Bosporus Strait

The Bosporus is a strait in Istanbul, Turkey, separating the European and Asian sides of the city and linking the Black Sea with the Sea of Marmara. The water flows through a narrow channel between hills on both sides, with the width varying from roughly 2300 feet (700 meters) at the narrowest point to over 1.9 miles (3 kilometers).

Persian and Greek armies crossed here on pontoon bridges as early as the 5th century BCE, and the area later marked the boundary of the Roman and Byzantine empires. The Ottoman conquest of Constantinople in 1453 turned the waterway into a strategic route controlled by Turkish naval bases for three centuries.

Local fishermen use traditional boats to work the waters of the channel, while tea gardens along both shores draw residents and travelers who sit watching the passing traffic. The name comes from Greek mythology and means ford of the cow, a reference to the story of Io, who is said to have crossed the water at this point.

Visitors can view the waterway from the water on excursion boats that depart from various piers on both sides, or from parks and promenades along the shores. The strongest currents occur during tidal changes, so the water appears noticeably more turbulent at certain times of the day than at others.

The waterway hosts a two-layer water flow, with surface water moving from the Black Sea toward Marmara while saltier water flows in the opposite direction below. This difference arises from the salinity gap between the two seas, with the Black Sea being less salty than the Sea of Marmara.