Nen Hithoel was a large lake upon the Great River Anduin amid the Emyn Muil to the east of Rohan. The lake was approximately 20 miles long from north to south, and 10 miles wide. The lake was formed by a narrow southern outlet and the small island of Tol Brandir, which created a natural dam. Upon the lake's northern approach from Anduin the Men of Gondor carved the huge pillars of the Argonath in the reign of Rómendacil II to mark the northern boundary of their realm, although by the time of the War of the Ring that boundary has long since receded. At the southern end of the lake stood three steep hills. Amon Hen, the "Hill of Seeing", was upon the western shore and Amon Lhaw, the "Hill of Hearing", was upon the east. The third hill formed the island of Tol Brandir. None has ever set foot upon the island due to the intense currents at the lake's south end, for the lake was drained by the Falls of Rauros.
In The Lord of the Rings
The Fellowship of the Ring arrived at Nen Hithoel on February 25, Third Age 3019, and made camp at Parth Galen close to Amon Hen. The fall of Boromir and the breaking of the Fellowship occurred soon after. Frodo and Sam took a boat for the eastern shore, Merry and Pippin were captured by Orcs, and after sending Boromir's body over the falls in another boat, Aragorn, Legolas and Gimli, calling themselves the Three Hunters, set out to track the Orcs and find the captive Hobbits.
Nen Hithoel means "Lake of Mist" in Sindarin (from nen = "river", hîth = "mist, fog", and oel = "lake, pool". By combining nen and oel, the name demonstrates how the lake is not a river source, but rather an extention of the Anduin channel).