Its two sources were the Hill of Himring, via the Little Gelion, and the Ered Luin, via the Greater Gelion. It then passed south, Estolad to the west and Thargelion to the east, then was crossed by the old dwarf-road at Sarn Athrad ("Ford of Stones"). It then passed into Ossiriand, the Land of Seven Rivers, where it was fed by Rivers Ascar, Thalos, Legolin, Brilthor, Duilwen, and Adurant, all rising in the Ered Luin.
Gelion then passed by Tuar-im-Duinath (the Forest between the Rivers) and into the Great Sea Belegaer.