It is used to access the Western Channel, and eventually Waterlogged Passage (hidden behind the waterfalls, opened by two levers). After breaching the Western Channel, it can be used to access Eastern Channel as well.
The door to the Ancient Waterway is locked and can only be opened by Zoltun Kulle. An attempt to unlock it without him will result in a message that it is sealed by a complex stone mechanism.
Inside, a projection of Hakan II appears.