New observations of mainly flow velocity, salinity, and temperature are used to show and discuss some of the physical conditions in ¿resund, the strait between Denmark and Sweden, one of three connections between the brackish Baltic Sea and the saline North Sea. The main geometric features are a contraction in the northern ¿resund and the shallow Drogden sill at the entrance to the Baltic. The observations show that the two-layer flows through the contraction are often hydraulically controlled. The observations also reveal details of the transition from subcritical to supercritical flow. In terms of the composite Froude number, on the basis of local flow parameters these details are that the flow may be subcritical as well as supercritical in different areas of some cross section. Existing theories on rotating hydraulics are unable to account for these circumstances, which are due to the strong influence of the Earth's rotation and the curvature of the streamlines. In the present study it is not attempted to explain these conditions, but the probable effects of rotation and curvature on the controlled flow rate are discussed briefly. Also, the possible effects of hydraulic control on the exchange of the Baltic are discussed. It is argued that the stratification in the Kattegat, the sea to the north, is more important than the presence of the Drogden sill for the amount of high saline water to enter the Baltic through ¿resund. This result is supported by observations of the stratification in ¿resund and the flow at the sill. The observations show that the interface in ¿resund rises significantly during flow to the Baltic and that the transport of saline water into the Baltic is closely connected to the shallowness of the interface in ¿resund. ¿ 2001 American Geophysical Union