VARIATIONS IN HOLOCENE SEDIMENTATION PATTERNS ACROSS FRESH-WATER TIDAL MARSHES, MATTAPONI RIVER, VIRGINIA

Variations in sediment characteristics across a regional system of fresh-water tidal (FWT) marsh surfaces suggest that a large number of factors can control sedimentation patterns. FWT wetlands exist along more than 35 km of the Mattaponi River where it passes through incised valleys of the Inner Coastal Plain of Virginia. Analyses of sediment collected during intensive tile-collection studies at two marshes and from pairs of 4m-long vibracores driven through five marshes suggest that overbank fluvial deposition from the main river channel dominate sedimentation patterns only in the uppermost portion of the system. In most of the FWT marshes, sediment patterns reflect many combinations of controls derived from river-estuary interactions, vegetation conditions, geomorphic settings, and geologic history. Increased silt-and-clay loads along the down-river half of the system may be due to the proximity of the estuarine turbidity maximum through the year. Vegetation density, and thus sediment baffling effects, changes significantly both seasonally and spatially during the growing season. Marsh elevation and flood depth also control sedimentation patterns. Inorganic suspended sediment collects more along the river edge in most marshes studied, but pulses of sediment from a small side-valley tributary basin seem to dominate accretion processes across the interior of one marsh. Stratigraphic analyses indicate that marsh and channel positions migrated little during the past 4000 years. At many marshes this stability may be the result of protective and anchoring cliffs upstream, formed during a Holocene(?) episode of meander migration and incision. Across the FWT system, the marshes accreted vertically at a rate similar to the global average sea level rise during the late Holocene (1-2 mm/yr). Existing sediment data suggest that the average position of the fresh-water-salt-water interface migrated very little during the same period.