Asymmetric current reversals and 3D inter-channel connectivity in a deep multichannel and macrotidal estuary

Multichannel estuaries exhibit complex tidal circulation, yet the mechanisms controlling tidal asymmetry and hydraulic connectivity in deep macrotidal systems remain insufficiently quantified. In particular, the transient dynamics governing tidal reversals and short-lived lateral exchanges between adjacent channels are poorly documented. This study analyzes hydrodynamic connectivity in the multichannel St. Lawrence Estuary using a high-resolution three-dimensional barotropic numerical model resolving intra-tidal variability. The model captures longitudinal and lateral velocities, reversal-front propagation, and momentum inter-channel redistribution over the separating shoal. Results show strong tidal asymmetry and pronounced inter-channel phase lags. Maximum longitudinal velocities reach 1.8–2.2 m s⁻¹ during flood and 1.5–1.9 m s⁻¹ during ebb. Lateral velocities over the shoal intensify during reversals, frequently exceeding 0.6 m s⁻¹, with peak lateral discharge between C1 and C2 reaching 6000–8900 m³ s⁻¹. Reversal fronts propagate at 25–39 km h⁻¹, with asynchronous transitions of 10–30 min between channels. These short phases generate transient shear zones, counter flows, and localized momentum redistribution over the shoal. These findings demonstrate that tidal asymmetry and inter-channel connectivity are governed by transient three-dimensional processes driven by reversal asynchrony and intensified lateral momentum transfer. They provide a quantitative basis for future investigations of sediment pathways, salinity intrusion, and tidal-network evolution.