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Abstract Seasonal sea ice impacts Arctic delta morphology by limiting wave and river influences and altering river‐to‐ocean sediment pathways. However, the long‐term effects of sea ice on delta morphology remain poorly known. To address this gap, 1D morphologic and hydrodynamic simulations were set up in Delft3D to study the 1500‐year development of Arctic deltas during the most energetic Arctic seasons: spring break‐up/freshet, summer open‐water, and autumn freeze‐up. The model focused on the deltaic clinoform (i.e., the vertical cross‐sectional view of a delta) and used a floating barge structure to mimic the effects of sea ice on nearshore waters. From the simulations we find that ice‐affected deltas form a compound clinoform morphology, that is, a coupled subaerial and subaqueous delta separated by a subaqueous platform that resembles the shallow platform observed offshore of Arctic deltas. Nearshore sea ice affects river dynamics and promotes sediment bypassing during sea ice break‐up, forming an offshore depocenter and building a subaqueous platform. A second depocenter forms closer to shore during the open‐water season at the subaerial foreset that aids in outbuilding the subaerial delta and assists in developing the compound clinoform morphology. Simulations of increased wave activity and reduced sea‐ice, likely futures under a warming Arctic climate, show that deltas may lose their shallow platform on centennial timescales by (a) sediment infill and/or (b) wave erosion. This study highlights the importance of sea ice on Arctic delta morphology and the potential morphologic transitions these high‐latitude deltas may experience as the Arctic continues to warm. 

In this study, we developed a 1D Delft3D-FLOW model to simulate the temporal development of the Colville River Delta, Alaska during the most active Arctic seasons: break-up/freshet (spring), open-water (summer), and freeze-up (fall). Simulations focused on the deltaic clinoform (i.e., the cross-sectional view of a delta) and used a floating barge structure to mimic the effects of sea ice on surface waters. Delft3D simulations were coupled with modules written in MATLAB and outputs were process in MATLAB. Arctic delta morphology is impacted by seasonal sea ice coverage which significantly limits wave and river influences and alters river-to-ocean sediment dispersal. However, our knowledge of the morphologic influences of ice on delta morphology is relatively limited. To assess the role of sea ice, our study used a model to explore long-term Arctic delta development under seasonal ice, river, and wave conditions. Delta developmental simulations (spanning 1500 years) included ice-free and ice-affected cases. These cases consisted of simulations with and without waves to separately examine and compare sea ice, river, and wave impacts on Arctic delta evolution. Long-term delta developmental simulations showed ice-affected deltas form a compound clinoform morphology – a coupled subaerial and subaqueous delta separated by a subaqueous platform that resembles the shallow 2 meter platform observed offshore of Arctic deltas. In the model, the presence of nearshore sea ice and river forcing promoted sediment bypassing during break-up, forming a depocenter up to 6 km away from the river mouth and were the key drivers behind platform formation. Furthermore, six varying sea-ice characteristics (extent and thickness) were evaluated to examine effects on delta development for the first 500 years. We found that the compound clinoform morphology modulated by sea ice was heavily dependent on ice conditions, with closer and thicker sea ice produced a more elongated subaqueous platform. In addition to long-term delta developmental simulations, we examined two future scenarios (spanning 450 years) to assess future Arctic delta morphology with less seasonal sea ice coverage and larger waves as predicted by climate models. Modeled future simulations showed Arctic deltas may lose the shallow 2 meter platform feature on centennial timescales by (1) sediment infill or (2) wave erosion. This study highlights the importance of sea ice on Arctic delta morphology and the potential morphologic transitions these high-latitude deltas may experience as the Arctic continues to warm. The dataset includes an example model run file (Delft3D-FLOW and MATLAB) and output of results from the described simulations. Files include: 1) Example Delft3D-FLOW model setup file, MATLAB run script, and ice files for a 1500-year simulation. 2) Processed MATLAB structures and metadata for model results A) Long-term Delta Developmental Outputs (1500-year simulations) i) Ice-free ii) Ice-affected iii) Ice-free with waves iv) Ice-affected with waves B) Varying Sea Ice Characteristics Outputs (500-year simulations) i) Ice matrix (six simulations) C) Future Arctic Delta Scenarios Outputs (450-year simulations) i) Scenario A ii) Scenario B