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Abstract The salt balance in estuaries is maintained by the outflow from the river, which removes salt from the estuary, and dispersive processes, which drive downgradient fluxes bringing salt into the estuary. We analyzed the salt fluxes in a realistic model of the North River, a tidal salt marsh estuary, using a quasi-Lagrangian moving plane reference based on the theory of Dronkers and van de Kreeke. Our study confirms their theoretical finding that in a plane moving with the tides, all landward salt flux results directly from shear dispersion, that is, the spatial correlation between cross-sectional variations in velocity and salinity. We separated cross-sectional variations in velocity and salinity not only based on their lateral and vertical components but also by distinct regions of the cross section: the main channel and the marsh. In this way, we quantified the salt flux contributions from vertical and lateral shear dispersion, as well as from trapping—the salt flux due to the difference between the mean velocity and salinity of the main channel compared to the marsh. Trapping accounted for up to half of the total landward salt flux in the estuary during spring tides but decreased to about one-quarter during neap tides. Within the channel, the primary mode of dispersion shifted from lateral shear dispersion due to flow separation during spring tides to vertical shear dispersion due to tidal straining during neap tides. These results demonstrate the important role of topographically induced dispersion on maintaining the salt balance, particularly in tidally dominated estuaries.