The Sirannon was the Gate-stream of Khazad-dûm, flowing westward from the cliffs near the West Gate of Moria through the Misty Mountains into Eregion. By the walls of Moria, the stream fell in a series of falls, known as the Stair Falls. While its precise course was unknown, the stream was presumably a tributary of the River Glanduin.

At the time of the War of the Ring, the Sirannon had been dammed and filled the valley in front of the west entrance of Moria. It was in the lake that the Watcher in the Water lived.