Controls of Groundwater Seepage on Channel Bifurcation Evolution: Implications for Groundwater–Surface Water Interactions and Sustainable Drainage Management

Understanding how groundwater flow influences river networks is essential for sustainable management of water resources and associated ecosystems, particularly in humid regions where baseflow sustains low-flow conditions and ecological habitats. Continental statistical analysis of river networks suggest that groundwater may exert a distinct control on channel morphology and bifurcation angles. However, direct process-based evidence for these controls remains limited, constraining our ability to predict groundwater availability and the response of fluvial systems to climate and anthropogenic change. To address this gap, we conducted a series of controlled sandbox experiments in which representative bifurcated channels were pre-constructed in a uniform sediment layer. Channel upward evolution under constant and uniform groundwater seepage was monitored. Meanwhile, a planar groundwater flow model and a coupled fluid-solid mechanical model were developed and parameterized based on the experimental conditions to simulate subsurface flow fields and assess stability. The results show that groundwater seepage can systematically modify initial bifurcation angles and thus reorganize channel branches. For symmetric bifurcations, new developed channel trajectories follow equipotential lines, with outward divergence at 30°, near-axial extension at 72°, and pronounced inward deflection at 120°. In asymmetric bifurcations, the branch aligned with the dominant subsurface flow persistently captures more discharge and stabilizes a robust drainage structure. Steeper hydraulic gradients promote wider and more symmetrically developed channels. These findings provide quantitative support that persistent groundwater flow can override initial geometric control and actively shape drainage architecture. They are helpful for predicting zones of focused groundwater discharge and the changes in river network structure caused by climate change, and for improving process-based models used in sustainable groundwater management.