VCR-LTER Student Conference, September 1996 Sediment Deposition on a Tidal Salt Marsh

Trine Christiansen Department of Environmental Sciences University of Virginia

A tidal salt marsh provides an ideal setting for studying flood plain deposition because over bank flow occurs every day during high tide. For this study we have selected a mainland fringing salt marsh in Hog Island Lagoon on the Eastern Shore of Virginia. The marsh is vegetated with Spartina Alterniflora (salt marsh cord grass). The tidal range in this area is 1-1.5 meters and the water depth on the marsh surface reaches 1 meter on the highest tides. Dating of sediment cores using 210^Pb isotopes indicate deposition of 1 to 2 millimeters per year. Sediment observed in suspension during the flooding tide is very fine grained (in the silt and clay range). Sediment is advected on to the marsh surface from an adjacent tidal creek. The advection velocities are very low (between 0 and 4 cm/sec). Magnitude and direction changes depending on whether tide is rising or falling. The velocities tend to be greater on the falling tides. We think that resuspension of sediment from the marsh surface may occur on the falling tide when velocities are greatest. We have used optical back scatter instruments to measure suspended sediment concentrations along a transect perpendicular to the tidal creek. Concentrations are highest on the rising tide, and decrease to a constant low level on the falling tide. Concentration also decreases with distance from the creek. It is expected that concentration levels and hence rate of deposition is related to to innundation duration and frequency of the marsh surface. Water levels on the marsh have been measured for one year. In addition we have access to long term records of tidal variation measured at a nearby tidal gauge and to wind speed and direction also measured in this area. We have used this information to determine the types of weather patterns that produce prolonged innundation of the marsh surface and thus are expected to enhance sediment deposition. North Easterly storms produce the highest water levels on the marsh and these storms are most common in the winter season between October and April.