Missing from the article is any explanation for why the statues were above water 600 years ago, then below water most of the time, and are now above water again

Missing from the article is any explanation for why the statues were above water 600 years ago, then below water most of the time, and are now above water again (at least briefly). Has there been some change in the course of the river? Was it generally drier 600 years ago? Is post-glacial rebound occurring (ie, if ice sheets existed a bit further north, pushing down land under them, and up south of them, with things now settling back down since the ice is gone)?