It may be supernatural, but magic doesn't last forever, does it? According to what I've seen in FfH, most spells either have a time limit (e.g. most buffs, some summons), unit limit (e.g. summoned tigers), or dispel themselves once the mage leaves (e.g. city buffs). Terrain modification spells however aren't consistent with this.
It would make sense if the terrain modification spells act like city buff spells, in that the mage/priest has to be in the area for the terrain to be kept modified. In this case, when the mage/priest leaves the area, the terrain can slowly revert.
However I can't think of an easy to implement this...