A salty wedge intrusion into the fresh river water is a relevant feature at the external zone of the Río de la Plata estuary. Accordingly, a particular sound velocity profile with a strong positive gradient very different from typical profiles at open sea areas, takes place. Upon these special conditions, using a Normal Mode Model the acoustic field is studied. The optimum frequency of propagation (f_op), i.e. the frequency in which there is a minimum in sound transmission losses (TL), is determined. The influence on f_op value of the seasonal changes in the sound velocity profile, the source and receiver positions and the change in the wind forces, is analysed. It is concluded that f_op values depend significantly on the source and receiver geometry. They have a distinct dependence on seasonal changes of the sound velocity profile and they remain invariant with increasing wind forces.