TIDAL CURRENT SHEAR IN A BROAD, SHALLOW, RIVER-DOMINATED ESTUARY

Approximately one year of current, temperature, salinity and pressure data were collected at a site near the mouth of the Mobile Bay estuary in Alabama. This estuary is broad, shallow (3 m) and relatively hat. Diurnal tides account for 85% of the current variance in the lower est uary. Tidal currents have significant vertical shear in both amplitude and direction. The principal diurnal constituents in the lower bay, O -1 and K-1, have amplitudes that range from 12 to 25 cm/s in the near- surface layer, 9 to 12 cm/s near the bed. The principal axis of the ne ar-surface diurnal ellipse is rotated 30 degrees clockwise relative to the near-bottom ellipse. This strong tidal current shear can exist be cause the estuary has a strong, persistent, haline stratification main tained by a continuous river input. The cause and strength of the tida l shear depends on a combination of factors, including differential to pographic forcing through separate entrances into the bay and spatial and temporal Variations in water column stability caused by changes in the magnitude of river flow and wind stress.