The Bosporus Strait in Numbers, 31 Kilometres, Two Currents and the Bends That Make It So Dangerous

On a map the Bosporus looks like a river. It runs about 31 kilometres from the Black Sea at Rumeli Feneri and Anadolu Feneri down to the Sea of Marmara at Sarayburnu, the point below Topkapı Palace. It is 3.4 kilometres across at its widest, near Büyükdere, and about 700 metres at its narrowest, between Kandilli on the Asian shore and Aşiyan on the European one. A good swimmer can cross it, and every summer a couple of thousand do, in the Bosphorus Cross-Continental Swim from Asia to Europe.

For a ship it is one of the hardest waterways on earth. Turkey's foreign ministry calls it "twisting and turning", which undersells it. Between Kandilli and Aşiyan a ship has to swing its heading by about 45 degrees; at Yeniköy, by about 80. A 250-metre tanker making those turns has little room to spare, and the water around it is moving.

Image: European Space Agency, Copernicus Sentinel-1, CC BY-SA 3.0 IGO, via Wikimedia Commons

A strait with two rivers in it

The Black Sea receives more fresh water from its rivers, the Danube, Dnieper and Don among them, than it loses to evaporation. The surplus pours south through the Bosporus as a surface current, about 16,000 cubic metres a second. Underneath it, heavier, saltier water from the Mediterranean and the Marmara flows the other way, about 11,000 cubic metres a second. The Black Sea's surface water is roughly half as salty as the Mediterranean's, and the two layers barely mix.

The first person to prove it was an Italian soldier and scientist, Luigi Ferdinando Marsigli, who lowered weighted lines into the strait while posted to Constantinople and published his findings in 1681. Fishermen had known for centuries: drop a net deep enough and it is pulled north against the surface flow.

Most of the time the surface current runs at a few knots. At the bends it can reach seven or eight, and when a strong southerly wind blows it can reverse in places, creating eddies that push ships sideways towards the shore. In 2010 a team from the University of Leeds mapped a channel on the strait floor carrying the deep current, so large that on land it would rank among the biggest rivers in the world.

Depth varies just as sharply. The strait averages about 65 metres, reaches about 110 metres between Kandilli and Bebek, and rises to 13 metres off Aşiyan Point. Ships of any size use a narrow deep-water lane.

Why the numbers matter

These are not trivia. Every rule that governs shipping through Istanbul follows from them (traffic and transit rules): the separate north and south lanes, the order to wait when the current runs too hard, the closures in fog, the pilots who board most big ships. The two deadliest tanker accidents in the strait, the Independenta in 1979 and the Nassia in 1994, both started with collisions in this geometry.

They also explain the strategic value. A 700-metre gap can be watched from a café table, mined in a night and covered by artillery from either shore. Mehmed II built Rumeli Hisarı exactly there in 1452 to cut Constantinople off from the Black Sea. The Montreux Convention of 1936 exists because whoever holds that gap decides who may enter the Black Sea.

The currents feed the city too. The two layers carry migrating fish, bonito (palamut), bluefish (lüfer) and mackerel, through Istanbul each autumn and spring, and dolphins follow them. The strait's fishing piers are crowded in October for that reason.

One strait, several names

The ancient Greeks called it Bosporos and told a story to explain it: Io, a lover of Zeus turned into a heifer to hide her from Hera, crossed here while fleeing a gadfly sent to torment her. The name was read as "cow's ford", or more loosely "ox ford", a comparison Oxford scholars have enjoyed ever since.

English uses two spellings. Bosporus is the older scholarly form, closer to the Greek, and the one most reference works and atlases use. Bosphorus, the variant with a ph, is just as old in English, appears in the text of the Montreux Convention, and dominates travel writing and Turkey's own English-language branding, from the old Bosphorus Bridge to the annual swim. In Turkish it is simply İstanbul Boğazı, the Istanbul Strait. This site uses Bosporus.

Thirty-one kilometres of water, and empires have fought over every one of them.