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 river to its final inflow into Anduin. Travelers were advised to avoid the marshy lands.
Although the areas west of Anduin belonged to Rohan, the splitting of Entwash was considered the border, so the delta itself was part of Gondor.
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 Wetwang.