Estuarine morphodynamics regime shift caused by catchment reservoir: Illustration from the Minjiang River Estuary

Human activities, particularly reservoir construction, have become a focal point in contemporary earth science research due to their profound impacts on flow-sediment-ecology systems of catchment. While some investigations have examined reservoir-induced alterations in fluvial-estuarine sedimentation patterns, the feedback mechanisms between morphological changes and tidal dynamics remain inadequately explored. This research focuses on the Minjiang River estuary, a macrotidal system in southeastern China, characterized as a medium-scale catchment (length: 562 km; drainage area: 60,000 km²). Employing an integrated approach combining field hydrographic measurements, multi-temporal digital elevation models, and Mike21 simulations, this study quantifies the decadal-scale interplay between anthropogenic-driven geomorphic evolution and tidal system adjustments over a 40-year period (1984–2022). The analysis reveals that reservoir operations and river sand mining have transformed the estuarine sedimentary regime from net deposition (0.4 cm/year, 1984–2005) to accelerated erosion (−2.6 cm/year, 2005–2022). This geomorphic transition has triggered significant hydrodynamic responses. During the deposition-dominated phase (1984–2005), sediment accumulation resulted in a progressive decline of 1.5 cm in the mean tidal levels and a 6.5 cm/s reduction in flow velocities. Conversely, the subsequent erosional period (2005–2022) saw a 2.2 cm rise in tidal levels accompanied by a 9.3 cm/s acceleration in flow velocities. The flow and sediment flux during flood and ebb periods in estuarine branches such as North Branch increased by 53 % and 57 %, respectively. Furthermore, the bed dunes in Maiwei and North Branch increased by 0.3m (13 %) and 0.1 m (8 %), respectively. This research establishes a fundamental principle of sediment dynamics: depositional processes attenuate tidal energy while erosional regimes enhance hydrodynamics. These findings provide critical insights into the cascading effects of human activities at the catchment scale on coastal hydro-morphological systems, offering a scientific framework for sustainable estuarine management under increasing anthropogenic pressures.