- Background: Tidal wetlands are important habitats for salmon and a diversity of other fish and wildlife
species. They also trap sediment, buffer coastal communities from flooding and erosion, and perform
other valued ecosystem services. Tidal wetlands currently exist just at and above sea level, and healthy
tidal wetlands are able to adapt to slow sea level changes. But if sea level rises too fast, tidal wetland
plant communities may not be able to persist at their current locations. To survive, these plants may
have to move to areas of higher elevation. These higher areas are called “landward migration zones”
("LMZs"); they are potential future tidal wetlands under sea level rise ("SLR"). This project modeled and
prioritized these LMZs. It was sponsored and supported by the MidCoast Watersheds Council (MCWC)
and the Pacific States Marine Fisheries Commission and funded by the Oregon Watershed Enhancement
Board and the U.S. Fish and Wildlife Service's Coastal Program.
Geographic scope: This project mapped LMZs for 23 estuaries on Oregon's coast south of the Columbia
River. From north to south these are: Necanicum River, Nehalem River, Tillamook Bay, Netarts Bay, Sand
Lake, Nestucca Bay, Salmon River, Siletz Bay, Yaquina Bay, Beaver Creek, Alsea Bay, Yachats River,
Siuslaw River, Umpqua River, Coos Bay, Coquille River, New River Area, Sixes River, Elk River, Rogue
River, Pistol River, Chetco River, and Winchuck River.
Modeling approach: This project used an elevation-based method (modified bathtub approach) to map
current and future tidal wetlands. Elevation was obtained from LIDAR; projected SLR was obtained from
recent, authoritative, and region-specific scientific literature. LMZs were modeled for six SLR scenarios
that could be expected between now and the year 2160, but this study did not assume any specific
timeframe for the scenarios modeled. Both lower and upper boundaries for LMZs were mapped, to
allow determination of areas that would be lost due to conversion to mudflat under each SLR scenario.
Wetland types mapped: This project mapped potential future tidal wetlands in three vegetation classes:
marsh, shrub and forested. We did not attempt to map the specific locations of each vegetation class,
because the necessary data are not yet available. The study did not map seagrass beds, because their
distribution is controlled not just by elevation, but also by other factors like water clarity and substrate
type. However, the mapping does show areas that transition from vegetated tidal wetland to mudflat
with rising sea level.
Diked and developed areas: The mapped LMZs are at appropriate elevations to support vegetated tidal
wetlands, but may currently lack a connection to tidal waters (e.g. they might be behind a dike or tide
gate). Mapping these areas helped identify lands vulnerable to SLR. The LMZ mapping did not exclude
developed areas such as roads, parking lots, urban, industrial, or residential areas. Developed areas
within LMZs may be at risk for inundation under SLR, but they are not likely to be suitable as future tidal
wetlands. We accounted for developed areas by separately summarizing the area of LMZs on
impervious versus non-impervious surfaces.
Sediment accretion: This study's LMZ maps did not account for rates of sediment accretion (although
results were compared to models that do account for sediment accretion). Accretion is an important
factor, but data on variability in accretion rates on the Oregon coast are lacking. Local groups may wish
to incorporate local accretion data when interpreting study results.
Products: This study's products include a PDF map of the 4.7 ft SLR scenario for each estuary; a PDF
prioritization map based on the 4.7 ft SLR scenario for each estuary; a presentation that explains the
project and its products; a project flyer; and geospatial data (shapefiles of LMZs for the six SLR scenarios
modeled, and a shapefile containing prioritization data and results). Products are available from MCWC: