One of the two great rivers of Beleriand, the Gelion was twice the length of the Sirion although not so broad or deep. It drained the lands of East Beleriand, particularly Thargelion and the woodlands of Ossiriand.

Among its many tributaries were the Rivers Adurant, Duilwen, Brilthor, Legolin, Thalos, Ascar, and the Greater and Little Gelion.

Its source and that of nearly all its tributaries were the Blue Mountains to the east.
Encyclopedia entry originally written by High_King_Of_Gondor