Gelion.

The longest river in Middle-Earth in First Age. There was two wells of Gelion who came from the high North and ran to the sea beyond Taur-im-Duinath, a part of the Southern Forest, in the South, in all more than 600 Miles ( On the map ). The river also divided Thargelion and Ossiriand from East Beleriand. The wells of Gelion was Little Gelion who came from the Southern hills of Himring and the March of Maedros, the other came from Ered Luin, and Mount Rerir; the Greater Gelion . Those two rivers soon joined there waters and begame Gelion . Now on the Western side of Gelion was the land Estolad in East Beleriand and on the Eastern was Thargelion until Gelion 150 miles South met river Ascar, the first of the six other rivers of Ossiriand, comming from Ered Lindon. With intervals of about 60 miles the others of the seven rivers, Ascar, Thalos, Legolin, Brilthor, Duilwen and Adurant joined Gelion on its way to the sea.