East Beleriand was the part of Beleriand that lay east of the River Sirion and west of the Gelion, and stretched a hundred leagues across at its widest. This region was ruled by Thingol from Doriath and (east of Aros) largely under the control of the Sons of Fëanor during the First Age.
Sirion descended from the north to the south. With its tributaries Mindeb, Esgalduin, Aros and its tributary Celon, they formed Neldoreth and Region, the forests of Doriath. North of Doriath, the rivers separated the lands of Dimbar, Nan Dungortheb, Dor Dínen and Himlad, just south from the northern highlands.
The eastern part of Beleriand was dominated by the river Gelion, which separated the main lands from Thargelion and the land of Ossiriand, marked by the river's tributaries (Ascar, Thalos, Legolin, Brilthor, Duilwen and Adurant). Further to the east, the Blue Mountains separated Beleriand from Eriador.
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Lost Road and Other Writings, "Part Two: Valinor and Middle-earth before The Lord of the Rings, VI. Quenta Silmarillion", p. 261
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Quenta Silmarillion: Of Beleriand and its Realms"
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Map of Beleriand and the Lands to the North"