As the width of a vertically well-mixed estuary increases, the longitudinal dispersion coefficient eventually reduces from a value associated with the transverse shear of the tidal current to a much smaller value associated with the oscillatory vertical shear. In this paper the final stages of this transition are investigated, with particular emphasis on buoyancy effects due to the salinity distribution. The central mathematical result relates the long-term longitudinal dispersion coefficient to the local unsteady horizontal dispersion coefficients and to the residual horizontal circulation. A useful consequence of this result is a demarcation of the parameter regimes in which the various mass-transport mechanisms are dominant. The Thames downstream of London Bridge is revealed to be buoyancy dominated.