Landward Transport of Bottom Sediments in Estuaries of the Atlantic Coastal Plain

ABSTRACT The estuaries of the Atlantic Coastal Plain, which are the mouths of rivers drowned during the latest eustatic rise in sea level, are being filled with sediment. River-borne sediment is partially trapped in the estuaries by the predominantly landward flow of estuarine bottom waters. The main evidence of this are measurements of the sediment flux (suspended-sediment concentration and water velocity measured at intervals of depth and through tidal cycles) that show sediment being moved progressively landward along the bottom. Comparisons of the loci of sediment deposition and the patterns of water circulation show that sediment accumulates in estuaries near the upstream limit of landward bottom flow. The movement of sands into the mouths of the larger estuaries from the continental shelf and nearby beaches is also suggested by several other lines of evidence. Bottom waters of the continental shelf move progressively into the mouths of estuaries, and they presumably carry bottom sediments with them. Beach sands move toward and into the mouths of some estuaries at rates of several hundred thousand cubic meters per year. And distinctive mineral components in the lower reaches of estuaries suggest that the bottom sediments were derived from offshore. The rates of filling of the estuaries have been different in the northern and southern parts of the Coastal Plain. The large northern rivers carry disproportionately small loads of sediment that have not yet filled the deep valleys which were cut during the ice ages. The southern rivers carry larger sediment loads relative to the sizes of their valleys, and consequently their estuaries are mostly filled with sediment.