The results of ten dye tracer experiments conducted in 2003–2006 to study the dispersion of the outflow of the Delaware and Hudson Rivers are presented. A fluorescent dye tracer was used to track the river plume and to measure directly the salt flux into the plume. A variety of flow regimes were encountered. During strong upwelling events, a salt flux of ∼3 ÷ 10–4 kg m s–1 at the leading edge of the plume implies a vertical diffusivity of Kz ∼ 3 × 10–4 m–2 s–1. Comparable salt flux was measured at the leading edge of a buoyancy-driven coastal current with Kz ∼ 6.3 × 10–4 m–2 s–1. For weaker wind events Kz was ≤10–4 m–2 s–1. Using a gradient Richardson number (Ri), these observations were replicated by a 1-D model of vertical salt flux to within a factor of 2. Upwelling events are the most efficient mechanism for dispersing the river plume water over the coastal shelf because the plume's offshore displacement is combined with a horizontal diffusivity measured to be ∼150 m–2 s–1 over the two-day period of each experiment.