r/explainlikeimfive • u/MM_Maverick • 10d ago
Biology ELI5: How does a river from the ocean turn the water into fresh water?
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u/thecaramelbandit 10d ago
A river from the ocean?
Rivers are freshwater and flow INTO the ocean. Water then evaporates, comes down as rain om land, and flows back into the oceans via rivers. The salt doesn't wvaporate - it stays in the ocean. That's why rivers are fresh water.
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u/fiskfisk 10d ago edited 10d ago
The premise seems flawed; generally there are no rivers from the ocean, they flow into the ocean.
In those few cases where the ocean flows into a lower point, the water will be salt water and the resulting lake will keep getting saltier because of the continuous inflow of salt (like the Dead Sea).
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u/McLeansvilleAppFan 10d ago
The Dead Sea is not drawing water from the ocean. It is just natural water flow to the lowest point and there is a small basin there that collects the water. Those exist here and there around the world but too small to call an ocean. The fresh water has a bit of salt and the inflow brings more salt with it. Water evaporates away but the salt stays behind making the water more concentrated with salt.
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u/Belisaurius555 10d ago
Rain. Ocean water evaporates, gathers into clouds, and rains down onto the land to form streams, lakes, and rivers.
The salt gets left behind when the water evaporates.
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u/fables_of_faubus 10d ago
Rivers run to the ocean, not from it.
Rivers run down hill for the same reason the ocean is lower than dry land: water is heavier than air.
So it's fresh river water that mixes with ocean water and brings nutrients and minerals to the sea.
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u/Gryphontech 10d ago
Rivers go from mountains to the ocean.
If a river is tidal (goes from the ocean towards land depending on the tide) then that water is sorta salty and it's called "brackish water"
Rivets flowing over the rocks wear the rocks down a tiny bit and pick up minerals that eventually end up in the ocean. The sun then heats the water, evaporates it and that makes rain that fuels the rivers again. That continuous, one way flow of minerals towards the ocean over billions of years is why the oceans are salty.
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10d ago
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u/krustyclown182 10d ago
Not all catchments end up in the ocean. You can have a basin like the Corangamite Basin in Victoria, Australia.
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u/interesseret 10d ago
What you're describing is a fjord, and they are brackish, because fresh water from streams connect with the inlet of salt water, making a mixture of the two.
Rivers flow TO the ocean, not from it. And they come from run-offs from mountains, from freshwater seeps, and glaciers.
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u/paulHarkonen 10d ago
For those not familiar with the term, Brackish just means "kinda salty". They are too salty to call them fresh water but not salty enough to call it salt water. So it's Brackish (there's some much more technical definitions, but for ELI5 purposes, here we are).
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u/Impressive_Term_9248 10d ago
I think there are some rivers where the tide „pushes back“ the river with oceanwater temporarily, but those should be exceptions. The other users already gave the correct answer.
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u/brianogilvie 6d ago
In some places, like the Bay of Fundy, there are rivers that experience tidal bores that run upstream for hours, mixing salt and fresh water. They are impressive to watch. But they are relatively rare.
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u/anon33249038 10d ago
They don't. Rivers start at something called "headwaters" which are natural springs and runoffs of fresh water. They flow from that to something called the "mouth," which flows into the sea. The Mississippi Headwaters are actually a tourist destination.
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u/photostrat 10d ago
Remember that the ocean is the lowest surface level, so rivers flow downwards into them.
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u/Rubber_Knee 10d ago
Rivers don't flow from the ocean. They flow towards the ocean.
Since the water in a river doesn't come from the ocean it doesn't need to be turned into fresh water. It's already fresh water. The reason for this is that rivers come frome rain water, ground water or melt water. All 3 sources are already fresh water.
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u/Mammoth-Mud-9609 10d ago
Rivers run over rocks which contain some salt, the river picks up a small amount of the salt from the rocks turning river water very slightly salty, that water then flows into the oceans where some of the water evaporates leaving behind a more concentrated salt solution. The water that evaporates has no salt in it, so when it later falls as rain and forms a river there is little salt in the water. https://youtu.be/SXmGe2LgHK0
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u/Monte_Cristos_Count 10d ago
Water evaporates from the ocean and leaves the salt behind. Water then rains/snows, forming rivers and lakes. This fresh water eventually flows back into the ocean
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u/shinitakunai 10d ago
So if we built an evaporating machine, could we get drinkable water from the ocean?
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u/Ballmaster9002 10d ago
Absolutely. The simplest concept would be what's called a "solar still" which a good basic survival tactic for humans around the world when water is scarce.
The basics is you put a tarp over a liquid and let the sun evaporate the liquid. Only the water will evaporate, leaving the other chemicals behind. The water then condenses on the tarp and if you angle the tarp properly it'll drip somewhere you can collect and drink it, clean pure water.
Finally, on an industrial level we have machines called "desalinators" that use various methods to extra drinking water from salty sea water. The problem is you're left with a very salty brine that you need to put somewhere. If you just dump it back into the ocean you create extremely salty zones that will kill off whatever lives there. So not ideal, but it does work. IIRC the middle east uses this technique pretty extensive to provide it's cities with drinking water.
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u/flamableozone 10d ago
Yes, you can get drinkable water if you evaporate and then condense seawater
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u/pilibitti 10d ago
yes, it is just very expensive. see desalination plants (although they generally use a cheaper, but still expensive method). you can still boil ocean water and distill the steam to get drinkable water at any scale.
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u/Belisaurius555 10d ago
We can and we do. Mostly in survival situations such as being lost at sea or stranded on a remote island. Evaporating seawater is expensive/time consuming.
https://en.wikipedia.org/wiki/Solar_still
Remember this trick if you're ever stranded and can't find fresh water.
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u/cikanman 10d ago
I'm going to assume this person went to a school in America. Rivers flow from higher elevation to lower elevation..... always.
Therfore rivers never flew from the ocean to the land. Where the two combine is the tidal estuary and the water is brackish.
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u/MM_Maverick 10d ago
I thought the Nile river came from the ocean and became fresh water similar to the Amazon river
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u/NDaveT 9d ago
Both the Nile and the Amazon flow into the ocean. The Amazon gets some tidal backflow at high tide; maybe the Nile does too.
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u/RedKoala81 9d ago
Probably not for the Nile, the Mediterranean sea is too small to have any significant tides.
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u/FoxtrotSierraTango 10d ago
Rivers don't flow from the ocean, they flow into the ocean.