Abstract

Small rivers that form at the apex of the coastal prism are common along many passive margins. They typically have drainage basins < 1000 km², and their headwaters are isolated to low-relief areas of the previous highstand shoreline. As sea-level fall exposes the apex of the coastal prism, incision and drainage-basin evolution of those small rivers should be strongly influenced by coastal paleotopography, but previous work suggests that paleodischarge is the dominant control on incised-valley dimension. To examine the controls on the morphologic evolution of small incised valleys, we examined 20 incised valleys in North Carolina situated close to the last interglacial shoreline, including coastal-prism-incised valleys, which have well-developed dendritic drainage networks on the highstand-shoreline topography, and tributary incised valleys, which form adjacent to and merge with larger piedmont incised valleys. Coastal prism (n  =  18) and tributary (n  =  2) incised-valley width, measured at the common bayhead-delta location, is proportional to drainage-basin area (long-term discharge); however, that comparison location overlaps with the knickzone only for the larger river systems. The knickzone of small river systems is located down dip from the modern bay-head delta, which is likely due to slower rates of knickpoint migration. We use geophysical and lithologic data from three incised valleys to highlight differences in regional valley morphology that can be used to distinguish between coastal-prism and tributary incised valleys. The Newport and North coastal-prism incised valleys connect to small Sangamonian drainage networks of 210 km² and 50 km², respectively. Those valleys widen and deepen at the knickzone, which is also where smaller tributary systems merge with the main trunk valley. The Scuppernong tributary incised valley has a drainage basin of 400 km² and can be differentiated from other incised valleys based on the absence of a compound fill and continuous seaward deepening and widening from the upstream limit of the knickzone. The knickzone is the junction of the Scuppernong with the adjacent larger Roanoke incised valley, the depth of which controls base level for the Scuppernong. The dramatic Scuppernong valley widening, in a down-dip direction, is principally due to highly erosive bay-ravinement processes associated with the adjacent much larger drowned Roanoke incised valley. These varying modes of valley evolution, specifically controls of paleotopography on valley incision and ravinement, explain the morphologies of small drowned river-mouth estuaries and improve understanding of basin-scale sediment-transport pathways.