Coupled Surface and Subsurface Controls on Flood-Driven River–Floodplain Connectivity in the Jinshan Gorge, Yellow River

Understanding river–groundwater exchange in confined gorge reaches remains challenging because hydrological connectivity varies during flood events and is governed by both surface and subsurface processes. We investigated river–aquifer interactions along a riparian transect in the middle reach of the Yellow River within the Jinshan Gorge using high-frequency groundwater observations, time-series analyses, and numerical modeling. Results show a strong hydraulic connection between river stage and riparian groundwater levels with no detectable lag. Riparian inundation occurred during 55% of the hydrological year and significantly enhanced river–aquifer exchange by increasing hydraulic gradients and expanding surface–groundwater contact. Modeling results indicate that inundation-induced riparian infiltration accounted for ~21% of total infiltration. Rapid recharge during inundation generated transient groundwater mounds that coalesced into a longitudinal ridge, inducing bank-return flow and sustaining subsurface connectivity beyond inundation. In contrast, flood-induced siltation produced a low-permeability surface layer that reduced riparian infiltration by an average of 46%, reaching 79% during large events. However, field observations indicate that siltation occurs only during rare, high-intensity, prolonged inundation events, while most events produce little to no sediment deposition. Importantly, siltation not only suppresses direct infiltration but also constrains groundwater mound formation, weakening subsurface connectivity. These findings demonstrate that river–floodplain connectivity is regulated by coupled visible and hidden pathways, where inundation enhances hydraulic exchange while siltation imposes direct and indirect constraints. Explicit representation of these coupled, event-dependent processes is therefore essential for accurately quantifying river–groundwater exchange, water budgets, and solute transport in large alluvial rivers.