The surface salinity of the sea averages about 22 parts per thousand, which is slightly greater than that of the Black Sea, but only about two-thirds that of most oceans. The water is much more saline at the sea bottom, averaging salinities of around 38 parts per thousand, similar to that of the Mediterranean Sea. This high-density saline water, like that of the Black Sea, does not migrate to the surface. Water from the Susurluk, Biga (Granicus) and Gonen Rivers also reduces the salinity of the sea, though with less influence than on the Black Sea. With little land in Thrace draining southward, almost all of these rivers flow from Anatolia.