A water mass formed in the Black Sea that flows into the Aegean Sea through the Strait of Bosporus, the Marmara Sea and the Strait of Dardanelles. The flux into the Aegean varies from 180-200 km³ yr^-1 to a maximum of 700 km³ yr^-1 between April and October. BSW is of primary importance to processes in the Aegean, but plays a secondary role in the overall water balance of the Mediterranean.

BSW is recognized by a surface salinity minimum, with the salinity off the mouth of the Strait of Dardanelles varying from 24 to 26 psu during the warm months and from 30-35 psu during the cold months. A pronounced halocline develops in the North Aegean, with the maximum depth ranging from 20-80 m. As it travels westward and southward, BSW is modified following the general cyclonic circulation of the Aegean. During the winter, it spreads westwards and then northwards, entering the Samothraki Plateau. It flows westwards over the plateau and then southwards along the eastern coast of the mainland to Evvia Island. If the thermohaline front in the Andros Strait is well developed, the BSW flows eastwards along the northern boundaries of the Kyklades Plateau, following the general cyclonic circulation. If the thermohaline front disappears, the BSW bifurcates, with one branch moving eastwards along the northern boundary of the Kyklades Plateau and the other southwards into the Saronikos Gulf, causing the winter salinity minima seen there. During the summer, the general cyclonic circulation pattern still prevails, with the Etesian winds causing the BSW to flow southwestwards to Evvia Island and then southwards. The low salinity waters flow through Andros Strait and create the second salinity minimum observed in the South Evvoikos and Saronikos Gulfs. It has been detected (by the surface salinity minimum) as far south as the Kithira Straits.