This study investigates the hydrodynamic and mixing processes at the estuary–shelf transition of a microtidal system, and the buoyant plume generated at the Patos Lagoon mouth (Brazil). Using measurements of turbulent kinetic energy (TKE) dissipation (<em>ϵ)</em>, current velocities, salinity, and temperature collected during a high-discharge period (∼9,400 m³ s<sup>-1</sup>), we characterize the spatial evolution of turbulence and mixing along the channel, from the source to the buoyancy-driven plume region. Observations show that the jetty-constrained inlet acts as a morphological nozzle, forcing the flow to remain supercritical (<em>Fri </em>> 1) for several kilometres onto the inner shelf. Despite strong stratification, intense shear-driven turbulence was observed, with TKE dissipation rates (ε) reaching 10<sup>-3</sup> W kg<sup>-1</sup> near the mouth, comparable to values reported in high-energy mesotidal and macrotidal systems. Analysis of the buoyancy Reynolds number (<em>Reb</em>) and the gradient Richardson number (<em>Ri</em>) indicates that inertial forcing overcomes buoyancy suppression, maintaining a predominantly turbulent regime (<em>Reb </em>> 200) at the plume front. These results demonstrate that, in narrow, high-discharge estuarine outlets, morphological confinement and sustained supercritical flow govern the near-field evolution of buoyant plumes, maintaining vigorous mixing even under pronounced density stratification.