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I'm looking for information on how feasible it is for a large, freshwater lake to become salty, at least to a level where the people, animals and plants that live around and rely on it can no longer drink or make use of it in their every day lives. My idea is that the lake in question (the Lake of Kurn) is a natural saltwater lake, and there exists a supernatural artifact within it that, when activated, purges the waters of salt and other contaminants. The process of doing so would have been occurring for many centuries.

The Lake of Kurn is located in a tropical savannah climate in the southern hemisphere (hot, semi-arid at its southern tip) and is roughly 58,000km² with an average depth of ~30m and a maximum depth ~78m; just smaller and shallower than the real-life Lake Victoria. The region it resides in, like most equatorial regions, has wet and dry seasons.

As I understand it, salt builds up naturally in any lake that does not have an outlet, and the process to change from fresh water to brackish/saline water can take years to decades to happen. What I am not sure of is how my particular lake could come about that change, or what conditions need to be met so that the lake requires maintenance. I'm not looking for the lake to become like the Great Salt Lake or Caspian Sea, I just want it to become salty or contaminated enough to no longer be healthy to drink, use for crops, etc. And I am looking for it to get to that point regularly, so that the ritual to purify the lake needs to occur every few years to prevent ecological and societal catastrophe.

In my search for information, the closest things I could find on the matter were Lake Bonneville, Lake Peigneur and the Salton Sea. None of which feel a fit for what I need, for one reason or another.

  • Lake Bonneville became salinized by the slow degradation of its outlets through the changing climate. In the case of the Lake of Kurn, it would be a freshwater lake with no outlets from the start, thereby being much more rapid in its salt accumulation. But I'm unsure how rapid.
  • Lake Peigneur became the way it is because of seawater flowing into it. The Lake of Kurn is over 100 miles from the ocean, so I cannot see a similar event occurring here.
  • Salton Sea almost matches what I am looking for, down to how rapid its contamination was. My major concerns for using the Salton Sea as a model are 1) the time period the story takes place in and 2) its geographical location. The story I am planning has a medieval-like setting. As such, the modern agricultural techniques that resulted in the Salton Sea's decline would not be present, or at least not to the scale needed for it. The Salton Sink was also located in a much more arid locale than the Lake of Kurn is, so the pre-existing conditions that created the saline lake would not be present to re-salinize a body of freshwater like the one I described, I wouldn't think.

My asks are:

  • What conditions would need to be present for the lake to shift from fresh water to more brackish waters, to the point it becomes noticeable?
  • How long would/could it take for this shift occur?
  • Would there be any additional ramifications from continually turning the lake fresh?

Any information on the viability of this would be appreciated. Whether it be deposits from tributary rivers, erosion from the soil, natural salt deposits in the lake bed, and so on.

Thanks!

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    $\begingroup$ if the lake has a thick salt crust built up on the bottom, like the great slt lake, removing the salt from the water will only last a while, but that salt resevior will eventually get used up. $\endgroup$ Commented Jan 12, 2025 at 0:13
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    $\begingroup$ What if the Lake discovered Reddit comment sections - that would be a good way to become Salty again (you are welcome) $\endgroup$ Commented Jan 12, 2025 at 18:36
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    $\begingroup$ +1 for a great question and loads of prior research $\endgroup$ Commented Jan 13, 2025 at 1:29
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    $\begingroup$ Isn't the real Question how a salt-water lake could have become fresh in the first place? Either way, the lake would go back to being salty because its tributary rivers went back to being salty. Consider the Aral Sea, which people have several times caused to dry up by diverting or damming its waters… $\endgroup$ Commented Jan 13, 2025 at 20:02

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Magic moves the salt, but not so far away that it can't run back into the lake, or there is a local source of salt.

My first thought was to use the magic removal of salt to prime the environment for its reintroduction to the lake. The salt doesn't disappear. Have it get moved and dispersed over a wide enough area that it does not become toxic, yet it still remains in the larger basin in which the lake resides. Over the years, precipitation and runoff transports the salt back into the lake, refilling the lake and adding salt until the ritual needs to be done again.

I don't really know if you need to crunch numbers for this. Just make the basin a large enough area in your story to make this seem plausible. The finite distance that magic can transport the salt can simply be a limit to its power. Maybe the magic loses it's affect outside the basin? Just make that limit a feature of your world.

My second thought was the basin could be located where an ancient sea used to exist. Salt deposits could exist underground that leach salt water into the lake. Every so often the salt needs to get removed. The advantage of this is that you don't need to explain where the salt goes if it isn't pertinent to your story. It just goes away, but the remnants of the ancient sea make it brackish in a number of years.

