Stratified flow over topography is examined in the context of its establishment from rest. A key element of numerical and steady state analytical solutions for large amplitude topographic flow is the splitting of streamlines which then enclose a trapped wedge of mixed fluid above the rapidly moving deeper layer. Measurements have been acquired which illustrate the development of this wedge. The volume of trapped fluid progressively increases with time, permitting the primary flow to descend beneath it over the lee face of the obstacle. Velocity structure of instabilities within the entrainment zone is observed and compared with laboratory measurements in which an altered vortex pairing interaction is observed. The measurements described here were acquired in a coastal inlet subject to gradually changing tidal currents; the observed mechanism for flow establishment also has application to atmospheric flow over mountains.