A lesser river or stream that flowed out of the White Mountains into the Gondorian land of Lebennin, a little to the west of Minas Tirith. It was a tributary of the larger River Sirith, though the details of its geography are a little unclear. The smaller-scale map that accompanies The Lord of the Rings shows it flowing into the Sirith from the east, but the large-scale contour map of Gondor shows it flowing from the west. A brief reference in Unfinished Tales suggests that the Celos was the next river eastward from the Gilrain, which would make the large-scale map correct. The river's name on that map, incidentally, is transliterated 'Kelos' rather than 'Celos', but the pronunciation is the same in either case.