Longitudinal estuarine fronts, aligned parallel to the axis of the estuary, are characteristics of the York River. Their time scale for genesis and decay is of the order of hours; they are usually located at the inner edge of the shoals or over the main channel. Field measurements have shown marked intratidal differences in density and current speeds across this estuary. These can be correlated with changes in depth and the strength of the advective processes. Differential advection between the channel and shoal regions, when acting upon a constant longitudinal density gradient, is shown to be of sufficient magnitude to generate strong lateral density gradients, and thus fronts, at particular times within the tidal cycle. ¿ American Geophysical Union 1988