Observations of the gravitational circulation over the sill at the entrance to Puget sound indicate a fortnightly modulation of the two-layer flow with a corresponding variation in stratification. Gravitational crirculation and stratification reach a maximum during neap tide and a minimum during spring tides. The modulation appears to be a function of the turbulence generation by tidal flow over the sill, which varies as a function of tidal amplitude. The enhanced gravitational circulation during neap tides augments the flux of dense water across the sill region and episodes of bottom water renewal of Puget Sound result. Intrusions of dense water in the main basin of the sound show a fortnightly periodicity with a phase lag appropriate for the propagation of an internal bore from the sill region, commencing at the neap tide. Observations indicate that the intrusions take the form of gravity currents, propagating at approximately 20 cm s-1 down the main basin. The detailed dynamics of the turbulent mixing processes that modulate the onset of inflow were not resolved in this study, but the integrated effect of tidal mixing on the low-frequency circulation is clearly demonstrated.