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In an Earth-like world, with a single Moon similar to ours, there is a mountain that is 1.000 meters high. On top of that mountain there is a crater with ca. 100 Km radius and 100 m of maximum depth. That crater was filled with rain water (it stands on a very rainy spot) during centuries and now it became a sea. Since the only water replenishment of this sea is from rain, it has become very salty (*) There is now a city built on the shores of said sea, almost at the sea level.

Now, for my storytelling, I would like to know three things:

  1. Would this sea have waves and tides?

  2. How could this sea drain its waters without flooding the nearby city?

  3. Would there be nocturnal fogs generated by this sea?

(*) EDIT: As a person in the comments has insightfully deduced, I'm describing an endorheic basin. People have pointed out that this may be tricky to accomplish. If you have answers or comments regarding the salinity of this sea, please post them here.

Now please, I urge people on this thread to stay on topic and answer the questions I actually posed.

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    $\begingroup$ " Since the only water replenishment of this sea is from rain, it has become very salty " ..? typically rainwater isn't ... salty? $\endgroup$ – Zxyrra Jan 16 '17 at 0:50
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    $\begingroup$ @Zxyrra The question appears to be trying to describe an endorheic basin, but getting some of the cause and effect mixed up. $\endgroup$ – SevenSidedDie Jan 16 '17 at 6:02
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    $\begingroup$ I'm rather amused by 1,000 meters being thought of as "high altitude" :-) $\endgroup$ – jamesqf Jan 16 '17 at 6:38
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    $\begingroup$ Compare the world's highest navigable lake, 3,812 m up: Lake Titicaca. $\endgroup$ – pjc50 Jan 16 '17 at 9:30
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    $\begingroup$ @PedroGabriel The title of your question states "is this plausible?" which suggests you're looking for feedback on the plausibility of your lake idea, but you're rejecting answers which talk about that plausibility. It's not unreasonable for someone to think you wanted feedback on the idea itself given the title. I suggest rewording the question title to make it more clear exactly what you do want. $\endgroup$ – barbecue Jan 16 '17 at 13:50
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  • Salt.

    Rain water is not salty at all; in unpolluted places it is the closest thing to distilled water to be found in nature. Lakes become salty only if they have no outlets; since this lake is in a very rainy region it must necessarily have outlets. It won't be salty, it will be a fresh water lake. I cannot think of any salt water lake in a very rainy area. The key to making a salty lake is to have no outlets, so that water loss is only by evaporation. Very rainy + high altitude + high evaporation make for a strange combination.

  • Tides.

    I'm afraid that there is no way to have tides in a lake only 200 km across. It's simply much too small. However, it can have seiches (and it most likely will). (OK, no tides is not strictly true; you may expect to see tidal amplitudes of about 1 cm or so.)

  • Drainage.

    The lake, being in a rainy region, has a constant influx of water. The water must go somewhere: it will flow out of the lake as a river. The outlet may be very spectacular -- see Blue Nile falls at the exit of Lake Tana. (As an aside, Lake Tana is quite similar to the lake in the original question, but it is not as deep. The lake in the revised question looks very much like Lake Victoria.)

  • Fog.

    Depends on climate. Lake-effect fog forms when the air is cooler than the water; for example, at temperate latitudes cold autumn mornings produce fog out of every little water surface. I would say that the high altitude makes morning fogs quite likely.

Edit: The question has been edited to specify that the lake lies in an endorheic basin. This contradicts the "very rainy" region. An endorheic lake in a very rainy region is not going to remain endorheic for long; the water level will increase until it will go over a sill or the lake will erode an outlet; especially a lake at an elevation significantly higher than the surrounding plain.

