Gelion

Gelion was a river of Middle-earth in the First Age, the principal river of East Beleriand.

Its two sources were the Hill of Himring, via the Little Gelion, and the Ered Luin, via the Greater Gelion. It then passed south, dividing Estolad to the west from 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 emptied into the Great Sea Belegaer.

Late in his life, Tolkien apparently decided to change the name "Gelion". The possible replacements he recorded were "Gelduin", "Gevilon", "Gevelon", "Duin Daer", and "Duin Dhaer".

Reference

 * The Silmarillion, pp. 121-124
 * The War of the Jewels, pp. 191, 336