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Abstract A study of transport mechanisms in the oligohaline reach of the Delaware Estuary examines several locations with rapid changes in cross-sectional area that increase the horizontal salinity gradient, the exchange flow, and the upstream salt flux. This study is motivated by the proximity of municipal drinking water intakes upstream of the oligohaline range of the estuary, which can advance to just below the City of Philadelphia, PA, during low flow events. A Regional Ocean Model System (ROMS) numerical model was used to analyze the potential mechanisms of dispersion in the tidal Delaware River. While the model domain is largely within the tidal-fresh upper estuary, the domain below Philadelphia becomes oligohaline (0.5–5 psu) during low-flow events. Results found that landward transport of salt is facilitated by a combination of steady vertical shear dispersion and tidal oscillatory salt flux, with the latter becoming increasingly important approaching the upstream extent of salt intrusion. Frontogenesis is also an important intrusion mechanism in the vicinity of specific bathymetric features between km 110 and 114 near the Delaware Memorial Bridge and near km 126 near Marcus Hook. Moreover, the tidal advection of these fronts near km 117 produces strong lateral gradients that drive secondary flows and produce stronger tidal oscillatory salt flux away from the regions of frontogenesis.