it would take a long time for that valley to regain any of its former natural beauty - if ever.
I don't think it would take that long. The walls and trees returning will take awhile but the ground floor would be prime for grass to take hold in a year or two. Then it will be a beautiful garden in the right season.
Nature will always find a way to make itself beautiful. We could not stop it by nuking the living crap out of it and you think a little water will defeat it?