A stream that rose in the western flanks of the Misty Mountains1 and flowed down past the West-gate of Moria, and then over the Stair Falls and on into the plains of Eregion, following the course of the road that ran out of old Khazad-dûm. It was known in Elvish as the Sirannon, a name that translates directly as 'Gate-stream'. For most of the history of Khazad-dûm it was a strong and powerful stream, but in the later Third Age its upper course was dammed. At this time the waters beneath the West-gate swelled into a stagnant lake occupied by a creature known only as the Watcher in the Water, and the stream itself was reduced to no more than a trickle.
The course of the Gate-stream after it flowed over the Stair Falls is not completely clear. We know that it ran along the road towards Ost-in-Edhil for at least some distance, but we don't know whether it actually flowed past or through the city of the Elves of Eregion (it would have at least been reasonable for Ost-in-Edhil to have been founded on a watercourse). It seems inevitable that the stream eventually flowed into the river Glanduin, the only major river in this part of Middle-earth, and then on down Gwathló into the Great Sea.
It is not in fact completely clear where the springs of the Gate-stream were, but when the Company of the Ring encountered the swollen stream waters at the West-gate, they found that they were fed by a creek running down from the heights to the north: this was presumably the source of the original Gate-stream before it was dammed.
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