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. Plain Language Summary River deltas are low‐lying coastal landscapes, making them vulnerable to sea level rise. Deltas are commonly composed of islands separated by river channels. These islands are formed and maintained by sediment supply delivered from the surrounding river channels. Here, we develop a numerical model to explain how sediment is delivered to islands, and how this sediment allows islands to build land vertically, potentially allowing them to keep pace with rising sea level rather than passively drowning. We find that small channels can deliver sediment from river channels to island interiors, allowing islands to keep pace with sea level. Our model predicts that these small channels undergo natural cycles of deepening and shallowing, consistent with observations. Additionally, our model predicts that large islands are eventually split by larger permanent river channels, whereas small islands tend to merge, potentially explaining the range of island sizes found in river deltas. Overall, our model shows that deltaic islands undergoing sea level rise do not passively drown, but rather activate secondary channels and change their size to allow vertical land building to keep pace with sea level. Key Points A 1D deltaic island model produces secondary channels that undergo autogenic discharge oscillations Secondary channels provide a mechanism for maintaining island sedimentation that keeps pace with sea level