Morphological Response to Sub-Seasonal Hydrological Regulation in the Yellow River Mouth: A 1996–2023 Case Study

River flow has historically been the primary force shaping the morphology of the Yellow River estuary. However, since the Xiaolangdi Reservoir began operating in 2000, the hydrological processes reaching the estuary have been significantly modified. To evaluate the morphological response of the estuary, we examined the evolution of the mouth channel from 1996 to 2023 using remote sensing, cartographic generalization, and hydrological analysis, supported by annual Landsat imagery, daily hydrological records, and field survey data. Our findings indicate that the channel extended slowly between 1996 and 2002, then advanced rapidly from 2003 to 2007, culminating in a natural avulsion between 2004 and 2008. Following the avulsion, the newly formed channel progressively extended (2008–2013) and, after 2014, developed into a multi-branch system. The development of this bifurcating system since 2014 is attributed to the sustained release of low-sediment-concentration flows from the Xiaolangdi Reservoir. In contrast, the earlier avulsion was triggered by the rapid discharge of a high-sediment-concentration flow in 2004. These results demonstrate that releases from the Xiaolangdi Reservoir with varying sediment concentrations at different timescales elicited distinct morphological responses in the Yellow River estuary, underscoring the need for carefully calibrated hydrological regulation.