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Geography and hydrology

The International Hydrographic Organization defines the limits of the Sea of Marmara as follows:

On the West. The Dardanelles limit of the Aegean Sea.

On the Northeast. A line joining Cape Rumili with Cape Anatoli (41°13′N).The sea's south coast is heavily indented and includes the Gulf of İzmit (Turkish: İzmit Körfezi), the Gulf of Gemlik (Turkish: Gemlik Körfezi), the Gulf of Bandırma (Turkish: Bandırma Körfezi), and the Gulf of Erdek (Turkish: Erdek Körfezi).

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