Salt gets into rivers when material that can't be dissolved is stripped away by erosion. This exposes new water soluble compounds to the water, where they dissolve into the water and are taken to the ocean.

Over millions of years erosion removes innumerable tons of material, essentially mining the subsurface soluble compounds and delivering them to the ocean. Once there, as you mention ,those salts remain in the ocean. On Earth, this process began billions of years ago and has been adding salt to the oceans ever since.

You can observe this happening in many rivers today. The Colorado River is a great one. If you measure is salinity at the headwaters (or heck, probably even the inlet of Lake Powell), and where it enters the Gulf of Mexico, you will observe an incredible increase in salt. There was an international treaty formed around the US delivering river water that is not too salty to grow crops in to Mexico. The US solved that problem by installing a desalination plant on the river!

However without that land based salt mining process, how salty would the oceans be? Lots of good clues in this thread, but I don't think anyone has offered a definitive answer.

This is all very interesting and pertinent. I was wondering about the hadean period, and whether you could actually get to an ocean world without first having continents with a water cycle. I don't know enough about planetary formation to conclude further. Thanks for pointing me to the hadean period, I will read more about that.

You might misunderstand my comment about the dead sea. The dead sea actually precipitates salt crystals onto the bottom of the sea. No land is required in this strange process. I don't think it's clear to say whether this happens because of the extreme salinity of the dead sea, or if the extreme salinity just makes it the only place we observe this rapid desalination on human time scales. I offered this as perhaps the most striking example that salts dissolved in water are not necessarily a stable state on a timeline of billions of years.

Running water causes a lot more erosion than stationary bodies of water. Consider lakes, which are still cycling water much like a river, but over thousands of years they deposit so much silt that they cease to exist. That's the opposite of erosion.

Underwater erosion is certainly a thing, but in comparison to downhill water erosion on land, it's pretty insignificant. It does not seem a given that it could significantly offset the processes that remove salt from salt water.

Running water causes a lot more erosion than stationary bodies of water. Consider lakes, which are still cycling water much like a river, but over thousands of years they deposit so much silt that they cease to exist.

Underwater erosion is certainly a thing, but in comparison to downhill water erosion on land, it's pretty insignificant. It does not seem a given that it could significantly offset the processes that remove salt from salt water.

Oh man! The fact that our current ocean isn't getting more salty implies that the addition by rivers is very significant to the total saltiness of the ocean! Over billions of years with no rivers, the ocean must get significantly fresher! Wow! That is strong confirmation that an entirely ocean world would at least be significantly less salty!

The presence of sodium and chlorine on the planet makes sense to me, but that doesn't necessarily mean it's dissolved in the water. I think the key understanding is if the water cycle is the key component of dissolving salt in water, or if the much less dramatic erosion on the bottom of the ocean is sufficient to make the water notably salty.

So far the best answer I've got is that water in comets and otherwise outside the planet might actually be something like salty, so maybe freshwater is just a temporary aberration of the water cycle.

At the same time, we know there are some processes that remove salt from oceans (e.g. the salt formations at the bottom of the Dead Sea), so in the end I think it would come down to where that balance of salt in vs salt out. It's not totally clear to me that without the continental influx of salt from rivers, that that balance would result in something like freshwater or saltwater. This thread has highlighted several factors that come in on both sides, so it may be something we won't know until we've explored more planets.

Thanks for all the detail! Your observation about comets is really pertinent. Saltwater is probably itself a purer form of water than comets. Maybe an ocean planet is actually more like a muddy swamp of nasty dirty water than a lake.

Exactly. If a planet ever had a salty ocean, adding more water probably wouldn't dilute it in any meaningful way, so it would need to be a planet that never had continents.

That is super interesting! I hope the Clipper gives us a definitive answer!

I was trying to figure out how much underwater erosion there is but if you compare the sandy and silty bottom of the ocean to like, Utah, it seems like continental erosion is orders of magnitude more significant.

Conversely, we know oceans deposit all sorts of stuff at their bottoms, which makes me think there is a small amount of salt being deposited. Would that cancel out significant underwater erosion?

Similarly, if underwater erosion was a big deal, wouldn't old lakes (in geological time) be notably saltier than young lakes? But the only salty lakes we have primarily lose all their water through evaporation, basically ultra concentrated river water.

And if you like Travelers, make sure to watch Sarah Conors Chronicle

Check out Travelers. They start out with a lot of agency but stuff gets wrong and it's more and more desperate.