Monitoring changes in surface elevation using deep Surface Elevation Table and Marker Horizon data at the Nisqually River Delta, Washington

Surface Elevation Tables and Marker Horizon (collectively SET-MH) datasets provide a unique opportunity to evaluate tidal marsh accretion rates compared with current and projected sea-level rise. SET is a tool that allows for accurate and repeatable measurements of marsh elevation, while Marker Horizon allows for the measurement of sediment that has deposited on top of the feldspar marker. SETs are deep rod benchmarks with an attachment for a portable leveling device (arm) at fixed directions. The distance from the fixed arm to the marsh surface is measured by lowering a set of pins (usually nine) from the SET to the marsh surface, providing a repeatable and accurate measurement of elevation change. Marker horizon data measure the amount of sediment that is deposited onto the marsh surface, is a layer of white feldspar clay applied to a 0.5x0.5m quadrats associated with each SET. Marker horizons are measured by extracting a plug from the marsh surface using a knife or cryo-core, and measuring the sediment deposited on top of the layer. Together, repeated measurements of SET-MH data separates surface deposition from shallow subsurface processes (e.g., root growth or shallow soil compaction). The ability of a tidal marsh to keep up with sea-level rise was largely due to relative sediment load and to a smaller degree it’s position within the tidal frame.