The River Entwash (Onodlo) is the most powerful of the tributaries of the Anduin. Its drainage basin was the entirety of Rohan, from Fangorn Forest and The Wold, south to the White Mountains.

The Entwash had its source in the interior of Fangorn Forest below the Misty Mountain peak known as Methedras. Leaving Fangorn, the Entwash flowed to the southeast for a while and acted as a southwest boundary for The Wold. It then turned south on the plains of Rohan and separated East Emnet from West Emnet. At the south of West Emnet it was joined by a tributary, the River Snowbourn, which flowed east from its springs in the White Mountains below the Starkhorn near Edoras.

The Entwash then flowed to the east and branched out into a huge delta, known as the Mouths of Entwash. After the river began to branch out into its delta it was joined by a second tributary, the Mering Stream, which flowed to the northeast from its source in the White Mountains below the peak of Halifirien.

The Mouths of Entwash were a vast system of ever-changing small islands and streams, full of pools and bogs. They were formed when the Entwash reached the flatlands below the Emyn Muil and the Falls of Rauros. It was a rich habitat providing for a vast array of animals and birds.

The Mouths of Entwash spread to a total width of about 100 miles by the time they reach the Anduin. The volume of water carried by the Entwash was so great by this time that the Anduin could contain it and its flow spread out over the Anduin's eastern banks, forming the vast, growing marshland known as the Nindalf (Wetwang).