Mouths of Entwash
From Tolkien Gateway
The Mouths of Entwash was a mighty delta formed where the River Entwash met the Anduin. The delta measured a hundred miles from the splitting of the Entwash to its final inflow into Anduin.
So great was the inrush of Entwash that Anduin could not carry all its water, and so a marshland formed on the Anduin's eastern bank, called Nindalf, or Wetwang in the Westron tongue.[1]