Why is seawater salty, when water from the lake isn’t?
In: Earth Science
As others have said, lakes usually have an output so that there is water (with a small amount of dissolved salts) constantly running through them. The exception would be [terminal lakes](https://en.wikipedia.org/wiki/Endorheic_basin) which do not run out to the sea and so do become salty to various different extents over time.
As for seawater, it’s not quite as simple as everyone else has made out, but not far off. It’s important to remember that the ocean is not the final resting place for salts, it’s just a way-station before they are passed on to the oceanic crust or the sediments which coat the crust. The length of time before this happens depends upon the individual ion in question — each one has its own cycle just like there is a water cycle and a carbon cycle.
This also explains why the balance of salts in seawater (which is the same pretty much everywhere in the oceans) is not simply a concentrated version of the river water which is supplied to the oceans. For example, although the dissolved ion content of rivers varies greatly (due to the particular bunch of rocks the rivers are running over/through before they get to the sea), chloride ions Cl^- are a fairly small part of the dissolved salts in all river inputs to the sea. Despite this, chloride is a major ion of seawater — along with sodium it makes up the vast majority of salt in the ocean. This is because chloride ions are removed *extremely slowly* from the oceans; far quicker than sodium ions Na^+ which are constantly being supplied *and removed* at a fairly high rate.
So seawater is salty not only because it has dissolved salts going into it (mainly from rivers), but also because some of those salts tend to hang around for a long time before getting removed. The level of any salt is a balance between the inputs and outputs of that particular salt. The oceans have been in a steady state balance for most salts for many millions of years, but this has not always been the case. It’s also not just been a case of building up all the salts over time, there’s a complex interplay between the factors which increase erosion and deliver salts to the sea (tectonics, climate, terrestrial volcanic activity) and the factors which remove salts (marine sedimentation, biological uptake, deep-sea hydrothermal activity, ocean circulation).
The history of ocean salinity has not been linear. The chemistry of the earliest oceans are extremely difficult to get a handle on, but we think that the oceans were significantly saltier than today until continents started to form, which helped to remove a lot of salts from the oceans (modern day salt flats and salt deposits within the crust — that all used to be part of the stuff circulating through the oceans).
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