Aircraft expendable bathythermograph, radar, infrared scanner, and visual data collected in October 1982, 1984, and 1985, as well as space shuttle photographs, indicate that an internal waveform (depending on flow conditions, either an internal hydraulic jump or a lee wave) is a semipermanent feature over the Camarinal Sill in the western approaches to the Strait of Gibraltar. Comparison of these data with both historic and recent current meter mooring data indicates that the waveform, generated at the interface between the Atlantic and Mediterranean waters, undergoes periodic changes wrought by interaction of the tidal current and the flow of the Atlantic and Mediterranean waters. We believe that the waveform is a major mixing mechanism between the two water masses.