It is part of the natural physics of water and other fluids to always find their level and remain flat. If disturbed in any way, motion ensues until the flat level is resumed. If dammed up then released, the nature of all liquids is to quickly flood outwards taking the easiest course towards finding its new level.

If the Earth is an extended flat plane, then this fundamental physical property of fluids finding and remaining level is consistent with experience and common sense. If...