SEDIMENTATION PATTERNS WITHIN TIDAL FRESH-WATER MARSHES, MATTAPONI RIVER, VIRGINIA

Tidal fresh-water marshes form inside meander bends along 35 km of the Mattaponi River. Sedimentation patterns in these marshes vary because of their position along the freshwater-estuarine continuum, their local geomorphic setting, and the region's geomorphic history. Hydrogeomorphic models that describe these changes in sedimentation patterns rely upon data collected during 24-hour (spring tides) and 30-day tile-collection studies in two marshes and from vibracores bored through five marshes. River suspended sediment load and sediment deposition were up to 10 times higher at the more downstream of the two collection study sites, possibly because of its proximity to the estarine turbidity maximum. Sediment deposition rates peaked during the summer at the downstream site, but were remarkably constant at the upstream site during a 15 month period. At the downstream site, sedimentation significantly correlated to plant density, elevation, and flood depth. In addition to these more important site differences, longer flood durations may increase sediment deposition. Analyses of 4 m vibracores 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. The marshes accreted vertically at 1-2 mm/yr, a rate similar to the global average sea level rise during the late Holocene. In four of the five marshes, inorganic suspended sediment accumulated mostly along marsh-river margins; in one marsh interior, cyclic sedimentation may indicate pulses from small tributary basins. Inorganic sediments reached a long-term maximum in most marshes approximately 1000-to-<400 years BP, probably because of increased clearing of fields. The high variability of down-valley patterns suggests that the upper Mattaponi River basin is not the principal source of marsh sediments in the study area.