Predicting Long-Term Trajectories of Coastal Marshes: Processes, Models, and Uncertainty.
Coastal marshes are ecologically and geomorphologically important environments that are increasingly threatened by climate change and human activities. Anticipating their long-term (decadal to centennial) trajectory is critical for assessing vulnerability and informing restoration strategies. A wide range of process-based models have been developed to represent marsh morphodynamics across spatial scales and levels of complexity, yet persistent challenges remain in translating field observations-such as suspended sediment concentration, settling velocity, and sediment porosity-into effective long-term model parameters. As a result, these models are often more successful at elucidating general mechanisms and feedback than at making site-specific predictions. Data-driven approaches, in contrast, are commonly used to assess marsh drowning and vulnerability at specific locations but typically neglect key feedback associated with evolving landscape geometry and sediment mass conservation. Tracking sediment mass trajectories under different forcing scenarios would facilitate comparison across models and support the transfer of insight between physics-based, exploratory, and data-driven models. In addition, as marsh trajectories are increasingly shaped by restoration and sediment management practices, future outcomes will depend not only on natural processes but also on resource management decisions. Here, we review fundamental considerations and various approaches to modeling marsh trajectory and highlight recent advances and research gaps.