In the ISWG, but not in older publications such as PFCS or Andoran, there is a river running from the 5 Kings Mountains to the Sennen River, namely the Dragonfly River.
It's not documented on the wiki and there's nothing about it in the ISWG except its existence on the map.
I figure there will probably be some logging on it, and maybe some trade from the 5 Kings to the Sennen, though it may be too steep and turbulent for that.
Presumably this was added for a reason beyond its geographical omission. Is it interesting?
