The question Historically, when (and why) did Ganga become a holy river? Is it after Sarasvati dried up? has two answers which are written from a purely Puranic point of view and does not address OP's question in the slightest. Despite flagging these two answers are still not deleted. No explanation was given by mods who handled the flags either in comments under the question or the two answers as to why the flags were rejected/disputed.
We have an ex-mod clearly explaining how this question should be answered.
That is not a duplicate. OP is asking when (historically) the Ganga was deemed sacred, not why (scripturally) the Ganga is deemed sacred. – senshin Jun 22 '14 at 6:29
In the other meta post there was a discussion about why a particular user's post was deemed NAA (not an answer) and promptly deleted.
Why aren't all NAAs treated equally on this site?