A stream or small river that rose from springs near the West-gate of Moria, and was therefore known as the Gate-stream (or, in Elvish, the Sirannon). From there it ran down a series of waterfalls known as the Stair Falls before continuing westward. The road from old Khazad-dûm to Eregion followed the course of the Sirannon for at least several miles, and it may be that Ost-in-Edhil, the chief city of the Elves of Eregion, was built on the river.1 The waters of the Sirannon presumably flowed out into the Glanduin, the only significant river in this part of Middle-earth, and thence into the Gwathló and on to the Great Sea.
In the closing years of the Third Age, after the occupation of Moria by creatures of Sauron, the upper waters of the Gate-steam were dammed and the Sirannon itself was reduced to a mere trickle. This created a great pool outside the West-gate, which was guarded by a dangerous and mysterious being known only as the Watcher in the Water.
We don't know exactly where Ost-in-Edhil stood, but the fact that the road towards it ran along the Sirannon, coupled with the clear practical value of building a city on a watercourse, gives at least some credence to the possibility that the city stood on the river.
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