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This image demonstrates what I'm trying to do. Basically the ocean "drops" down to a lower level via a waterfall, with different sea levels, sort of like how we have "locks" on our rivers. Is there any way this could be possible, or is it just too far out there?

enter image description here

You can't see a bottom in the image, but in my idea, there is another ocean way down there, and a massive "wall" leading up to the higher ocean. I'm not sure if water can circulate in this way, but maybe evaporation could constantly redistribute it as it falls down.

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    $\begingroup$ Wouldn't the upper one lose all its salt and become a lake rather than sea - given sufficient time? WHat sort of time-scale are you looking at? $\endgroup$ Mar 31 at 8:36
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    $\begingroup$ @Sam Moon with out magic no it is impossible. The water on the upper edge would just drain. But if you have multiple oceans completely cut off from one another than yeah they can have different sea levels. But as you have shown it it's impossible. $\endgroup$ Mar 31 at 8:48
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    $\begingroup$ It would seem that this is exactly what happened when the Mediterranean emptied/filled a few times over the last few million years. (As a answers have mentioned.) However, it only lasts for perhaps two or three years and that's it. There are endless articles on this, just google "how long did it take the Med to fill" scientificamerican.com/article/… $\endgroup$
    – Fattie
    Apr 1 at 13:28
  • $\begingroup$ Actually here's a more up to date info on that ... science.org/content/article/… .. it may have taken as little as months (!!) for the bulk of the Med. filling. $\endgroup$
    – Fattie
    Apr 1 at 13:33
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    $\begingroup$ Interestingly, this exists on a small scale. The Pacific Ocean is about 20 cm higher than the Atlantic at the site of the Panama Canal. A sea-level canal would have a constant flow of water. $\endgroup$
    – Wastrel
    Apr 1 at 14:28

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An example on smaller scale of what you want can be seen at the Bøsdalafossur waterfall, in the Faroe Islands.

enter image description here

However in that case it's a lake forming the waterfall in the ocean, not an ocean.

Something similar involving oceans has happened in the past when the Gibraltar strait opened up again and the Atlantic started falling down into the dried up Mediterranean sea, during the Messinian salinity crisis.

And I believe that's the only way in which it can happen in your case: a transitory phenomena, when an ocean pours down in a water deprived region until it fills it.

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    $\begingroup$ I doubt that Gibraltar opening up looked very much like the image in the question. Probably rather like a huge river with enormous rapids, but stretching over some distance. $\endgroup$
    – gerrit
    Apr 1 at 16:44
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This has happened already multiple times in geological history. For example, the Mediterranean dried up multiple times and then refilled. Likewise, the Black Sea used to be a deep valley with a lake and then the Bosphorus opened up. When the Atlantic Ocean first started opening up, it was significantly lower. At present, the Atlantic Ocean is about 20 cm lower than the Pacific. https://psmsl.org/train_and_info/faqs/#3

Large differences are always "temporary" but can last for years. They happen when the land to be filled sinks first and then opens to the ocean. The speed at which they fill up depends on the size of the opening to the rest of the world's oceans.

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    $\begingroup$ The opening to the rest of the world will deepen rapidly. Catastrophic flood events (of which there are many in the geological record) erode rock at enormous rates; the rate at which the base of the channel drops is faster than the rate at which the level of the surface of the ocean drops, so you get a runaway process (until things are mostly equalized). $\endgroup$ Mar 31 at 19:59
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    $\begingroup$ @DonalFellows Catastrophic floods erode rock by rubbing other rocks against them. There isn't as much erosion at the sill as there is further downstream. Estimates of the erosion at the sill of the Strait of Gilbralter is that it eroded at less than a meter per day (which is still fast) en.wikipedia.org/wiki/Zanclean_flood $\endgroup$
    – David R
    Apr 1 at 14:46
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    $\begingroup$ @DavidR if flow rates are high enough you'll get cavitation effects which are bad news even for very hard bedrock in clear water. Not that it'll remain clear for long, of course. $\endgroup$ Apr 2 at 12:33
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This is Impossible

The equation is simple: water lost to the "lower ocean" must be replaced by rainfall (if we assume no magic).

If the waterfall is not limited to a very, very small area (and the upper ocean is otherwise bounded), you would effectively need a waterfall's worth of water constantly falling on the upper tier.

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In addition to the already-mentioned problem with a viable water cycle you have the problem that waterfalls are always temporary--they erode upstream over time. In the long run, water always beats rock.

However, if you'll accept that this is an artificial construct it doesn't take that much of a handwave. That wall of water is actually a very thin layer over a wall of unobtainium that's tough enough not to erode. And that lower ocean is so far down that it's extremely hot, much of the water evaporating as it lands. (This requires the energy of the fall to exceed the energy to raise the water temperature and the heat of vaporization.) Extremely violent weather, though!

