Control of river discharge on large‐scale estuary morphology

Abstract Estuaries are dynamic landscapes with complex bar and channel patterns formed by interactions between tidal and fluvial currents. River discharge dampens the tidal wave, enhances the ebb flow, and supplies sediment to the estuary. However, it is largely unknown how river discharge influences overall estuary morphology. The objective of the current study is to quantify the control of river discharge on bar and channel dimensions and sediment transport throughout the estuary. To this end, a long‐term and large‐scale Delft3D‐2DH estuary model was designed with a suite of model runs undertaken where discharge systematically varied. Results show that tide‐dominated estuaries with significant river discharge can develop towards a dynamic equilibrium with a constant tidal prism through adjustment of channel dimensions to accommodate the supplied river discharge. It is essential to account for this morphodynamic adjustment when considering the transition from tide‐dominated estuaries to aggrading river‐dominated estuaries. After this transition, the estuary evolution depends on the discharge‐to‐width ratio. Tidal prism either decreases with higher river discharge as the tidal flow is dampened and the estuary aggrades, or increases when the estuary widens as it adjusts to the increase in total discharge. Additionally, results show that a higher river discharge increases the difference between the limit of flood‐dominant sediment transport and the limit of flow reversal, which has important implications for the preservation of the tidal signal in the stratigraphy. Estuary dimensions and channel patterns can be described as a function of river and tidal discharge. These findings indicate that the dynamic spatial component in numerical models is crucial in predicting trends in long‐term estuary morphology as well as in inverse predictions from stratigraphy.