When we think of rivers, we sometimes imagine crystal clear waters flowing as a stream, gradually growing on its journey to the sea. On its way, it may flow into a lake, or down a waterfall, both of the banks shaded by trees in some places and cities in others. Eventually it flows into the ocean, repeating the water cycle or travelling the oceanic highway.

But what if a similar process happened within the depths of the ocean itself? After a surface river has reached the ocean, flocculation removes any sediment brought along. It binds particles together into larger and larger flocs, which then fall to the ocean floor. Here is where an underwater river begins. Undersea Canyons are key to this, possibly carved by the repeated processes, or naturally formed to cause this phenomena. A mix of sediments and dense, saline water travels down as far as 3.2 kilometres and thousands of miles long, eventually spilling out onto the abyssal plain and the deep ocean. Channels like this are found all over the world, etched into the seabeds.

3-D view of Astoria Canyon, west of the Columbia River entrance showing locations of science transects (appear as colored ribbons on canyon walls)
Author: NOAA Photo Library

 

One of the least understood occurrences on the planet, undersea rivers are fundamental to the survival of many deep-sea communities.  A fairly well explored area is the Bosphorus Straight. Saline water from the Mediterranean Sea flows through the Straight, and into the Black Sea. Sonar scanning had previously found channels carved into ocean floors, and were suspected of being caused by ‘river-like’ activities. With the discovery of the Black Sea river, uncertainties were made certain. The streams were unpredictable and powerful at times, and so an AUV (autonomous underwater vehicle) was used to investigate the channel. It measured 60km long, 0.97km wide, and 35m deep. It flowed at a speed of 4mph, with 22,000 cubic metres of ‘water’ travelling through it per second. It carried unto 10x more water than the Rhine (one of Europe’s biggest rivers). Typical surface river features were also discovered. River banks, floodplains, waterfalls and rapids all exist, with one interesting difference. When the river rounded a bend, the currents spun in the opposite direction, compared to rivers on land. Due to the sediments and the higher salinity it was comprised of, the river acts as a density current.

Black Sea – screenshot from NASA’s globe software World Wind (Bosphorus Straight – lower left)
Author: Bogdan

Another interesting example is Cenote Angelita (Spanish for Little Angel) in the Yucatan Peninsula, Mexico. Here, instead of saline water like in the Black Sea, a layer of Hydrogen Sulphate/Sulphide separates from the saltwater. Complete with ‘trees’ and ‘leaves’, the area makes for a surreal setting. The Hydrogen Sulphate/Sulphide is most likely produced by bacteria from decomposition of organic matter.

A Halocline – Cenote Angelita (Spanish for Little Angel), Mexico
Author: Jeffsery

While some of these rivers can be extremely important for the survival of organisms and communities, some deep-sea ‘water features’ can end up being literal death-traps. Brine pools (and in some cases ‘brine lakes’) are hyper-saline (~4-5x) ponds for only the well adapted. Microorganisms, such as bacteria, are the only living things tough enough to withstand the super-concentrated concoction. Common in the Gulf of Mexico, the pools form a characteristic “shoreline and surface“. Those that cannot live in the pools adapt to live around them. Chemosynthetic bacteria have a symbiotic relationship with mussels that encrust the pools. Any organisms unfortunate to fall into a pool will meet a ‘pickled‘ fate. The pools are so dense that a submersible can rest upon the surface without falling through.

A mussel bed at the edge of the brine pool (Deep sea : in “Green Canyon 246” (GC246)). Image : Lophelia II 2010 Expedition, NOAA-OER/BOEMRE.
Author: NOAA, from Expedition, NOAA-OER/BOEMRE

All of these unique and newly discovered environments still hold many questions. It has been theorised that by uncovering more about such strange habitats, that they may provide us with possible models for what we may encounter on other planets.

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