The term “underwater river” refers to dense, flowing bodies of water that appear distinct from the surrounding seawater, creating a visual boundary on the seafloor. While the phrase suggests a traditional, continuous flow, it actually encompasses two distinct phenomena: sediment flow and highly concentrated saltwater.
Clarifying the Concept of Underwater Rivers
The phrase “underwater river” generally describes two separate geological processes. The first involves powerful, episodic turbidity currents. These are not permanent rivers but fast-moving flows of sediment-laden water that surge down continental slopes and through submarine canyons. Driven by the weight of suspended mud, sand, and debris, these flows are denser than the ambient seawater. They can reach high speeds, sometimes exceeding 70 kilometers per hour, and possess immense erosive power. Turbidity currents are responsible for carving out large canyon systems and can snap transoceanic telecommunication cables.
The other, more visually striking phenomenon is the deep-sea brine pool, a stable, persistent body of hypersaline water collected in depressions on the ocean floor. Unlike the transient nature of turbidity currents, the density difference between the brine and the overlying seawater is so extreme that the two water masses do not mix. This creates a visible, defined boundary that mimics a shoreline. The remainder of this discussion will focus on brine pools, as they are the primary subject of inquiries regarding localized danger due to their unique chemical and physical properties.
Formation and Unique Properties of Brine Pools
Brine pools are formed through specific geological processes that create water with a salt concentration far exceeding that of normal ocean water. One common mechanism involves the dissolution of large, ancient salt deposits, or evaporites, buried beneath the seafloor. As water seeps through cracks, it dissolves the underlying salt, creating a highly concentrated brine that is then forced out into a basin. Another formation method involves geothermal heating, where super-saline water is discharged from tectonic spreading centers. The resulting brine is typically three to eight times saltier than the surrounding ocean.
The intense density difference between the brine and the normal seawater establishes a sharp separation layer called a pycnocline. This stable boundary prevents the dense bottom water from mixing with the lighter water above it. Because the brine layer is effectively isolated from the oxygenated upper water column, it quickly becomes anoxic, meaning it is completely devoid of free oxygen.
The chemical composition of the brine layer makes these pools highly hostile to most life forms. In addition to the crushing salinity and lack of oxygen, the water often contains high concentrations of toxic compounds. Hydrogen sulfide and methane are commonly found within the brine, sometimes produced by chemosynthetic organisms or by decaying matter trapped within the dense layer.
Assessing the Hazards to Divers and Marine Ecosystems
The hazards associated with brine pools are primarily chemical and physiological, rather than physical risks like being swept away by a current. For deep-sea technical divers, physically entering the pool would not involve a strong current, but the extreme salinity could quickly damage specialized diving equipment and seals.
The immediate and most severe threat to any organism is the chemical toxicity and anoxia of the brine itself. The hydrogen sulfide and methane concentrations are lethal to most aerobic life forms, and the complete lack of oxygen means instant suffocation. In some cases, brines associated with hydrothermal activity can also be significantly warmer than the surrounding deep-sea water, adding a thermal hazard.
For marine ecosystems, brine pools are effectively “dead zones” where life cannot be sustained within the pool itself. The hypersalinity causes an almost immediate osmotic shock to any fish or invertebrate that accidentally crosses the visible boundary, causing cells to rapidly lose water. The anoxic and highly saline environment then acts as a preservative, leaving the bottoms of these pools littered with the intact carcasses of organisms that ventured too far.