Bosphorus

The Bosphorus (Turkish: İstanbul Boğazı) is a narrow strait located in northwestern Turkey. It connects the Black Sea and the Sea of Marmara, which in turn connects via the Dardanelles Strait to the Mediterranean Sea. The strait divides the city of Istanbul into two parts: the European side and the Asian side.

The Bosphorus is one of the world's most strategically important maritime passages for international commerce. It is approximately 30 kilometres long, though at some points it is extremely narrow. For this reason, large oil tankers and cargo vessels require specialized navigation assistance, and a system of traffic management and control is in place.

Geography

The strait runs in a north-south direction and has several curves. Both sides feature hills and high plateaus, which historically served as defensive positions and fortress sites. Water flows in two layers: an upper layer from the Black Sea with lower salinity, and a lower layer from the Marmara with higher salinity. This mixing affects marine life and fisheries.

History and strategy

The Bosphorus was for many centuries a vital nexus of imperial power. The city of Byzantium—later renamed Constantinople and then Istanbul—built its control of the empire on command of the strait. Both the Byzantine and Ottoman empires used the strait for defence and commercial revenue. Control of the strait has also been a subject of numerous conflicts and international agreements.

Legal framework

Traffic through the Bosphorus and Dardanelles is regulated by the Montreux Convention, signed in 1936. The convention grants Turkey the right to manage the straits and establishes limits on warships of non-Black Sea states—restrictions on tonnage, numbers, and duration of passage. In wartime, Turkey has broader powers of control.

Bridges and tunnels

The two sides are connected by several major bridges and by an underwater tunnel carrying rail transport. Ferries carrying passengers also form an important part of Istanbul's daily transport network.