Currently, the raise terrain and lower terrain spells do exactly that - immediately.

I think that they would be more interesting if they were treated as expensive yet temporary enchantments rather than a single-turn effect.  That is, the spells may take 5-10 seasons to actually alter the terrain, during which they constantly eat mana and use up a champion enchantment slot.  This gives opposing spellcasters opportunity to counter when these spells are used offensively.  I think that this should be very important tactically.  After all, drowning a city or putting it on top of an uninhabitable mountain peak may be an extremely effective way of dealing with a city without invading it, in theory.  Not to mention earthquake effects.