Résumé : A 3D hydrodynamical model has been set up to describe the distribution and variability of the salinity in Belgian coastal waters. Particular attention was paid to determining the relative impact of the Scheldt and Rhine/Meuse freshwater plumes and testing the hypothesis that the salinity of Belgian waters is primarily a mix between salty offshore water and freshwater from the Scheldt Estuary. Attention was also paid to determining whether the Seine has significant impact on the Belgian zone. The 3D hydrodynamical model, based on COHERENS, has been applied to the Channel and the Southern Bight of the North Sea using a 5′ (longitude) by 2.5′ (latitude) grid. The model has been run for the years 1991–2002. Real river runoffs have been taken into account for the main rivers within the domain: the Scheldt, the Rhine/Meuse, the Seine and the Thames. Model tracers were used to characterise the signature of water masses in terms of Atlantic and riverine waters. Results indicate that the salinity of Belgian waters is dominated by inflow of the Channel water mass which mixes with freshwater originating mainly from the Rhine/Meuse with a much smaller contribution from the Scheldt Estuary. This conclusion is further supported by simulation results obtained when each river discharge is separately set to zero. Thus, the ‘generally accepted’ hypothesis of a ‘continental coastal river’ with fresher coastal water flowing north-eastward up the French-Belgian-Dutch coast and picking up freshwater from successive outflows seems inappropriate for Belgian waters where horizontal dispersion of Rhine/Meuse water in the opposite direction is significant.