Duilwen was one of the seven rivers of the Ossiriand in Lindon during the First Age. The Duilwen began from Blue Mountains, it was one of the six tributaries of the great river Gelion, and lay between the two other rivers of the Ossiriand: Brilthor and Adurant. According to Karen Wynn Fonstad's The Atlas of Middle-earth, it was nearly one-hundred and fifty miles in length.