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I am currently creating a map. It has two landlocked inland seas. However, I want to make them interesting. I am still learning, so please bear with me regarding my formatting.

The approximate height of the inland seas is around 600 M above sea level. They are inside of two ancient calderas that are around 100 metres deep. The mountains around it are around 1.8 KM above sea level on average, with ~200 metre fluctuations here and there depending upon the peak. Large and ancient glaciers cap them, which melt and refreeze yearly as winter and summer come around. Annual rainfall is around 650 mm per year. The seas are separate from each other. They feed into small rivers going North and Northwest, which are filtered of the salt to be natural freshwater by abundant sediments.

Please let me know what I may be missing. I would like to know the specific template of formatting for future posts if I make a mistake. Additionally, please let me know if I am too broad, in which case I will tighten whatever is missing.

Thank you for your contribution.

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    $\begingroup$ In what important way is this fantastic sea different from the real world Lake Tanganyika? Surface area 32,900 sq km (12,700 sq mi), max depth 1,470 m (4,820 ft), surface elevation 773 m (2,536 ft) above sea level. Or Lake Victoria, with an even larger surface area of 59,947 sq km (23,146 sq mi), surface elevation 1,135 m (3,724 ft). Or Lake Baikal for that matter. $\endgroup$
    – AlexP
    Commented May 6 at 23:21
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    $\begingroup$ "Large and ancient glaciers cap them, which melt and refreeze yearly." That kinda implies that the ice fields are entirely melting and reforming yearly which contradicts the 'ancient' $\endgroup$ Commented May 6 at 23:22
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    $\begingroup$ Seas are salty. which implies much less water exiting via rivers then what is coming in by whatever source. If that condition is met then plausible. $\endgroup$ Commented May 6 at 23:30
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    $\begingroup$ you said how deep but how wide are these lakes, generally they need to be far larger than any caldera to be called seas. $\endgroup$
    – John
    Commented May 7 at 0:31
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    $\begingroup$ To help with some clarity. It sounds like your main question is not if an inland sea of this size can exist. Rather that the sea you describe can exist, and spacificicly, be classified as a freshwater inland sea. Is this correct? $\endgroup$
    – Gillgamesh
    Commented May 7 at 15:57

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Lake Titicaca on the Bolivian-Peruvian border is 3,812 meters above sea level (over six times your sea's elevation). This lake is situated in the Andes Mountains, making it an excellent analogy.

Another comparable body of water is Tilicho Lake in northern Nepal (4,919 meters above sea level).

Lake Titicaca is 8,312 km2, and Tilicho lake is 4.8 km2. Your inland sea could be a larger version of those.

Judging from the climate you have described in the second paragraph, this inland sea is glacial like Tilicho Lake.

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Crater Lake, Oregon

  • Max. length 6 mi (9.7 km)
  • Max. width 5 mi (8.0 km)
  • Surface area 20.6 sq mi (53 km2)
  • Average depth 1,148 ft (350 m)
  • Max. depth 1,949 ft (594 m)1
  • Water volume 4.49 cu mi (18.7 km3)
  • Residence time 157 years
  • Shore length1 21.8 mi (35.1 km)
  • Surface elevation 6,178 ft (1,883 m)

Entirely refilled by rain, snow, and glacial melt. The elevation and rainfall are both about 3 times what you are proposing, but Crater lake is at the top of the mountain, where as your proposition puts your inland seas more at the base. I would think that more than accounts for the viability if the mountains are the primary collectors and the seas fill from the combined run-off the surrounding mountain range. Crater lake is also significantly deeper than your proposed seas (averaging 350 m, but nearly 600m at the deepest). So, assuming the same total volume of water, they could have a much larger total area.

https://en.wikipedia.org/wiki/Crater_Lake

enter image description here enter image description here

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Is it entire necessary for these volumes to be salty seas and not just giant lakes? Unless the salt itself is somehow important to your story, make them giant elevated lakes, which happen all the time.

The problem with making these saltwater, is:

  • if the rainfall> than water loss, then the sea will turn into a sweetwater basin.
  • if the rainfall< than water loss this sea will turn into briny marsh, then saltpan.

The only way around this problem is coming with a clever idea that the mountains themselves are somehow salty already, and continuously enrich the water with salt at the same rate the water is replaced. Im not sure if this is scientifically plausible.

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  • $\begingroup$ Eh? No, no, I don't want them to be salty. I want them to be not salty. $\endgroup$
    – Rory 02
    Commented May 9 at 12:07
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First: Let's talk semantics

While some sources will say that inland seas and salt lakes are synonymous or that it is just a matter of size, there is one key difference that sometimes comes up. Salt lakes form when a river terminates in a lake with no outlets whereas an inland sea is connected to the ocean by a narrow straight allowing for a mutual exchange of salt water. Your description of a caldera filled with water would technically make it a lake because it is too high to mutually exchange water with the ocean. Even if it is connected to the ocean by a short river, the elevation difference will prevent a mutual flow of water.

Seas are also very large. The smallest body of water that is considered a sea is the Marmara sea (11,350 km2), but the largest high elevation lake formed by any mountain caldera is Yellowstone Lake (342 km2)... which is not nearly big enough to be considered a sea even by the looser definition.

Why mountain caldera lakes are size limited

The funny thing about Yellowstone Lake is that it fills up less than 10% of the actual land area that is the Yellowstone Caldera. There are several mountainous calderas on Earth that are in the 2000-4000 km2 range that could conceivably be considered small seas if filled but, but none of them are able to completely fill up.

While smaller calderas like Laguna de Quiltoa can form nice symmetrical circular ridgelines, very large calderas like Yellowstone that form along mountain ranges create very uneven calderas that can not create a perfect boundary due to the original geography of the surrounding mountains. As such you can not get lakes much bigger in a mountain caldera than the original mountainous geography would have allowed for anyway.

This unevenness also means that large mountain calderas, even very young ones like Long Valley, tend to quickly develop rivers that cross through them. These rivers break down the ridgelines even further keeping them from filling up.

In conclusion, if by "inland sea" you are okay settling for a decent sized lake, then sure, it's totally doable, but the fact that you are in the mountains will limit its size regardless of whether you place it in a large caldera or not.

Use an Intermontane Basin or Continental Rift instead.

While mountain calderas are volcanically blown out holes in existing mountain ranges, there are much larger geological structures that can form flat lowlands at higher elevations.

Most intermontane basins are formed due to large areas of the Earth's crust having slightly different general compositions than those adjacent to them in an area where mountain formation occurs. In these places, the softer rock erodes away faster than the surrounding mountains and then fills up with sediment causing them to flatten out. These areas can be much bigger, and much flatter than mountain calderas which allows them to form much bigger high elevation lakes. Lake Titicaca for example is an elevated lake that formed in such an intermontane basin with a surface area of 8,372 km2 making it bigger than any mountain caldera and almost as big as the smallest sea.

But the real winner is going to be Continental Rifts. These are special kinds of intermontane basins where the crust is splitting which can lead to truly massive depressions in a mountain range that can span entire continents. The Albertine Rift in Africa is responsible for many of the world's largest lakes including Lake Tanganyika which has a surface area of 32,900 km2 (bigger than the smallest sea). It is the largest lake in the world outside of the Great Lakes, but unlike the great lakes, it is at an elevation of 773m (over 600m) making it fit your settings requirements.

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