The Gladden Fields (in Sindarin Loeg Ningloron) were a marshland in the vale of the Gladden river, and on the banks of the river Anduin. It was there that the One Ring was lost by Isildur and found again a long time later by Déagol the Stoor.
The fields started near the mouth of the river Gladden, and went on beyond where it joined the Great River, extending onto its eastern banks. It was a region of many dark pools and islets, with beds of reeds and rushes and many wildflowers - a particularly common flower being the Yellow Iris, which grew there in profusion.
In ancient times when the Silvan Elves were in the region, there was a pool or lake at the intersection of the two rivers; however, over time this receded, forming the marshes.
It was in the Gladden Fields that Isildur and his sons were travelling to Rivendell when they were assailed by orcs in TA 2 . Isildur, in an attempt to escape, jumped into the river, using the Ring to keep hidden. However, the Ring slipped from his finger as he swam away, landing in the riverbed, and Isildur was slain by the orcs' arrows on the bank of a small islet.
Twenty-five centuries after the ambush, in TA 2463, the Hobbit Déagol found the ring when he was fishing with his cousin Sméagol: an especially large fish pulled him under the water and, spotting it amongst the weeds, he took it to the bank. Ensnared by the Ring's power, the two fought over it, and ultimately it went to Sméagol, who killed Déagol and later became the creature called Gollum.
Later, around TA 2851, Saruman began searching the wetlands for the Ring. He found the Elendilmir, the token of Royalty of the North-kingdom, which Isildur had been wearing at the time of the attack. At the meeting of the White Council that year, Saruman overruled Gandalf's suggestion that they purge Sauron from his stronghold in Dol Guldur.
Sauron also sent his servants to search for the Ring. When the White Council learned of this, they agreed to an attack on Dol Guldur to prevent Sauron from finding the Ring, in TA 2941. At the meeting of the White Council in TA 2953, Saruman pretended that he had learned that the Ring had been carried downriver to the Sea.