This paper describes the results of an experimental study of freshwater which flows two-dimensional1y over a still pool of saltwater. This type of flow can be observed when heated cooling water is discharged from a power station horizontally onto the surface of a lake, reservoir [4, 6, 12, 30, 31, 33] or in an estuary where the outflow of freshwater mixes with underlying saltwater; the flux of brackish water increases with the distance downstream from the head of the estuary [6,9, 10, 19]. In these types of flow the depth of the lighter fluid at the mouth is usual1y high, being of the order of 2-3 m; and the mouth densimetric Froude number (defined later) is about unity orless [4,28,36], indicating that the mixing process of the surface layer is dominated mainly by buoyancy rather than momentum. In most previous laboratory studies measurements were carried out for jet-type flow, for which the densimetric Froude number was high with a shal10w depth of flow at the inlet [5, 8, 15, 18]. The object of the present investigation is to study the cases in which the densimetric Froude number is low, varying between 0.5 and 2 and, the depths of freshwater, which are relatively large, vary between 0.08 m and 0.12 m. It is assumed that the flow structure and the turbulent mixing process that occur in this study are similar to those occurring in nature. Experimentally freshwater was released two-dimensional1y from a wide channel onto the surface of stationary saltwater with density of 1025 kg/m held in a wide, deep flume. During the experimental runs saltwater was supplied gently at a constant rate along the bottom of the flume in order to obtain a steady surface flow. Mean velocity and density promes, together with their turbulence intensities, were measured at various sections downstream from the inlet. Entrainment was measured directly from saltwater supply (Fig. 1) and it was also determined from density promes.