Intrinsic and extrinsic controls on sedimentation and the morphology of late Pleistocene to modern estuaries in North Carolina

Coastal morphologies along passive continental margins are the product of complex interactions between erosional and depositional processes that respond to upstream and downstream forcing mechanisms over varying spatial and temporal scales. Upstream controls regulate river-discharge conditions whereby downstream controls govern the distribution of coastal accommodation space and basin hydrodynamics. Understanding how these processes interact over varying timeframes is important for developing accurate models of past and future coastal evolution.Specifically, three aspects of late Quaternary coastal evolution are investigated, including: (1) controls on late Pleistocene valley incision at the highstand shoreline, (2) connectivity between a coastal-plain watershed and its estuarine depositional basin, and (3) influences of coastal setting and respective land-use modifications on fringe-marsh shoreline evolution.Research into controls on incised-valley size and shape shows systems at highstand passive-margin shorelines are equilibrated to the size of their respective drainage basins, which reflect long-term discharge. These findings negate widely accepted models that attribute shelf gradient as being the dominant control on valley morphology and offer tools for investigating the data-limited ancient rock record.The correlation between valley dimension and drainage-basin size degrades for small coastal fluvial systems as slope processes override discharge as the primary mechanism of valley incision. These differences have an effect on resulting highstand river morphology. Large valleys have high accommodation in their low-gradient extensive floodplains and small watersheds on the lower coastal plain have higher fluvial gradients and much less storage space along route. Contrary to large estuaries that are well buffered from changes in sediment load, estuaries associated with small lower coastal plain rivers are directly connected with their watersheds and are more sensitive to climate and land-use changes.Since riverine landscapes of the lower coastal plain are highly connected to estuaries, land-use changes in the basin quickly facilitate mineral accretion and marsh progradation in upper bay environments. In contrast, marshes distally located to these systems receive little of this sediment and are less likely to maintain a favorable intertidal elevation and stable shoreline position with respect to rising sea level and human impacts.