The Glanduin was a river on the southern border of Eriador. It was also known as the Swanfleet river meaning the river that flowed into the Swanfleet.
Course
It flowed from the Hithaeglir south of Khazad-dûm swiftly for approximately a hundred miles westwards, then made a bend and continued to the northwest for a few miles where it flowed for approximately seventy miles through a fenland in the plains that was known as the Swanfleet (Nîn-in-Eilph) until it joined the river Mitheithel only a few miles northeast of the town of Tharbad. At their confluence the two rivers formed the river Gwathló.
There was an old ford across the Glanduin east of the falls where the river dropped down into the lowlands where the Swanfleet marshes were located.
Note
In an annotation to Pauline Baynes, J.R.R. Tolkien had R. Glanduin placed near the Mountains, and the lower part labelled "Swanfleet". It was misunderstood in first printings of A Map of Middle-earth that "R. Swanfleet" was the name of the lower part of the river, but it was pointed out as wrong by Christopher Tolkien (Swanfleet being the fens the river flows into) and it was corrected in later printings.
The river received its name Glanduin in the Second Age when it formed the southern border of the elven realm of Eregion, beyond which were the unfriendly people of Dunland. By the time of the War of the Ring, the name Glanduin only applied to the swift upper course of the river, and it was referred to as the Swanfleet river.
After the War of the Ring, Frodo, Sam, Pippin, Merry, Gandalf, Elrond, Celeborn, Galadriel and their company of Elves from Rivendell and Lothlórien came from the south, crossed the old ford over the Glanduin into Eregion on September 6 T.A. 3019.