In the intial phase of MARSEN (marine remote sensing experiment), three main water masses and North Frisian Wadden Sea Water were found in the German Bight in the late summer of 1979. This is shown by t/S diagrams of the CTD data obtained with the RV Friedrich Heincke and the RRS John Murray. For each t/S pair the percentage composition of the three main water masses is estimated by applying the linear mixing theory. The distribution of the main water masses indicates a frontal structure in the German Bight. A clearly separated water mass, which originates from the mixing of North Sea bottom water and Elbe estuary water, is found in the frontal area. This frontal water is apparently trapped by the vertical current shear and the topography of the post-glacial valley of the Elbe.