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.

“The upper surface of a fluid at rest is a horizontal plane.  Because if a part of the surface were higher than the rest, those parts of the fluid which were under …