Why is sea water salty while most lakes are not?

@scott-martin asked an interesting questing during the @metcalph & @Joerg exchanges in the Pent thread about Hot Lake (page 372):

Given that elemental Water isn't salty - what is salt water in Glorantha? Why are the seas salty? Is actually any water salty other than that specifically stated? are the seas actually not salty?

The Sean Pantheon section on page 151 says:

Quote

Merman philosophers teach that the world was once a single current owing through the cosmos. For some reason the current divided into fresh and salt water, then hot and cold, and in other ways, eventually forming the present generation of sea deities in the world.

Perhaps @Jeff can help