Effects of Increasing Coastal Sediment Loads on Tidal-creek Levee Morphology

Salt marshes act as metronomes of sea-level rise, recording changes in inundation within their soils. When flooded for too long, marshes drown Yet without water delivering sediment and nutrients they cannot thrive. The ecosystem services they provide, such as nutrient filtering, carbon sequestration, and storm surge attenuation, hinge on their ability to keep pace with rising sea levels. One way in which marshes persist is through the trapping of sediment delivered by tidal creeks. The preferential deposition of sediments close to these conduits commonly produces vertically distinct natural levees. Such widely observed features have a higher sediment supply but are inundated for less time than the lower elevation salt marsh interior and are also thought to accrete at the rate of sea-level rise. Due to their positioning relative to sediment sources and within the tidal frame, however, levees potentially record changes in sediment loading differently than marsh interiors. To examine the function of salt marsh levees as buffers to sediment transport we collected transects of creek-normal, 6-meter spaced cores at multiple sites from the creek into salt-marsh platforms. We then developed detailed records of sediment and mass accumulation rates (cm y-1 and g cm-2 y-1) in North Carolina since ~AD 1900 using radiometric dating techniques. Using these rates, we determine whether the elevations of levees in various watersheds have recently increased as a result of tidal-creek sediment loading and if this negatively correlates with interior marsh sedimentation rates, a predictor of marsh vulnerability to sea-level rise. We contextualize these rates by relating them to patterns of land-use change recorded from remote imagery within their small coastal-plain watersheds, estimating salt marsh response to future development-induced sediment loading.