h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Salt level is constant within a few percent. Salt had to come from deposits in earth. Rivers run through earth and into sea. Rivers are essentially 'fresh water'Question is Why is salt level in sea constant and why don't rivers absorb salt from earth on their journey to sea.?

Because, it's dirt on the river bottoms, and most of the dirt that was going to come up already has... Look at the grand canyon, it took millions of years to form...one piece of dirt at a time...

Its not exactly constant, there are some places where there are much more concentrated areas in the sea.

If all the ice melts will this also have an effect on salt levels - I'd assume it would go down in percentage if the salt volume is constant?

Also how does human salt mining effect levels going into the system?

River water isn't salt free. It just has less than the sea as it flows and is constantly replaced by stuff from the sky.