You also don't need to wait for toxic levels of salt. Consider performing the ritual more often to keep the waters tasting fresh. You don't need to wait for people to get sick. The water just needs to taste a little off. Amounts over 180 milligrams per liter and your water tastes salty.

Really, the best explanation is not needing to explain the physics of this. Just come up with something plausible. The salt doesn't go far enough away and runs back into the lake because that's simply a rule limiting the magic in your world, or some local source keeps replenishing it.

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    $\begingroup$ Another twist to throw in: The "wet season" is what causes the salt to be washed back in. Therefore, you do your artifact activation most years, typically at the end of the wet season. $\endgroup$ Commented Jan 12, 2025 at 23:56
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    $\begingroup$ Or, if you want it less frequent, after each particularly bad wet season. Or maybe even after every particularly bad storm in the wet season (as in after a 100 year storm). $\endgroup$ Commented Jan 13, 2025 at 1:39
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    $\begingroup$ If the main local subsistence crop is a halophyte, then the entire purpose of the artefact may be to move salt from the lake, to the soil of specific farmlands, to help the plants grow. And then the rains and rivers gradually wash the salt out of the soil, and back into the lake. Of course, that partly counteracts the "too salty to use for crops" — however, other crops may not be halophytes, and pumping water uphill to irrigate the fields can be inconvenient. $\endgroup$ Commented Jan 13, 2025 at 13:02
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    $\begingroup$ That last bit is probably the best advice of all lol $\endgroup$ Commented Jan 14, 2025 at 1:17
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The artifact doesn't remove the salt, it just separates it

The salt stays in the lake. The supernatural artifact merely prevents it from being dissolved in the water. When the artifact is active, the salt stays at the bottom of the lake, being magically separated from the water. As soon as the artifact stops doing its job, the barrier disappears, and the salt will quickly be dissolved again.

The ritual just means that the artifact will be powered again for the next x months or years. If you need the process to be slower when the artifact loses power, just make the barrier become weaker and weaker, instead of disappearing immediately.

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    $\begingroup$ I like this. In a still lake, water can form layers with different properties that don't normally mix. Salt water is heavier than fresh, so it could be that the salt is just pushed to the deeper layers. It would then mix slowly, perhaps enhanced by storms, earthquakes, or even just people dropping things in the lake. (Consider perhaps the Lake Nyos disaster.) $\endgroup$ Commented Jan 13, 2025 at 13:50
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    $\begingroup$ Huh. This idea would thematically fit the story I was planning, actually. $\endgroup$ Commented Jan 14, 2025 at 0:58
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Perhaps there was a salt plain in the area a long time in the past, which became covered by other sediments, and salt tectonics has caused the formation of salt domes in the area.

Other tectonics may have resulted in the area becoming a water trap - i.e. a lake - but erosion had first exposed the top of one or more salt domes.

So, this Lake of Kurn would be above salt domes that are slowly dissolving into the fresh water. For greatest effect, the magic that desalinates the lake would be forcing the salt back into the salt domes, but even if it was removing the salt from the water entirely, there could be enough salt in the domes to keep the lake salty for millenia.

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Just add salt.

  • What conditions would need to be present for the lake to shift from fresh water to more brackish waters, to the point it becomes noticeable?

Add enough salt.

Brackish water [...] can range in salinity anywhere from 0.5 to about 30 ppt.

oceanconservancy.org - What are Brackish Water Environments?

  • How long would/could it take for this shift occur?

As much time as it takes to add the salt. Even cold water can dissolve salt in less than 10 minutes.

In cold water (around 0-10°C or 32-50°F), salt can take anywhere from 1 to 10 minutes to dissolve, depending on the amount of salt and whether the water is stirred.

quora.com - How long does it take to dissolve salt

  • Would there be any additional ramifications from continually turning the lake fresh?

If by that you mean the lake would be repeatedly changing from fresh to brackish and back then you'd kill any wildlife that depended on the water being fresh and any wildlife that depended on the water being brackish. You'd select for wildlife that could handle both. Humans, being what they are, would likely find a way to turn it into a tourist attraction.

If, however, you only mean ramifications of continually magically turning a lake that should have been brackish to fresh the biggest ramification is confusing geologists who don't know you've been playing with the laws of physics.

As for how to add the salt, the ocean isn't the only source of salt. There are other salty lakes (one could empty into your lake after an earth quake), rivers that deposit dissolved salt regularly, and this magical doohickey of yours that could be depositing the salt somewhere handy.

