To provide a better comprehension for the processes that occurs inside estuary, it was made a physical classification of the estuary into homogeneous zones. This classification was made using all available salinity values (Average Salinity Distribution) from hydrodynamic model.

There is three different zones according to salinity  values: Seawater zone (salinity > 27 psu), Mixing zone (salinity  between 27-1 psu) and Tidal Freswater zone( salinity < 1 psu).

Seawater zone (20% of Douro Estuary total area)

Mixing zone (21%  of Douro Estuary total area)

Tidal Freswater zone (59%  of Douro Estuary total area)