I vaguely remember that Aleroth was changed during some sort of cataclym during which the fjords came to exist, too ... I read that somewhere as an explanation during the Divinity 2 times ...
What I find most interesting, however, is this expression :
I remember that it ran dry ? at one point before Divinity 2 took place, which was why people (like one certain NPC in the first or second village of Divinity 2) had begun looking for replacements ...
I never played Divinity 2 / FOV to the end, and D:OS only to a part, so I'm not familiar with additional information.