This study represents a detailed examination of intratidal variations in boundary-layer structure in a relatively narrow, straight section of the Hudson river estuary during high river discharge. The presence of active mixing is inferred from estimates of the Richardson number. The maximum height above bed to which mixing occurs is comparable during flood and ebb; however, the regions of mixing differ in density structure. During the flood, active mixing is confined to a well-mixed near-bed layer, whereas during the ebb, active mixing occurs throughout regions of significant stratification. Tidal straining maintains stratification during the ebb, while it promotes the growth of a uniform bottom, mixed layer during the flood. In general, maximum and minimum near-bed stratification occur during late ebb and flood, respectively, reflecting the dominant role played by tidal straining in determining intratidal variation. Vertical stretching was also found to be an important term in the stratification balance during the ebb. ¿ American Geophysical Union 1996