On the basis of a 301 day acoustic Doppler current profiler mooring in an estuarine tidal strait the strength and structure of secondary circulation in a region of flow curvature is related to variations in tidal forcing and river discharge. During low-flow conditions the structure of secondary flow is consistent with a centrifugally forced helical flow, with bottom flow toward the inside of the bend and surface flow toward the outside of the bend. The strength of secondary flow increases linearly with tidal range and is consistent with a vertical eddy viscosity that is linearly dependent on tidal current speed. During times of high river discharge the strength of secondary flow is significantly reduced, and its vertical structure undergoes a fundamental change over the spring/neap cycle. During spring tides the classic helical flow pattern is evident, albeit weaker than during low-flow conditions. However, during neap tides a more complex two-cell structure is evident. The change between these two states occurs with a spring/neap transition in the subtidal flow, indicating that it is also accompanied by changing stratification. Simple scaling analysis suggests that during weakly stratified conditions, secondary circulation will influence stream-wise dynamics and dispersion for channels with widths on order or less than 0.1 H/Cd, where H is the water column depth and Cd is a quadratic bottom drag coefficient. In contrast, during highly stratified conditions, lateral excursions due to secondary flows are limited to approximately one tenth of the channel's width and are an ineffective lateral mixing agent.