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I am doing worldbuilding on a region centered around a gigantic lake in the sub-tropical region in the northern hemisphere of a planet. I want the lake to incredibly huge, like around 2 million square kms. My question is, would it make sense for this lake to be freshwater at all? If no, would this now saltwater lake need to be connected to the sea/ocean and can it be underground?

I tried searching for any answers, but I was not able to find any answer to if it is possible for it to be freshwater at all. The idea for the biomes would be wetlands next to the lake itself, followed by jungles which turn in shrubland and then into a sort of savannah followed by a desert as we go further away (I am talking about a 5000 kms * 6000 kms sort of area). Would this make sense at all? I hope someone can help me with this dilemma!

Thanks to all who take time to read this. Cheers! :)

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3 Answers 3

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The larger the lake the harder it is to generate the required conditions for it to be fresh water. You are asking for a lake area similar to the size of the mediterranean sea. This is not impossible, it's just improbable that the oceans, mountains and uplands are arranged in such a way that it can exist.

It would also require a vast drainage area to keep it topped up. The mediterranean sea was itself cut off from the Atlantic a few million years ago and was reduced to a few hyper-saline lakes which were then flooded to produce larger brackish lakes until the Atlantic Ocean finally broke through. So a second issue is getting sufficient rain into the lake to keep evaporation at bay.

In summary it's possible but unlikely. The bigger the lake and the hotter the climate the less likely it is.

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  • $\begingroup$ Thank you for your answer! So it would have to be either - a saltwater lake or a sea that's been cut off from a bigger ocean? What if the region was surrounded from the north, east and south by mountain ranges and the southwest was connected to an ocean? Would that help in trapping the evaporated water in the region at all? Sorry if my questions seem a little dumb, I just recently started reading on all this. :) $\endgroup$
    – Ravenquothe
    Sep 19, 2022 at 10:59
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    $\begingroup$ Ideally the lake level would be higher than the ocean and would drain into it. If connected to the ocean in any meaningful way the waters would mix. Salt water is heavier than fresh water so would flow along the bottom of any channel and fill the lower levels of the lake whilst fresh water would flow out of the lake in the surface waters. Given time, currents, temperature changes and tides the waters would eventually mix. No such thing as a dumb question, only a dumb answers although some may be incoherent yours seems fine although site rules state that you can only ask one question at a time $\endgroup$
    – Slarty
    Sep 19, 2022 at 11:17
  • $\begingroup$ Thanks again :) That makes a lot of sense. Guess I'm scrapping the freshwater idea. $\endgroup$
    – Ravenquothe
    Sep 19, 2022 at 11:22
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    $\begingroup$ Consider the salt and freshwater lakes on Earth. None of the salt lakes drain to the ocean. That is why they are salty. If a lake is constantly draining and replenishing it's water with rain water, then it will be fresh. The big oceans and seas of the planet are salt because the only way water leaves them is via evaporation. I suppose, however, that you could arrive at a salt lake that drains somewhere if that drainage is minimal. $\endgroup$
    – jmarkmurphy
    Sep 20, 2022 at 15:43
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Salt Dynamics:

Salt builds up over time in bodies of water that don't drain out anywhere else. Lakes always have new fresh water draining into them bringing tiny amounts of salts with them. A lake with an outlet loses salt through the outlet and that water is replaced by fresh. A lake with no outlet gradually accumulates salt as the fresh water evaporates, leaving any salt behind.

So for a large body of water to be fresh, it needs to drain into a lower body of salt water. Very small lakes can be transient enough for this not to matter. But a BIG body of water has a lot of mass and volume behind it. The flows needed to outflow that much fresh water will rapidly cut rock and soil alike. While it is possible to have such a large body of fresh water, it is unlikely and probably fairly short-lived. The Great lakes, for example, formed due to glaciation damming up the water, pushing down the land, and providing a huge influx of fresh water to form the lakes. They are gradually disappearing as the depressed land from the glaciers gradually rises and Niagara falls slowly cuts it's way back further and further.

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  • $\begingroup$ This is awesome, thank you! I guess it makes more sense to have either a salt water or a brackish lake similar to the Caspian sea, than a gigantic freshwater one. This helps, thanks again. :) $\endgroup$
    – Ravenquothe
    Oct 17, 2022 at 12:26
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I got your giant lake right here!

lake agassiz

Lake Agassiz was an immense lake that existed in north-central North America during the last ice age. It is named for Louis Agassiz, the first scientist to realize it had been created by glaciers acting as dams. Larger than many modern seas, its waters were fresh, not salt. At its greatest extent it covered an area larger than California (see map right) and held more water than is today contained in all the freshwater lakes of the world combined.

http://www.macroevolution.net/lake-agassiz.html

140,000 square kilometers is smaller than the Mediterranean sized lake you want. But you could make your lake the same way our world made Lake Agassiz - lots of glaciers, melting. During the ice age your world had more glaciers than ours and when they melted, your worlds geography let the water accumulate in your vast lake.

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  • $\begingroup$ Very interesting! I do have some huge mountain ranges planned to the north of the lake, so I could play around with that, I think. Thank you for your answer! :) $\endgroup$
    – Ravenquothe
    Oct 17, 2022 at 12:28

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