Waterfalls will spawn near mountains, increased percentage on catching fish, find random chests underwater filled with loot. If in Ithilien you touch the water from waterfalls there will be a chance that rangers will spawn and attack you or rangers around you will attack.
Im not sure if the last part is possible, but if so it should be similar to how ents and huorns react when u destroy trees. Maybe replacing touching the water with fishing in it instead would make more sense and it would be easier to code maybe.