The Celduin River System
The Celduin River (River Running) springs from the gates of Erebor, in the Lonely Mountain and flows many hundreds of miles south and eastwards to its outflow into the Sea of Rhun. The major tributaries of the Celduin River include: Carnen, Enchanted River, and the Forest River. The Celduin forms the eastern boundary of Rhovanion.
River Carnen (Redwater) springs from the Iron Hills, far to the northeast of Middle Earth. Its meandering path takes it southwards for more than two hundred miles where it joins the Celduin. The upper reaches of the Carnen do have a distinctive red-orange color, which is probably do to the high iron content of its source waters.
The Forest River begins in the Grey Mountains, just a few miles east of the sources of the River Anduin, and flows east-southeast into Long Lake, near Lake-town. The Forest River is so-named because the majority of its length runs through northern Mirkwood. The Forest River's tributaries are the Enchanted River and one unnamed stream that flows beneath Thranduil's halls.
The Enchanted River is the dark river that begins in the Mountains of Mirkwood, and flows northwards in a great loop to meet the Forest River under the canopy of the trees of Mirkwood. Its water is black, and carries a curse that gives the river its name - any person coming into contact with its enchanted waters immediately falls into a long, deep sleep. About halfway along its length, the Enchanted River flows across the Elf-path that leads west to east through Mirkwood to Thranduil's halls, a ferry is maintained there, for the use of the Wood-elves.
Sea of Rhun is the inland sea in the land of Rhun into which the Celduin empties its waters. Its northeastern shores are densely forested, and to the southwest lie a mountainous region. The mysterious land of Dorwinion lies on its northwestern coasts.