The river Lhûn, or Westronized Lune, was a river of north-western Eriador.
It had two tributaries: one was the Little Lune arising from the Ered Luin, and an unnamed other one beginning in the Hills of Evendim north of the later capital of Arnor, Annúminas. It was not connected to Lake Nenuial, the origin of the Baranduin.
In the First Age, the course of the river is not known. The Gulf of Lune was not created until the War of Wrath so the river must have had a different lower course. Possibly it connected with the Baranduin further South.
The meaning of Lhûn is not known. In an article "The Problem of Lhûn", published in Vinyar Tengwar 48, Patrick H. Wynne lists all the scraps Tolkien ever wrote about the name, and finds that the only thing he can say for certain is that it is not related to luin, "blue", despite being originally perceived as so. Lhûn, and especially the initial /lh/, may have been valid in Noldorin, it was not so in Sindarin, so Tolkien had to rewrite the etymology. He constructed the following:
- CE Slōna, "floody"
- CE Slōnā, "in flood, full of water" (during melting season)
- CE Slounā, "flow freely"
- CE Slōno, "deep of water, applied originally to the Gulf!"
- A renaming to Sîr Luin
- CE Slōn, "sound"
- A Khuzdul origin, salôn or sulûn, "fall, descend swiftly"
It is unknown what Tolkien's final intent was.