The determination of the longitudinal distribution of a substance in natural channels is based on the mass balance equation. In this equation the dispersion term accounts for the longitudinal mixing which results from the combined effects of turbulent diffusion and the shear-induced velocity distribution in both the transverse and vertical directions. In estuaries, due to the tidal action and large scale gravitational circulation associated with salinity intrusion the process is more complex. Two data collection campaigns were carried out during a neap tide and during a spring tide in the Scheldt estuary. Five measurement points were situated on a cross-section of the estuary and two others at the upstream and downstream of the cross-section. The measured quantities were velocity and its direction, conductivity, salinity, temperature and concentrations of sand and mud. The results of these measurements were used to investigate whether the increase in the Taylor-Elder dispersion coefficient found by Fischer in rivers due to the transverse velocity distribution is applicable to estuaries with usual large width to depth ratio. This research was also extended to find out the effect of flow oscillation in estuaries on dispersion and the possibility of prediction of the magnitude of dispersion coefficients in tidal waterways.