The Sirannon or Gate Stream was a river of Eriador.
Beginning at the Silvertine, it flowed past the gates of Moria (hence its name) to Ost-in-Edhil, the old capital city of the Noldor of Eregion, where it was met by the river Glanduin. The Sirannon encountered some falls under the gates of Moria, and its sound was heard from miles around.
During the War of the Ring (by which point both Moria and Eregion had been abandoned), the Fellowship of the Ring found that the Sirannon had been dammed by someone or something, and before the gates of Moria there was now a foul lake. In this lake dwelt the Watcher in the Water, who nearly caught the Ringbearer Frodo Baggins.