Note that even this will eventually fail as your upper ocean will silt up. To make something truly permanent you need to look to the Ringworld, pump the silt from the ocean bottoms up to the mountain tops.

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  • $\begingroup$ Great call on it being a very thin layer, that changes everything $\endgroup$
    – Fattie
    Apr 1 at 13:29
  • $\begingroup$ What if that obtanium was some type of coral. that is constantly building up the dam against the erosion... and feeding off of the silt that is caught in it's dam? $\endgroup$
    – Questor
    Apr 2 at 21:59
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Short form: This doesn't work long-term.

The Horseshoe Falls at Niagra passes 2.8 million liters (~750 thousand gallons) of water every second. Your ocean picture is a couple of orders of magnitude wider at a quick guestimate, with a similar flow rate, so maybe 280 million liters (~75 million gallons) every second. That's... a lot of water.

But the problem you're facing isn't just the volume, it's raising all that water up at some point for it to be able to fall down again.

Evaporation might help, assuming that you have a way to ensure that the rains only fall outside the lower ocean. The unintended result of this is that your upper ocean will eventually become a very large fresh-water lake while the lower ocean will become super-saturated with dissolved salts and so on.

So really what you need is a way to force water to run up-hill somehow, in large enough quantities to balance the flow rate from your waterfall. Which would be a hard enough trick using the best engineering we can currently manage, let alone through some putatively natural process.

Maybe you can do something with underwater tunnels filled with volcanic vents which super-heat the water and push it through to a higher level. Assuming the tunnels are somehow shaped like Tesla Valves that might work. You just have to explain how that shape came to be... but I'm going to leave that to you.

The next issue you'll have to face is sedimentation, both from particulates carried over the waterfall and from erosion due to the flow. Given a long enough time the lower ocean will fill with sediment until the bottom raises to the level of the upper ocean. It'll be slow, and increasingly slower as the level approaches even, but unless something is removing that sediment buildup it's inevitable. Of course your water-raising formation will probably be choked up long before that happens, so the upper ocean may empty before the levels equal out. Best case scenario is that you end up with a massive channel cutting through to your pump formation.

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    $\begingroup$ It's a lot of water, yes, but it's less than the historic peak flow of the Amazon River. $\endgroup$
    – Mark
    Apr 2 at 22:38
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    $\begingroup$ @Mark Yeah, but the Amazon River is fed by tributaries capturing water - fresh water - from amost 7,000,000 square km. And it's currently averaging less than that 280 million liters per second. It doesn't have falls for a few reasons, but imagine the erosion if it did :D $\endgroup$
    – Corey
    Apr 3 at 2:16
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On Earth, we have one big body of water, commonly known as "the Ocean" which covers most of the surface and a number of smaller bodies of water, usually called lakes.

These smaller bodies of water can be above or below the Ocean, and this difference is maintained because water evaporates in one place, moves with the wind and rains down somewhere else. Weather can move big amounts of water.

Often there will be rivers moving the water back where it came from and they can have spectacular waterfalls.

If we move to another planet, there is nothing stopping you from saying there are multiple really big bodies of water.

On Earth, most of the planets surface is covered with water. This leads naturally to one big ocean with multiple continents. On a drier planet, where less than half of the surface is ocean, you will naturally get one big landmass, with multiple big bodies of water, possibly continent-sized.

Weather can move water from one to another to give the water level difference you want.

As Donal Fellows notes in a comment to another answer, waterfalls cause erosion, so the city in your image would not last very long.

To counteract this erosion, your planet will need to be geologially active, with continental plates moving around making mountains and such. (Like Earth)

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Strong Tide

It might be possible with a strong tide caused by a large/close moon.

The tide pulls water up on a smooth side, and then falls over the waterfall.

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Oscillating Tectonics

The city finds itself on a tectonic platelet that is sheared out of the main ocean and rises and drops with the tide.

The planet, which has a prograde moon with an orbital period very close to a day, has an outer crust made out of a naturally occuring form of graphene that resist rupture and tearing.

As its moon orbits the planet (very slowly from the perspective of anyone on the surface as it is nearly in geocentric orbit), the few areas of the crust not made from this graphene-like material find themselves subject to a concentration of hydraulic forces from magma in the outer mantle.

With nowhere for the magma to push but on these "islands", those areas periodically rise above the ocean, creating magnificient waterfalls that last for half the moon cycle.

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You could make it work, on a planet with a wobbly orbit. Meaning its a hot summer one month, winter the other.

The summers evaporate oceanwater which drops at the poles upon mile high glaciers. The glaciers freeze and move forward every winter. Then a brutal yet short summer arrives, melting them away- into freshwater oceans, that drop down towards the saltwater oceans.

These waterfalls would be seasonal though and they could never loose more water then the rain/snow transports to the glaciers.

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