Loeg Ningloron was the region where the Gladden River flowed down from the Misty Mountains to meet the Great River Anduin, south of the Carrock. In ancient times a lake had formed at their meeting-place, but by the beginning of the Third Age the lake had been replaced by a land of marshes and islets. These were the Gladden Fields (named from the giant yellow iris flowers that grew there in hosts). In Elvish, the Gladden River was known as Sîr Ninglor, from which derived the Elvish name for the Fields: Loeg Ningloron.
In the first years of the Third Age, after the first defeat of Sauron, Isildur was marching close to the Loeg Ningloron when he was set upon by a host of Orcs. His soldiers were overwhelmed, and he escaped into the wetlands only to be shot down by an Orc-arrow. There he was lost, and with him the One Ring that he was carrying north, which lay hidden in the marshes of Loeg Ningloron for nearly two and a half thousand years.