HIGH SEDIMENT CONNECTIVITY BETWEEN SMALL COASTAL WATERSHEDS AND ESTUARIES INFLUENCED BY LATE PLEISTOCENE INCISED-VALLEY MORPHOLOGIES

The late Quaternary evolution of two small coastal-plain river systems in eastern North Carolina was investigated from core and seismic data to reveal how incised valleys formed at the last interglacial shoreline in the absence of large watersheds and to assess linkages between valley morphology and modern landscape-estuarine connectivity. Studied systems include the Newport River, which connects to a 210 km² drainage basin predating the Sangamon, and the adjacent North River, which lacks a modern watershed and originates at the last interglacial shoreline.

The subaerial exposure surface beneath the Newport estuary, interpreted as the sequence boundary that formed during the last glacio-eustatic cycle, is terraced, houses a single meandering thalweg, and underlies basal river deposits, suggesting that fluvial incision was the dominant process of valley formation. A structure map of this subaerial exposure surface beneath the North River estuary shows a network of small converging gullies; these channel features likely formed a late Pleistocene watershed not unlike the modern Newport's and may serve as an analogue for understanding the North River's past. Lacking fluvial sand and deltaic deposits, the North estuary is largely filled with bay mud, but possibly contains fluvial deposits down-gradient.

Valley morphologies created by small river networks as sea level fell below the last interglacial highstand shoreline yield relatively steep modern coastal gradients. This morphology facilitates connectivity between landscape and estuary. The modern Newport River bay-head delta has advanced around 0.5 km in the last 100 years. High estuarine sedimentation is confirmed to have begun with the onset of silviculture in the watershed with no measurable time lag, as suggested by ¹³⁷Cs- and ²¹⁰Pb-records. The watershed of the North River has been entirely inundated by sea-level rise, making the estuary more isolated from landscape changes than the Newport. Large river systems, defined by gentler fluvial gradients and higher buffering capacities in the lower coastal plain, are not as well connected with changes to the landscape going undetected at river termini over human timescales.