The Mering Stream formed the border between the realms of Gondor and Rohan, together with the White Mountains and the Mouths of the Entwash.  It flowed through the forest of Firienholt, which grew about the mountain Halifirien in Anórien in Gondor, joining with the River Entwash just below Snowbourn. The last of the Warning beacons of Gondor stood on its edge observed by Beaconwatch on its west bank in the Eastfold in West Rohan.
- J.R.R. Tolkien, Christopher Tolkien (ed.), Unfinished Tales, "Cirion and Eorl and the Friendship of Gondor and Rohan"