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    $\begingroup$ @PedroGabriel: Changed the answer to indicate Lake Victoria as a very similar real-geography lake. If fits very well. $\endgroup$ – AlexP Jan 15 '17 at 22:54
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    $\begingroup$ Concerning salt, it's also important that it's not being fed by any rivers - water entering the lake would not have a chance to pick up salt and bring it to the lake. $\endgroup$ – Rob Watts Jan 15 '17 at 22:56
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    $\begingroup$ Lake Victoria is very very much like this lake. Surface elevation 1100m. Maximum depth 83 m. Surface area 68000 sq. km (about double than the lake in the question). Fed by rainfall over a small area. $\endgroup$ – AlexP Jan 15 '17 at 23:07
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    $\begingroup$ @PedroGabriel: A lake remains fresh as long as it has an outlet; the lake accumulates salt if water is lost only by evaporation (because water evaporates and salt doesn't, so in time the concentration of salt increases). Examples: Salt Lake, Caspian Sea, Dead Sea. Salty lakes are tricky, because they live at the narrow edge where the influx of water exactly compensates evaporation. See how the Dead Sea has shrunk in recent decades. $\endgroup$ – AlexP Jan 15 '17 at 23:10
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    $\begingroup$ You can have bigger tides if you make the moon bigger. Of course, this means that oceanic tides will be even more significant... $\endgroup$ – IndigoFenix Jan 16 '17 at 7:18
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One big problem that nobody seems to have noticed:

This lake is on a mountaintop. That means its catchment area is little more than the lake itself—any other water goes down the outside of the mountain, not into the lake.

Thus you have in effect a giant-sized rain puddle that never dries up. I have a hard time picturing this.

As others have said, the normal means of making a salt lake can't work here. However, I don't consider this a showstopper, let's make a salt lake by a different method:

Long, long ago there was a massive magma intrusion in the area, perhaps there was some actual vulcanism but that's irrelevant. A huge area of granite was formed. As the millenia went by the material above this granite eroded away. (Granite only forms when the magma cools very slowly—which means it must be deep. The same material on the surface forms basalt—not nearly as hard. There's also an intermediate between these whose name I have forgotten in the decades since school.)

Now a supervolcano erupts, blowing a huge caldera—the size of your lake. This is lowlands, though, not a mountain. A salt lake forms, then it's opened to the sea and a fairly small amount of ordinary sedimentary rock is laid down on top.

Now the area is uplifted to your desired height, the wind chips away at the soft rock on top but it doesn't eat it all—the salt layer is still underneath. It reaches your desired height and the climate turns rainy for some reason. Now you have a basically freshwater lake on top of salt with a thin and damaged barrier—at some point the water reaches the salt and dissolves enough to make it salty.

If you will accept a somewhat greater deviation from your description:

While this is one mountain it's actually the foothills of an even higher mountain range that has arisen (uplift and vulcanism are often found together) The actual catchment area of the lake includes a decent chunk of those mountains, the water is flowing underground into your lake and through the salt.

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    $\begingroup$ I like this answer, because it explains how most salt lakes / rivers form up, trought underground salty passages. Salty caverns of cardona, where water still flows in small, very salty, and super cold, river streams. eyeonspain.com/userfiles/image/mac75/parc_06.jpg $\endgroup$ – CptEric Jan 16 '17 at 7:26
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    $\begingroup$ Than you for your answer. Unfortunately it is off topic. As soon as I can, I'll post another question regarding the saltiness of this lake and I ask you to copy-paste this answer there, for it might be the chosen answer. $\endgroup$ – Pedro Gabriel Jan 16 '17 at 7:48
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    $\begingroup$ @LightnessRacesinOrbit Our water sources are replenished mostly by water runoff not by direct rain. Direct rain only will have a hard time keeping up with evaporation. $\endgroup$ – John Jan 16 '17 at 14:58
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    $\begingroup$ @LightnessRacesinOrbit rainfall falling on a march larger basin than the body of water, this is not the case here, the drainage basin is the same size as the lake, which means it is receiving very little incoming water. Normally lakes are fed by rain falling on a land area hundreds of times the lakes surface area. $\endgroup$ – John Jan 16 '17 at 15:43
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    $\begingroup$ @kingledion Yes lake victoria would be a good model. $\endgroup$ – John Jan 16 '17 at 22:35
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A lake of 100 km radius will have an area of 31,000 km$^2$. That is about the size of Tangyanika or Baikal, or a little bigger than Lake Erie.

If it were 100m deep, then its volume would be about 3,100 km$^3$. That is about the volume of Lake Huron or Lake Victoria.

