Intratidal and fortnightly variability of turbulence at the mouth of a macrotidal estuary is explored in this study. Profiles of turbulent kinetic energy dissipation were estimated from a Vertical Microstructure Profiler, and velocity shear measurements and current velocities were collected with a vessel‐mounted Acoustic Doppler Current Profiler. Gradient Richardson numbers were quantified using density measurements from a Conductivity, Temperature, Depth profiler and squared vertical shear quantified from current velocities. All measurements were collected over one complete semidiurnal tidal cycle in both neap and spring tide conditions. Vertical eddy viscosity (A_z) was quantified from the available data and was used as a proxy for vertical mixing. Results showed intratidal asymmetries in bottom‐generated turbulence during neap with A_z larger during ebb tide (∼10⁻² m²/s) than flood tide (∼10⁻³ m²/s) with the opposite occurring during spring tide with larger A_z during flood (10⁻¹ m²/s) than ebb (10⁻² m²/s). Lateral processes produced elevated near‐surface mixing decoupled from bottom‐generated turbulence at the end of flood and ebb tide during neap and spring. The secondary flows were driven by Coriolis, and these flows either enhanced (neap tide, end of flood) or maintained (spring tide, end of ebb) total shear through slack tides producing near‐surface to midwater column turbulence. These results are the first of their kind to show midwater column mixing from lateral circulation driven by Coriolis in a well‐mixed system, which vary from the lateral processes previously shown to destabilize water columns in estuaries more influenced by stratification.

Transverse variations of tidal and subtidal flow were investigated in a macrotidal and convergent estuary. This was accomplished by combining data analysis of current velocities and water density with numerical modeling at the mid‐reaches of the Gironde Estuary (France). Nonlinear mechanisms responsible for overtide generation and hence subtidal flows were found to vary across the estuary and from neap to spring tides. Subtidal flows were driven by a combination of internal asymmetry, tidal advective accelerations, nonlinear effects of water level variations, quadratic friction, and river discharge. The quarter‐diurnal overtide band (D4) in the flow was generated by internal asymmetry and tidal advective accelerations during neap tide. The ratio of quarter‐diurnal to squared semidiurnal bands (D4/D2²) was largest (>0.3) in sections of the channel showing subtidal outflow. River discharge increased from neap to spring tides causing a subsequent increase of seaward subtidal currents. During spring tide, D4 was generated by tidal advective accelerations and quadratic friction combined with river discharge, rather than by internal asymmetry. The sixth‐diurnal overtide (D6) in the flow was comparable to D4 for both neap and spring tides. Largest D6/D2³ ratios were found in the shallowest cross‐channel locations during both neap and spring tides.