The Entwash was a great river in Rohan, notable for its huge inland delta.
Leaving Fangorn Forest, the river flowed past the Wold of Rohan, and then headed south, dividing Rohan into Westemnet and Eastemnet. At the latitude of Edoras it was joined by the river Snowbourn, and then flowed west towards Anduin, which it joined just south of the Falls of Rauros, in the huge inland delta known as the Mouths of Entwash. The Mering Stream or Glanhír met one of the Entwashes' arms there. The influence of water was so great that at the other end of the Anduin the marshes of Nindalf (Wetwang) and the Dead Marshes arose.
Entwash is a modernization of Old English Entwaesc (waesc "flood water"), so modernized because it was recognizable by speakers of Westron. Gondorians used that name, assimilated to their own language.