Autocyclic secondary channels stabilize deltaic islands undergoing relative sea level rise

Understanding what sets the size and stability of deltaic islands is critical for predicting how deltas will respond to sea level rise. Models of overbank sedimentation produce an exponentially decaying sedimentation profile, seemingly incompatible with island stability, which requires uniform sedimentation balancing sea level rise. However, secondary channels provide a mechanism for delivering sediment deeper into island interiors, potentially stabilizing islands. Using a 1D morphodynamic model, we found that autogenic secondary channels allow islands or parts of islands to maintain a stable profile dynamically through cycles of channel incision and aggradation. However, when islands are too large, secondary channels grow to become stable, primary channels, thereby bisecting the island, resulting in smaller, stable islands with more connectivity to the channel network. Rather than passively drowning, our results indicate that deltaic islands can respond to sea level rise through morphodynamic feedbacks that act to enhance island accretion.