In Pratchet, one of the inherent forces of nature is magic, with its help many mismatches are explained, and the water really fell from the edge of the disc, but here is about the fact that behind the turtle was left a shelf of water is not a word. That is to say, from the end, she fled, but not from the system.
It always seemed to me that the disc in this regard is like a large fountain, from which water seems to flow beyond the edge, but in essence the system is closed.