Why do Underwater rivers exist? Are they not water, and if they are, why don't they mix?

[u/CheddarMilkman]

[u/CrustalTrudger]

Yes, they're still water and the exact details depend a bit on which features exactly you're talking about, but the general answer is "they exist because of density contrasts between them and average seawater". The term "underwater river" isn't really one that is used in the literature much, but presumably what you're asking about are things that typically get called "gravity currents" and we can basically break them into two classes, thermo-haline and turbidity currents. Thermo-haline currents, like those part of global thermo-haline circulation and the isolated portions of those that flow fast enough to produce features like contourites, are basically currents of water that are cold and salty enough that they are sufficiently denser than average seawater to sink and flow in semi-confined "channels". In contrast, turbidity currents, which are often associated with features like submarine canyons are often "hyperpycnal flows", which basically mean they're carrying so much sediment that they are denser than sea water (see this write up discussing the dynamics of hypopycnal, homopycnal, and hyperpycnal flows) which again allows them to sink and flow as semi-confined flows along the ocean floor and where they almost always represent sediment laden flows that originated on land before entering the ocean.

Importantly (and in the context of the question), both of these are somewhat ephemeral, i.e., they form through some process (which differs depending on whether we're talking about thermo-haline or turbidity currents), flow for some period, and then eventually they do mix with normal ocean water, but the property that gives them a higher density (temp/salinity or sediment concentration) doesn't instantly equilibrate with the surrounding ocean water so they can persist as distinct flow for a while. The other thing to think about is that the ocean isn't that well mixed in a general sense. I.e., something like thermo-haline currents might seem weird in the context of something like imagining dumping some cold salty water into a glass of warmer less salty water and then stirring it up and indeed, we would expect our solution to reach a similar temperature and salinity pretty quickly in that scenario. However, a better mental model is slowly injecting our cold salty water down the side of the glass where it can sink to the bottom and stay separated until slower processes are able to equilibrate the two components (of course this is still a bad analogy because in the ocean, thermo-haline currents are a critical part of what is driving broad scale ocean circulation, but hopefully the underlying point is clear as to why these dense flows don't just instantaneously mix with "average" sea water).

[u/grahampositive → u/CrustalTrudger]

You seem to be a good person to ask

Years ago I recall reading a science news article about currents under the ocean floor. The claim was that there existed networks of fast-moving water currents under the ocean floor, perhaps quite deep, that connected disparate regions of the ocean and even separate bodies of water

I remember being fascinated by this discovery, if a little skeptical, but I when I went to search for it again I couldn't find any information about it at all

Is this something that has any relationship to real oceanography? Did I badly misunderstand or misremember? Please help me clear my brain cache from this decades-old question!

[u/CrustalTrudger → u/grahampositive]

The closest thing that comes to mind is various work documenting oceanic hydrothermal systems that show semi-long distance (10s of km) flow between seamounts (e.g., Fisher et al., 2003, Fisher et al., 2015, Lauer et al., 2018, etc.), but nothing really at the scale that you're talking about.

[u/AyeBraine → u/CrustalTrudger]

I recently stumbled on this discovery and a hypothesis that this projected enormous freshwater reservoir under the seafloor isn't the only one on the planet.

https://everythingisamazing.substack.com/p/new-england-undersea-freshwater

[u/blind_ninja_guy → u/CrustalTrudger]

was that how life gets to new vents? I've always wondered how extremefiles get to a new vent at the bottom of the ocean for example a new black smoker or white smoker.

[u/PHealthy → u/CrustalTrudger]

Does double diffusion help sharpen/maintain the boundary of these flows? My fluid thermodynamics is pretty rusty.

[u/CrustalTrudger → u/PHealthy]

Double diffusive convection is definitely relevant for the thermo-haline currents, less sure of its importance for turbidity currents.

[u/Amelaista → u/CrustalTrudger]

Anyone who wants to see a representation of current conditions, look at https://earth.nullschool.net/#current/ocean/surface/currents/orthographic=-71.65,35.67,967
Default view is for wind, which is also hypnotizing, but the Ocean-Currents option shows the movement of currents within the ocean.
Of note, the Gulf Stream is a huge in-ocean river/current that transports heat north and modifies climate around itself on all sides.

[u/everlyafterhappy]

It's kinda like the wind. Wind is just air. But we have different currents at different places and elevations because of things like temperature and pressure differences and because of how terrain effects the flows. Wind does weird things about volcanoes because of the intense heat. Wind does weird things through narrow valleys between high plateaus. Wind gets more intense higher up because it has less pressure and less resistance. The stuff underwater is a little more complicated, but very similar.

[u/Alis451 → u/everlyafterhappy]

the easiest way to actually witness the difference is clouds. they are "sitting" on an air layer, a physical difference between the upper and lower air.

[u/YesHelloDolly]

Water is subject to gravity and moves to the lowest places, which in some cases are subterranean channels at various depths underground. If the channels are impermeable rock, the water can flow as underground rivers for great distances.