Carnen — Sindarin for Redwater — was a river in the north-east of Middle-earth.
Course
The Carnen flowed from its source in the Iron Hills about 250 miles to the south until it flowed into the Celduin, which flowed into the Sea of Rhûn.
It roughly formed the border between the "The North" (the northwestern region of Middle-earth) and the vast "East" (the area called Rhûn).
Between the time when Thráin I founded the dwarf-kingdom in the Lonely Mountain (in T.A. 1999) and the arrival of Smaug (in 2770) the Northmen living between the Celduin and Carnen grew strong and repelled all enemies from the East. It is possible that this military success was the result of the acquisition of weapons and armour from the Dwarves of Erebor. The area became depopulated after the arrival of the dragon but again filled with Northmen after Smaug's demise.
For a long time Easterlings, which were allied with Sauron threatened the borders of the Kingdom of Dale. On 14 March T.A. 3019, during the War of the Ring an army of these Easterlings host of Sauron crossed the Carnen and drove the forces of King Brand back to Dale. A great battle between the army of the Easterlings and the army of King Brand and their allies the Dwarves of Erebor lasted for three days at the feet of the Erebor. On 17 March T.A. 3019, the Easterlings defeated their opponents and killed King Brand and King Dáin Ironfoot. The surviving men of Dale and Dwarves retreated into the Lonely Mountain and were besieged by the Easterlings. The Easterlings were distressed when they received news that Sauron's army had been defeated in the south. On 27 March T.A. 3019, the defenders of the Lonely Mountain attacked and defeated the Easterlings. The surviving Easterlings feld back to the East and did no longer trouble Dale.