Sirion was a river of Middle-earth in the First Age, the principal river of Beleriand. During most of its course it was the border between East and West Beleriand. Karen Wynn Fonstad estimates its length as 390 miles in her Atlas of Middle-earth.
The Sirion's source was at Eithel Sirion on the eastern side of the Ered Wethrin, which lay in between Ard-galen (later Anfauglith) and Mithrim and Hithlum. It was guarded by the tower of Barad Eithel.
Sirion flowed southwards along the border of the Ered Wethrin, passing through the Fen of Serech before entering the narrow steep-sided valley between the Ered Wethrin and the Echoriath named the Pass of Sirion. Sirion then continued south into Beleriand, with the Forest of Brethil to the west, and Dimbar and then Doriath to the east. After leaving Doriath at Aelin-uial (the Fens of Sirion) it plunged below ground in the Falls of Sirion at Andram (the Long Wall), where the ground fell steeply. Three leagues southwards the Sirion exited the underground caves at the Gates of Sirion. It then flowed southwards through Nan-Tathren until it reached the Bay of Balar, part of Belegaer, at the Mouths of Sirion.
In order from north to south, the principal tributaries of the Sirion were the Rivil, flowing from Dorthonion until it met Sirion in the Fen of Serech; the Mindeb, which had its source in Nan Dungortheb and the Ered Gorgoroth, the Teiglin, the Esgalduin of Doriath; the Aros flowing south from Dorthonion (and also containing the waters of the Celon from the Hills of Himring and Himlad) which met at Aelin-uial, and the Narog, which joined Sirion in Nan-Tathren.
Barad Eithel, at the source of the Sirion in the Ered Wethrin, was a chief fortress of Fingolfin and his son Fingon, probably guarding a pass into their realm of Mithrim. Further south, in the Pass of Sirion, lay Tol Sirion (see below). After the Coming of Men, the Edain of the House of Haleth made their home in the Forest of Brethil, and the Sindar ruled by Thingol lay secure within the Girdle of Melian in their realm of Doriath. At the outflow of Sirion into Belegaer, after the destruction of Eglarest and Brithombar, the Havens of Sirion were built by Círdan and his people.
The most important island in Sirion was Tol Sirion. The original Minas Tirith was built here by Finrod Felagund in the strategic location controlling the Pass of Sirion where it entered Beleriand between the Ered Wethrin and the Echoriath. Shortly after Dagor Bragollach it was captured by Sauron and the island became known as Tol-in-Gaurhoth.
Sirion means something like "Great River". It's name in Khuzdul was Gabilân