GEOMORPHIC EVOLUTION OF A FRESH-WATER TIDAL ESTUARY DURING LATE QUATERNARY SEA LEVEL AND CLIMATE CHANGES

Variations in sedimentation style and in landform patterns indicate that changes in sea level and climate influenced the Late Pleistocene and Holocene evolution of the lower Mattaponi River, eastern Virginia, USA. In the fresh-water tidal (FWT) section of the river, large wetlands with thick (>3m) mucky silt or clay lie inside sinuous meander bends. Some wetlands may have formed as the channel migrated laterally, but most marshes accreted vertically, particularly at sites downstream of obdurate cliffs that influence the thalweg at high flows. Proximity to the main channel controls only minor variations in marsh sedimentation at most sites, usually recognized as either a slight coarsening of the silt fraction or an increase in clastic sediment. Significant sedimentation from high fluvial flows is recognized only in the upper reaches of the FWT zone, but lower reaches may receive considerable volumes of fine sediment moving with the estuarine turbidity maximum. Across the FWT zone, meander bends change from symmetric (tide-dominated) to asymmetric (river-dominated); farther upstream across the Coastal Plain, the river appears underfit with the sinuous river channel flowing in a valley with large moderately sinuous meanders. All of these active meanders impinge on fluvial and marine terrace sediments, the youngest having been deposited by low-sinuosity streams transporting mostly sand and gravel.

Interpretation of these observations suggests that a relatively straight Mattaponi River transported mostly bedload from the crystalline Piedmont uplands during the most recent Late Pleistocene (stage 2?), a period of pronounced cryoturbation in that upland. Later, during the same period of lowered sea levels, large sinuous meanders developed as the river channel incised both downward and outward. The causes of the wider channel (e.g. higher discharges?) are unknown. As sea level rose during the late Holocene, most channel meanders in the FWT system maintained their position as the marshes accreted vertically at a rate similar to the global average sea level rise (1-2 mm/yr). Existing sediment data suggests that the average position of the fresh-water-salt-water interface migrated very little during the same period.