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  • $\begingroup$ And salt domes in the earth that used to hide oil. Give any number of earth changing events and a salt dome can be exposed. $\endgroup$ Commented Jan 12, 2025 at 15:13
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Flooding?

Especially if the lake is near the "southern tip" of the continent, then some natural or supernatural event causes the ocean to overflow the natural boundaries and into the lake. Perhaps there is a volcano offshore that produces tsunamis periodically. Perhaps your world has many moons or a comet-like object which raises tides exceptionally every few centuries. Perhaps an angry god spawns Noah-like random events. Perhaps the merfolk have gone to war again.

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  • $\begingroup$ "Perhaps the merfolk have gone to war again." I hate it when that happens. ;^) $\endgroup$ Commented Jan 12, 2025 at 18:39
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Lake Peigneur

Lake Peigneur is the perfect way to make it salty again. The oil rig drilled into a salt mine beneath, causing a singhoke as the water rushed to fill the gap. Afterwards the flow of water was from the sea to the now much deeper lake, adding salwlt water.

Say your magical artifact is in fact filtering the salt, then storing it underground under the lake. The moment the water erodes the ground enough to reach that salt, it can mix with the water again. Fully dependent on the contact area between the salt and the water it can go fast or slow. For example, it can just be an area not bigger than your arm. It could also be a tunnel with salt edges, set on an area with much moving water, allowing tons of salt to dissolve in a day.

You can substitute the magically transported salt with a salt vein if you like.

You should be able to notice it before it reaches 0,5 parts of salt per thousand parts of freshwater. That is the lower limit between fresh and brakish water.

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A Geothermal Feature Spits Out Salt

In the Dallol province of Ethiopia, there are geysers and volcanoes that produce salt water, salty mud, and leave salt cones. Import the salt geysers and the underlying volcano from Ethiopia, and tune the volume and frequency of eruptions, and the various chemicals produced, to meet the needs of your story.

Ref: https://en.wikipedia.org/wiki/Dallol_(hydrothermal_system)

(Note: I was originally looking for mud volcanoes, well, actually mudpots (as seen in geothermally active places like Yellowstone and Iceland), which could plausibly produce saline mud, but stumbled across Dallol which was just so much weirder and more interesting.)

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Salinity (on Earth) is measured in a unit called parts per thousand (ppt), which refers to the concentration or percentage of salt in the water. Less than 0.5 ppt is the average for fresh water, while 35 is roughly the level of sea water. Brackish is roughly anything in between.

So you could go from fresh to salty merely by evaporation: the actual water leaves the lake and the salt stays behind. (Note that this is basically how salt was harvested for centuries.) Basic math tells us this would require 69/70th of the lake to evaporate, to turn it truly salty. Some level of brackishness might require less, but I'm guessing you'd need at least half to evaporate before it gets noticed. This of course creates a lot of additional environmental pressures, which you might not desire.

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Without magic there's examples of this as a natural phenomenon. These are called Meromictic lakes. Apparently the Black Sea is the largest known meromictic environment.

Similar in principle to making a traffic-light cocktail where you have layers of different density fluids sitting on top of one-another. Salt water is more dense than fresh water, so fresh(er) water will "float" on top of salt water.

In a lake, such as you describe with no natural outlet, as described you get evaporation of any incoming water, leaving the salt behind. You could have surrounding hills (possibly distant) that every so often experience a large dump of water. This causes an influx of fresh water that sits on the top of the salty water, leaving the lake as fresh water. The resulting re-salting (sorry, couldn't resist) could happen by a few mechanisms - turbulent mixing would be the most likely - you'd get some with the in-flow, followed or in combination with, evaporation, and of course diffusion. Diffusion alone would be very slow, as would evaporation.

For an example of fresh sitting on salty water - see any of the meromictic lakes in the Wikipedia link in the first paragraph. Also the Amazon River has a fresh water plume that pushes out over the salty Atlantic Ocean for about 200 km (~124 mi) from the shore. To quote Wikipedia:

The river pushes a vast plume of fresh water into the ocean. The plume is about 400 km (250 mi) long and between 100 and 200 km (62 and 124 mi) wide. The fresh water, being lighter, flows on top of the seawater, diluting the salinity and altering the colour of the ocean surface over an area up to 2,500,000 km2 (970,000 sq mi) in extent. For centuries ships have reported fresh water near the Amazon's mouth yet well out of sight of land in what otherwise seemed to be the open ocean.

For a magical explanation of how the lake becomes salty again rapidly. A monster lives in the bottom of the lake. It occasionally needs placating, or it becomes angry and stirs up the lake. This mixes the salt layer in. The monster is then placated and the lake goes back to getting fresh water sitting on top of salty.

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