The River Sirion was the greatest of the rivers of Middle-earth during the First Age. It provided the border between East and West Beleriand for the whole of its length. The river rose at Eithel Sirion and skirted round the edge of Ard-galen before flowing southwards for 130 leagues.

The river had many tributaries, of which the Rivil was the most northerly, and the Narog, the most southerly. Their names were Narog, Rivil, Teiglin, Mindeb, Esgalduin, and Aros.

The Sirion was unusual in that it ran nine miles of its course underground - from the marshland of Aelin-uial beneath the high land of the Andram.

The Sirion met the Sea at the Bay of Balar in a wide sandy delta scattered with marshes and isles of reeds - the Mouths of Sirion.
Encyclopedia entry originally written by atalante_star