How do water flows through the Bosporus work?

Water flow through the Bosporus is characterized by a complex, two-way exchange between the Black Sea and the Sea of Marmara.1

This system functions as follows:

• Surface Outflow: A surface current flows from the Black Sea toward the Sea of Marmara, carrying less saline water out of the Black Sea basin.12
• Deep Inflow: A countercurrent exists below the surface, which carries saltier, denser water from the Sea of Marmara into the Black Sea.12

This exchange is a type of density current, where gravity drives the movement of water based on differences in salinity. The process is further complicated by surface winds, particularly in the shallow sill or threshold between the two basins. Because this water exchange is relatively slow, it is estimated that a complete recycling of the Black Sea's waters takes approximately 2,500 years.31