The estuarine zone is an area of strong interaction between fresh and salty water. Dynamics in these areas is complex due to the interaction of the forcing mechanisms such as wind, tides, local coastal currents and river discharges. The difference of density between fresh water and salted water causes the formation of the buoyant plumes which have been investigated by means of numerical models and field studies. Plumes play a significant role in the transport of pollutants and the ecology in the frontal areas where density gradients are strong. Therefore, in order to investigate the horizontal and vertical dispersion of salinity and temperature the YAXUM/3D baroclinic numerical model was developed. The model is validated and applied for two particular cases. The first one consists of modeling the discharge of a jet of hot water where the gradients of temperature prevail and the second to study the discharge of the mouth of the estuary Leschenault toward the Koombana bay, Australia where salinity gradients are analyzed. The results derived from the YAXUM/3D are satisfactory and in agreement with other models which have been already validated.