The Bosporus Strait (/ˈbɒspərəs, ˈbɒsfərəs/;[a] Ancient Greek: Βόσπορος Bosporos [bós.po.ros]; Turkish: İstanbul Boğazı ‘Istanbul strait’, colloquially Boğaz) or Bosphorus Strait is a natural strait and an internationally significant waterway located in Istanbul in northwestern Turkey. It forms part of the continental boundary between Asia and Europe, and divides Turkey by separating Anatolia from Thrace. It is the world’s narrowest strait used for international navigation.
Most of the shores of the Bosporus Strait, except for the area to the north, are heavily settled, with the city of Istanbul‘s metropolitan population of 17 million inhabitants extending inland from both banks.
Sections of the shore of the Bosporus in Istanbul have been reinforced with concrete or rubble and those sections of the Strait prone to deposition are periodically dredged.
The name of the strait comes from the Ancient Greek Βόσπορος (Bósporos), which was folk-etymologised as βοὸς πόρος, i.e. “cattle strait” (or “Ox-ford”[b]), from the genitive of boûs βοῦς ‘ox, cattle’ + poros πόρος ‘passage’, thus meaning ‘cattle-passage’, or ‘cow passage’. This is a reference to the Greek mythological story of Io, who was transformed into a cow and condemned to wander the Earth until she crossed the Bosporus, where she met the Titan Prometheus, who comforted her by telling her that she would be restored to human form by Zeus and become the ancestor of the greatest of all heroes, Heracles (Hercules).
Io supposedly went ashore near Chrysopolis (present-day Üsküdar), which was named Bous ‘the Cow’. The same site was also known as Damalis (Δάμαλις), as it was where the Athenian general Chares had erected a monument to his wife Damalis, which included a colossal statue of a cow (the name δαμάλις translating to ‘heifer’).
The English spelling with -ph-, Bosphor is not justified by the ancient Greek name, and dictionaries prefer the spelling with -p-[a] but -ph- occurs as a variant in medieval Latin (as Bosphor, and occasionally Bosphorus or Bospherus), and in medieval Greek sometimes as Βόσφορος, giving rise to the French form Bosphore, Spanish Bósforo and Russian Босфор. The 12th-century Greek scholar John Tzetzes calls it Damaliten Bosporon (after Damalis), but he also reports that in popular usage the strait was known as Prosphorion during his day, the name of the most ancient northern harbour of Constantinople. In English English the preferred spelling tends to be Bosphorus.
Historically, the Bosporus was also known as the “Strait of Constantinople”, or the Thracian Bosporus to distinguish it from the Cimmerian Bosporus in Crimea. These are expressed in Herodotus‘s Histories, 4.83; as Bosporus Thracius, Bosporus Thraciae, and Βόσπορος Θρᾴκιος (Bósporos Thráikios), respectively. Other names used by Herodotus to refer to the strait include Chalcedonian Bosporus (Bosporus Chalcedoniae, Βοσπορος της Χαλκηδονιης [Bosporos tes Khalkedonies], Herodotus 4.87), or Mysian Bosporus (Bosporus Mysius).
As a maritime waterway, the Bosporus specifically connects the Black Sea to the Sea of Marmara and thence to the Aegean and Mediterranean seas via the Dardanelles. It also connects various seas along the Eastern Mediterranean, the Balkans, the Near East, and Western Eurasia. Thus, the Bosporus allows maritime connections from the Black Sea all the way to the Mediterranean Sea and the Atlantic Ocean via Gibraltar, and to the Indian Ocean through the Suez Canal, making it a crucial international waterway, in particular for the passage of goods coming from Russia.
There is one very small island in the Bosporus just off Kuruçeşme. Now generally known as Galatasaray Island (Galatasaray Adası), this was given to the Armenian architect Sarkis Balyan by Sultan Abdülhamid II in 1880. The house he built on it was later demolished and the island became a walled garden and then a water sports centre before being given to the Galatsaray Sports Club, hence its name. However, in the 2010s it was completely overbuilt with nightclubs which were torn down in 2017. It reopened to the public in the summer of 2022.
The exact cause and date of the formation of the Bosporus remain a subject of debate among geologists. One recent hypothesis, dubbed the Black Sea deluge hypothesis, which was launched by a study of the same name in 1997 by two scientists from Columbia University, postulates that the Bosporus was flooded around 5600 BCE (revised to 6800 BCE in 2003) when the rising waters of the Mediterranean Sea and the Sea of Marmara broke through to the Black Sea, which at the time, according to the hypothesis, was a low-lying body of fresh water.
Many geologists,[who?] however, claim that the strait is much older, even if relatively young on a geologic timescale.
The limits of the Bosporus are defined as the line connecting the lighthouses of Rumeli Feneri and Anadolu Feneri in the north, and between the Ahırkapı Feneri and the Kadıköy İnciburnu Feneri in the south (“Fener” is Turkish for lighthouse). Between these limits, the strait is 31 km (17 nmi) long, with a width of 3,329 m (1.798 nmi) at the northern entrance and 2,826 m (1.526 nmi) at the southern entrance. Its maximum width is 3,420 m (1.85 nmi) between Umuryeri and Büyükdere Limanı, and minimum width 700 m (0.38 nmi) between Kandilli Point and Aşiyan.
The depth of the Bosporus varies from 13 to 110 m (43 to 361 ft) in midstream with an average of 65 m (213 ft). The deepest point is between Kandilli and Bebek, at 110 m (360 ft). The shallowest locations are off Kadıköy İnciburnu at 18 m (59 ft) and off Aşiyan Point at 13 m (43 ft).
The southbound flow of water is 16 000 m3/s (fresh water at the surface) and the northbound flow is 11 000 m3/s (salt water near the bottom). Dr. Dan Parsons and researchers at the University of Leeds School of Earth and Environment describe a Black Sea undersea river.
The Golden Horn is an estuary off the main strait that historically acted as a moat to protect Constantinople from attack, as well as providing sheltered anchorage for the imperial navies of various empires until the 19th century, after which it became a historic neighbourhood at the heart of Istanbul.