Freshwater plumes are important flow structures that influence the dynamics and water properties of coastal regions and continental shelves. Turbulence in plume regions is mainly driven by shear instabilities at the interface between plume and oceanic waters, which, in turn, depend on the geometry and outflow of a specific plume region. The Southern Brazilian Shelf presents a highly variable hydrographic distribution modulated by the seasonal wind variation and the freshwater discharge from the La Plata River estuary, which has a significant impact on the continental shelf circulation. This buoyant plume creates strong density gradients and interacts with local water masses resulting in a complex hydrographic pattern. In this study, high resolution hydrography and microstructure measurements were obtained in order to verify the effect of freshwater stratification on vertical mixing in this highly dynamic continental shelf. Results show that the plume is highly stable at southern portions of the shelf, as density displacements, or Thorpe displacements, δ<sub><i>T</i></sub>, heat diffusivity, <i>K<sub>T</sub></i>, buoyancy flux, <i>B<sub>f</sub></i>, and density gradient ratio, <i>R<sub>p</sub></i> are reduced when compared to the northern areas. Moreover, hydrographic data suggests that the large-scale La Plata River plume has a dynamic mid-field region due to instabilities generated when reaching the shelf break.