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River mouth bar formation, a key process in fluvial-deltaic morphodynamics, is subject to both river discharge and waves. Given the increasing variability of both forcings under continuous climate change and human interventions, assessing their combined effects on mouth bar formation is an imperative issue. In this study, an extensive set of combined high and low river flows coupled with varying wave conditions and sediment grain sizes was assumed for numerical experiments conducted in Delft3D-SWAN. The results suggested that three regimes existed for mouth bar formation, namely, stable, ephemeral, and absent. These regimes corresponded to consistently weak, initially-weak-then-strong, and initially strong relative wave strengths, respectively, during the onset and reworking stages. Suppression of mouth bar formation further led to the inhibition of deltaic distributary networks. These findings have important implications for water and sediment management strategies, such as water diversion and dam regulation, in estuaries and deltas to prevent coastal erosion.

Plain Language Summary River discharge and waves are important forces that shape the morphology of deltas. Under continuous climate change and human interventions, both forces tend to be more variable, and predicting their combined effects on the formation of river mouth bars, a key morphological unit in a delta front, becomes important. To address this issue, we carried out extensive numerical experiments to reproduce the formation of river mouth bars under hypothetical unsteady river discharges and wave conditions and explored the inherent patterns of mouth bar formation. Based on the numerical results, we found that mouth bars may form or be suppressed from the start, and the formed mouth bar may subsequently persist or diminish as it continuously evolves, all dictated by the wave strength relative to the variable river discharge. We further showed that the different patterns of river mouth bar formation have cascading effects on the evolution of deltaic distributary networks, another prominent morphological feature of deltas. Our findings have important implications for water and sediment management strategies, such as water diversion and dam regulation, in estuaries and deltas to prevent coastal erosion.