Sea of Rhûn
The Sea of Rhûn, according to Tolkien's maps, covers roughly 400 square miles (1,000 km²). The Celduin (River Running) flows from the northwest into a northwestern arm of the sea, while several unnamed rivers flow from the northeastern shore. West of the Sea of Rhûn is the land of Dorwinion and roughly 200 miles (300 km) to the South is the eastern end of the Ered Lithui.
Christopher Tolkien and others have speculated that the Sea of Rhûn might "...be identified with the Sea of Helkar, vastly shrunken" (The War of the Jewels, pg. 174). Karen Wynn Fonstad adopted this assumption in The Atlas of Middle-earth. In The Peoples of Middle-earth there are references to the Sea of Rhûn existing in the First Age, but no indication as to whether it should be equated with the Sea of Helkar or not.