estimate future marsh elevation at each site under three SLR scenarios.
We found that accretion rates across the PNW were positively correlated with long-term freshwater discharge into the estuary and strongly influenced future trajectories of marsh elevation. Using the LEAN method, we reduced initial lidar error by an average of 58%. WARMER projections suggest that marshes will transition from high marsh to low marsh habitat under moderate SLR (+63 cm after 100 yrs), while under high SLR (+142 cm) most will transition to mudflat. The two sites with the highest accretion rates (Grays Harbor and Willapa Bay) were projected to maintain elevation over the next 100 years. We found that using uncorrected lidar or non-local accretion data had substantial effects on model outcomes, with large differences in projected wetland elevations relative to site-specific data. Overall, we found that site-specific data reduce uncertainty in long-term projections of SLR impacts. Current rates of marsh accretion are greater than recent rates of SLR, but accelerating sea-level rise poses substantial risks to tidal marshes and the services they provide.