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Tidal straining is proposed as a key mechanism influencing the magnitude and timing of the horizontal mass flux of nutrients, terrestrial carbon and anthropogenic contaminants across Regions of Freshwater Influence (ROFIs), the critical interface between estuaries and continental shelf seas. Evidence for this hypothesis is presented in estimates of the long-term residual current profile, obtained from five years of continuous ADCP measurements, taken at the Proudman Oceanographic Laboratory Coastal Observatory mooring in Liverpool Bay. The observed mean residual velocities are found to be three times larger than those predicted by Heaps’ (1972) ‘classical’ solution. The strongest residual currents from the coastal observatory mooring are shown to occur when the water column periodically alternates between a well mixed and stratified state, a consequence of tidal straining, rather than simply related to the density gradient. These results are used for the further development of the POLCOMS 3D numerical model to improve the simulation of physical processes that drive the off-shore flux of freshwater.