Where do coastlines stabilize following rapid retreat?

We present a numerical model that shows that the transgressing upper shoreline of wave-dominated estuaries (bayhead deltas), which commonly contain populous urban and industrial centers, stabilizes, and their rate of retreat decreases at tributary junctions. The decreased rate of retreat across a tributary junction is caused by a decrease in the total accommodation, while sediment supply remains conserved. Our model predicts that bayhead deltas from smaller systems will be located closer to tributary confluences than their larger counterparts. An examination of the modern bayhead deltas in Albemarle Sound, U.S. Atlantic Coast, reveals that bayhead deltas from smaller tributaries are located closer to tributary confluences than bayhead deltas associated with larger tributaries, supporting our model prediction. Our results highlight the importance of antecedent topography created during falling sea-levels on shaping the nature of transgression during the ensuing sea-level rise. In particular, tributary junctions act as pinning points during transgression. Key Points Bayhead deltas stabilize at tributary junctions during transgression Inherited topography impacts the nature of subsequent transgressions Smaller deltas retreat at slower rates within flooded valley networks