THE SOURCE OF ALLUVIUM IN LARGE RIVERS OF THE LOWER COASTAL-PLAIN OF NORTH-CAROLINA

Despite high erosion rates in the North Carolina Piedmont and extensive sediment delivery to Coastal Plain reaches of Piedmont-draining streams, sediment budget studies and estuarine sedimentology suggest that relatively little Piedmont-derived sediment reaches the lower Coastal Plain. Dominant sources of alluvium in the lower Coastal Plain reaches of Piedmont-draining rivers were inferred from alluvial floodplain soils. Piedmont-derived alluvial soils are characterized by mixed mineralogy and presence of mica flakes. Alluvial soils derived from erosion of Coastal Plain uplands are characterized by siliceous mineralogy and an absence of mica flakes. The Coastal alluvial soils which occur in fluvial/estuarine transition zones being drowned by rising sea level are organic or buried organic soils. The distribution of these soils along the Tar and Neuse Rivers in the lower Coastal Plain shows almost complete domination of fluvial-estuarine transition zones by soils of the Coastal group, and by soils of the Coastal Plain and Coastal groups for 40 km or more inland of the upper estuaries. No soils of the Piedmont group were found in fluvial-estuarine transition zones or lowermost fluvial reaches. This suggests that the Piedmont is a relatively insignificant source of alluvium in the lower reaches of the rivers. The absence of Piedmont sediment in the lower Coastal Plain is attributed to extensive sediment storage upstream, dilution by Coastal Plain sediments, and Coastal Plain erosion rates which may be higher than was previously believed.