Lake Victoria is larger by surface area than your lake, about the same by volume, and has a surface elevation of 1133m. So you are basically talking about Lake Victoria in this question, except salty. The closest high altitude salt lake I can think of is Issyk Kul, at about 1/5 the surface area, 1/2 the volume (its very deep) and at elevation 1607 meters. However, if the water inflow to Lake Victoria dropped, it could easily become an endoheric lake due it its massive surface area and high evaporation. In that case, it would become salty over time.

Lake Victoria does not have tides (of any appreciable size), and drains to the sea through the Nile River. It can generate noctural fogs, but more importantly it generates huge areas of rainfall. There are thousands of square kilometers of savannah turned to rainforest conditions on both its East and Western shore.

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  • $\begingroup$ Up-voted for answering the questions in the topic. $\endgroup$ – Pedro Gabriel Jan 16 '17 at 19:57
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You need to be looking at Lake Bonneville and Lake Lahontan. The Great Salt Lake is a real salty lake at 1283 m at 4400 km2 that is a remnant of Lake Bonneville.

The lakes mentioned don't drain, they don't/didn't have tides, I don't believe that Bonneville was salty, but lower lake levels were, and the Great Salt Lake does produce fog.

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    $\begingroup$ Bonneville did drain, and rather dramatically :-) en.wikipedia.org/wiki/Bonneville_flood The remnants of the pluvial lakes of the Great Basin vary: the Great Salt Lake is, of course, salty, as is the Bonneville Salt Flat. Mono Lake is also quite salty, but Walker & Pyramid lakes are only slightly so. The playas (or at least the ones I'm familiar with) that are usually dry are more alkaline than salt. $\endgroup$ – jamesqf Jan 16 '17 at 6:33
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You have a really fundamental problem with wanting a mountaintop lake of that size. Mountains, as you may or may not have noticed, are kind of pointy on the top, which means there's not a lot of room up there to put a lake. Also non-volcanic mountains tend to occur in fairly linear bands - ranges, IOW - which again limits the area on top.

I can think of maybe 4 ways to get something vaguely like the lake you want.

1) A volcano which experienced a catastrophic eruption, leaving a caldera which is then filled by a lake. Oregon's Crater Lake, formed by the eruption of Mt. Mazama about 7500 years ago, is the classic example: https://en.wikipedia.org/wiki/Crater_Lake The problem here is that there are limits on the size of a stratovolcano, which means that the resulting lake isn't going to be that much bigger than Crater Lake.

2) A valley between mountain ranges which is dammed somehow. Lake Tahoe is a good example of this: the valley formed by uplift/downdrop between the Sierra Nevada & Carson Range was dammed by volcanic eruptions: https://en.wikipedia.org/wiki/Lake_Tahoe#Geology The problem here is that it isn't on the mountain TOP, and is surrounded by a larger catchment basin that feeds it through rivers & streams.

3) Endorheic lakes, like Lakes Lahontan https://en.wikipedia.org/wiki/Lake_Lahontan & Bonnevile of the Pleistocene Great Basin. These can be at the required elevation, but aren't located on mountain tops, but within higher elevation plateaus. (The lowest points in the Great Basin are more than 1000 elevation.) Lahontan had a high elevation of ~1500 m, but is surrounded by (and enclosed as islands) mountains of 3000 m and more. They also have large catchment basins, feeding them from rivers & streams.

4) Meteor crater lake, like Manicouagan: https://en.wikipedia.org/wiki/Manicouagan_crater These aren't going to be on mountaintops, either. If the impacting body did happen to land on top of a mountain, that mountain would be removed. along with much of the underlying rock. One large enough to form a lake of the size you want would likely fill with magma to about sea level, maybe below, e.g. large lunar craters.

Bottom line is that AFAIK there's no way to get a lake meeting all your requirements.

As for your questions, #1 has been adequately answered already. For #2 you have two choices, either the water rises until it finds an outlet (e.g. Tahoe), or it's endorheic because enough water evaporates before the level rises to an outlet. #3 depends on your local climate, e.g. the Tahoe Basin and Great Basin valleys can be filled with winter fog/low clouds (as it is today), but be clear in summer.

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