Observations of density, velocity, and acoustic backscatter obtained over the sill in Observatory Inlet, British Columbia, during the ebb tide illustrate several features of internal hydraulic control in a tidally forced stratified flow. Following high water slack, the flow field was initially determined by critical control of the second internal mode at the sill crest. As the flow speed increased, a transition occurred, and control changed to an ''approach control'' of the second internal mode located upstream of the sill crest. Whereas the critical control represents a minimum energy condition at the crest, the approach control corresponds to a maximum energy condition upstream of the sill crest. Later, when the ebb currents were just past their maximum values, the control condition changed to critical control of the first internal model. The concept of approach control of flow over topography has not previously been discussed in the literature. The data described here are believed to represent the first field observations of this phenomenon. The changing hydraulic conditions are analyzed in terms of two- and three-layer hydraulic models. Model results are consistent with the evolution of the flow patterns, although the model cannot reproduce the apparent hysteresis in the response after maximum ebb. The concept of an approach control is demonstrated with a simple two-layer laboratory model.