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 (fop), i.e. the frequency in which there is a minimum in sound transmission losses (TL), is determined. The influence on fop 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 fop 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.

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