The Bosphorus () or Bosporus (; , Bósporos; ) is a strait that forms part of the boundary between Europe and Asia. The Bosporus, the Sea of Marmara, and the Dardanelles strait to the southwest together form the Turkish Straits. The world’s narrowest strait used for international navigation, the Bosporus connects the Black Sea with the Sea of Marmara (which is connected by the Dardanelles to the Aegean Sea, and thereby to the Mediterranean Sea.) The limits of the Bosporus are defined as the connecting line between the lighthouses Rumeli Feneri and Anadolu Feneri in the north and between the Ahırkapı Feneri and the Kadıköy İnciburnu Feneri in the south. Between the limits, the strait is long, with a width of at the northern entrance and at the southern entrance. Its maximum width is between Umuryeri and Büyükdere Limanı, and minimum width between Kandilli Point and Aşiyan. The depth of the Bosporus varies from in midstream with an average of . The deepest location is between Kandilli and Bebek with . The most shallow locations are off Kadıköy İnciburnu on the northward route with and off Aşiyan Point on the southward route with . The Golden Horn is an estuary off the main straits that acted as a moat to protect Old Istanbul from attack, as well as providing a sheltered anchorage for the imperial navy until the 19th century. Most of the shores of the strait are heavily populated, straddled as it is by the city of Istanbul (with a metropolitan area population in excess of 12 million inhabitants) which extends inland from both coasts. It has been known since before the 20th century that the Black Sea and the Sea of Marmara flow into each other in an example of a density flow and in August 2010 a continuous ‘underwater channel’ of suspension composition was discovered to flow along the floor of the Bosporus which would be the sixth largest river on Earth if it were to be on land. The study of the water and wind erosion of the straits relates to that of its formation. Sections of the shore have been reinforced with concrete or rubble and sections of the strait prone to deposition are periodically dredged.
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