Short-Term Accretional and Erosional Patterns in a Virginia Salt Marsh

We estimated 3-year average rates of accretion and erosion in different vegetation zones of a juvenile Spartina alterniflora salt marsh at Wallops Island, Virginia, by precise releveling of a fixed grid. Seaward of the marsh there was extremely variable accretion and erosion in tidal flat, as a result of winter ice scouring and transport. At the lower limit of the marsh, tall Spartina edge marsh accreted at about 6.2 mm yr-1 , well in excess of relative sea level rise, supplied by mineral sediments. At the upper limit, levee Spartina and high marsh accreted at about 1.6 mm yr-1, in equilibrium with sea level rise. Accretion there was supplemented by organic sediments from tidal wrack. At mid-elevations, medium Spartina middle marsh eroded slightly at about -0.6 mm yr-1, and low-density Spartina and bare soil eroded rapidly at about -5.3 mm yr-1. These zones may be relatively sediment-starved. The most severe erosion resulted from vegetation diebacks beneath tidal wrack. Patterns of accretion and erosion show that this site is maturing topographically from a juvenile foreshore marsh to a creek-drained marsh.