RESPONSE OF BAY-HEAD DELTAS TO SEA-LEVEL RISE

Bay-head deltas (BHD) are located at the head of drowned river-mouth estuaries and encompass the ecologically important tidal freshwater zone. The Holocene sedimentary record along the North Carolina Atlantic and northern Gulf of Mexico coasts show that BHDs shifted landward rapidly (back-stepped) during a period of time when sea level was rising at a rate similar to what is predicted by the year 2100. Back-stepping events are commonly attributed to decreases in the rate of sediment accumulation and increases in the rate of sediment accommodation. Sediment accommodation is largely modulated by the rate of sea-level rise and the morphology of the area being inundated. In Mobile Bay, Alabama the bay shoreline transgressed at a rate of ~100 m/yr beginning around 8.2 ka. During this time, the extensive delta plain and marsh were eroded and replaced by a central-basin environment. This change occurred as a threshold in the rate of accommodation-space creation was crossed during the early Holocene in which low-gradient antecedent topography was rapidly flooded by sea-level rise.

Tributary junctions may be an important, but previously overlooked threshold in BHD evolution. Most drowned river-mouth estuaries are fringed by smaller drowned tributary estuaries. Prior to inundation, these tributaries merged with the main trunk valley contributing their discharge to a larger BHD depositional environment. As tributaries are inundated, their drainage basins are isolated from the main trunk valley. This causes an abrupt decrease in discharge to the BHD, bisection of the BHD, and BHD inundation and rapid landward retreat. This conceptual model needs further testing, but research in Galveston and Matagorda bays along the Northern Gulf of Mexico and Albemarle Sound and the Newport River Estuary along the mid-Atlantic margin supports applicability of the model. Many modern BHDs are located at or near tributary junctions that are at an elevation below 50 cm, making this depositional environment particularly vulnerable to future sea-level rise.