Sirion arose on the eastern side of the Ered Wethrin, then ran south through the narrow steep-sided valley between the Ered Wethrin and the Echoriath. In this valley was an island in the river named Tol Sirion; it was here that Finrod Felegund built the first Minas Tirith. Sirion then continued south into Beleriand, with the Forest of Brethil to the west, and Dimbar and then Doriath to the east. It received the River Teiglin from the west, then continued south, receiving the River Esgalduin that passed by Menegroth and then bending to the southwest. The River Celon that arose in the Hills of Himring emptied into Sirion in a marshy area known as the Fens of Sirion.
After the Fens the river dropped in the Falls of Sirion, where it disappeared into the rock and flowed through three Tunnels of Sirion for nine miles, re-emerging below the Andram (Long Wall) from the three Gates of Sirion. The river continued south, receiving the River Narog at the Nan-Tathren (Valley of Willows) before emptying into the Bay of Balar at the Mouths of Sirion.