Where the Mouths of Entwash meet the Anduin, below the Falls of Rauros, they carried more water than the banks of the Anduin could hold and that resulted in the formation of a huge expanding area of wet meadow and marsh known as the Nindalf (Wetwang) on the eastern bank.

During the Third Age the Nindalf spread to cover part of the gravesites from the Battle of Dagorlad, resulting in the formation of the Dead Marshes.
Encyclopedia entry originally written by PotbellyHairyfoot