The Morgulduin was a small river whose source was high up in the Ephel Duath. It flowed right through the Imlad Morgul ( the Valley of Morgul), past the gates of Minas Morgul, and through Ithilien before reaching the River Anduin.

Faramir told the Hobbits (Frodo and Sam) not to drink its water because it was poisonous.