Why aren’t rivers/estuaries/lakes etc salty like the sea ?
In: Planetary Science
Rivers/lakes/estuaries are mainly run off from rain fall. Rain is made from water vapor. When the water evaporates from the sea, it doesn’t take the salt with it. The run off collects salt that is in the ground though, thats probably how the sea got salty to begin with. In most areas there isnt that much salt near the surface anymore, so you only get trace amounts. But if the water collects in a lake with no outlet, it begins to collect that trace salt. As the water evaporates the salt builds up over time. This is how you get salt lakes like the Great Salt Lake in Utah and the Dead Sea in the Middle East.
When water form into clouds, they don’t take the salt with it. Then it rains on land, that water makes its way into rivers, and eventually into the ocean (or evaporated out of an inland lake again). If it runs through/over salt it will dissolve it and take it on the journey.
So no matter where the salt starts, it’s perpetually washing downstream to a lake bed or—more often—the ocean. Once salt gets there, there’s no mechanism to move the salt anywhere else. Do that for billions of years, and you get salty oceans and fresh rivers.
The salt in the ocean came from those rivers.
The river flows, and dissolves salts from the rocks they flow over. That water and salt gets dumped into the ocean.
Water evaporates from the ocean and rains back down on land, leaving the salt behind in the ocean. This process repeats over and over again, but there is never salt being returned to the rivers, so the rivers never accumulate salt, so it all sits in the oceans
Make a pot of salt water.
In a different cup, fill with some tap water.
Now, pour a bit of water from the tap water cup into the salt water pot.
Does the cup become salty? No, because the salt water is staying in the pot and isn’t doing anything to the cup.
This is the interaction between rivers and the ocean; it is almost entirely one way (from fresh water to salt water), so there’s no reason for the saltiness to go against that direction.
Freshwater bodies- rivers, lakes, etc have teeny tiny amounts of salt in them. They’re not salt free. The salt is dissolved from minerals in the rocks they pass over.
Once they reach the ocean, both the water and salt they contain is dumped into the ocean.
The water eventually evaporates back into the atmosphere, is transported over land, comes down as precipitation, and starts the cycle anew.
But the salts cannot evaporate. They just build up in the ocean until they start getting deposited on the bottom (salt mines are old ocean salt deposits!).
So rivers act as conveyor belts for water and salt to get to the ocean. In the case of water, it’s a loop- the water evaporates from the ocean and rains back down on land, keeping the river going. But in the case of salt, it’s a one-way process.
Thus the oceans have a balance of water- incoming from streams versus losses to evaporation. But for salts a river is a one way street into the ocean- the oceans maintain a rough equilibrium of salt concentration through sedimentary processes- salts get turned into rock on the sea floor at the same rate they enter the ocean from streams.
This is why there are salt lakes like the Great Salt Lake in the USA and the Dead Sea in the Middle East- the same processes are at work, just with different equilibrium points for the salt concentration.
Rivers and lakes are fed from rainwater running from high ground to the sea. While rainwater does mostly come from the sea, it has to evaporate first. Evaporated water can’t carry salt, so it gets left behind in the ocean.
If you put a bunch of salt in a pan or water and boil it dry, you’ll see all the salt deposited on the bottom of the pan.
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