Using average sections along the Bosphorus taken in September 1994 with a loosely tethered profiler and an acoustic Doppler current profiler, Gregg et al. <1999> found the exchange flow between the Sea of Marmara and the Black Sea to be quasi-steady but far from satisfying the hydraulic control conditions for two-layered flows. Here we examine synoptic sections and use images from an acoustic backscatter system to provide the first detailed look at the flow and water mass changes in the Bosphorus and to assess how well the flow satisfies the hydraulic assumptions. Thirty kilometers long, 28--100 m deep, and 0.75--3 km wide, the Bosphorus has bathymetry for more complex than that used in analytic or numerical models of exchange flows. The particulars affect dynamics in important ways. For instance, owing to changes in channel shape, the narrowest section, known as the contraction, is not also the minimum in cross-sectional area. Rather, it is a transition between the wider northern half and the narrower southern half of the strait, and some places south of the contraction have slightly smaller areas. Sharp bends occur throughout the strait and often produce flow separations as well as directing upper and lower flows to opposite sides of the channel. Never <28% of the water column, the interface thickens to 75% in the southern half of the strait as a result of intense mixing downstream of the contraction. As a consequence of the strong mixing and numerous flow separations, we conclude that the exchange flow may be at least partly controlled by friction instead of being a simple hydraulic flow.