What is the Black Sea underwater river?

The Black Sea underwater river is a remarkable phenomenon: a powerful current of particularly saline water that originates from the Mediterranean Sea, flows through the Bosphorus Strait, and continues along the seabed of the Black Sea. This unique current behaves much like a terrestrial river, complete with banks, floodplains, and even waterfalls.

What is the Black Sea Underwater River?

Discovered in 2010 by scientists from the University of Leeds, this undersea current is an actual flowing body of water on the seafloor. Unlike typical ocean currents that are driven by wind or temperature, this "river" is primarily a density current, caused by the significant difference in salinity and density between the waters of the Mediterranean Sea and the Black Sea.

The Mediterranean Sea's water is much saltier and, therefore, denser than the fresher, less saline water of the Black Sea. When this denser Mediterranean water enters the Black Sea through the narrow and relatively shallow Bosphorus Strait, it sinks to the bottom, forming a distinct, fast-flowing current that carves its own path along the seafloor.

Formation and Characteristics

The formation of this underwater river is a classic example of density stratification in an enclosed or semi-enclosed basin.

• Salinity Difference: The Mediterranean Sea has an average salinity of about 38-39 parts per thousand (ppt), while the Black Sea's surface waters are significantly less saline, around 17-18 ppt, due to the inflow of numerous large freshwater rivers.
• Density Driven Flow: This density difference causes the heavier Mediterranean water to sink upon entering the Black Sea. It doesn't mix immediately with the lighter Black Sea water but flows underneath it.
• River-like Features: Sonar mapping has revealed that this underwater current has distinct features mimicking a river on land:
  • Channels: It flows through channels up to 37 miles (60 km) long, 0.6 miles (1 km) wide, and 115 feet (35 meters) deep.
  • Banks: The edges of these channels act as banks, containing the flow.
  • Rapids and Waterfalls: There are areas where the current accelerates over drops, creating features akin to rapids and waterfalls, pushing sediment along with it.

Discovery and Significance

The existence of such a large, distinct underwater river was a significant scientific discovery, challenging previous assumptions about deep-sea currents and the hydrodynamics of straits.

• Advanced Mapping: The river was mapped using a robotic submarine, or autonomous underwater vehicle (AUV), equipped with advanced sonar and sampling equipment.
• Volume and Speed: Estimates suggest the river carries about 22,000 cubic meters of water per second, making it one of the largest undersea currents known. Its flow rate can reach speeds of about 4 miles per hour (6.5 km/h).
• Ecological Impact: This deep-water inflow plays a crucial role in the Black Sea's oceanography. It contributes to the anoxic (oxygen-depleted) conditions found in the deeper parts of the Black Sea by bringing in dense, saline water that prevents vertical mixing of the water column. This stratification isolates the lower layers from atmospheric oxygen.

Key Characteristics at a Glance

This underwater river offers valuable insights into deep-sea hydrological processes and the complex interactions between different water bodies. It highlights that the ocean floor can host dynamic and visually striking features, similar to landscapes found on land.