User blog comment:D2r/Unanswered Questions of Redwall/@comment-1861166-20100202000427

The Raven Quin says: The Pond is seen in Redwall. The dungeons are at a lowe elevations, so the woodlanders divert the River Moss to flood the dungeons and (eventualy) Kotir. It takes awhile for the water to drain, so a smal pond is formed. Also, in one of the books, the south wall of the Abbey almost sinks because it is built on part of the old foundation of Kotir and there is a river running beneath it. I thnik this is where the pond coemes from.