Called Onodló by the Elves, the River Entwash rises from springs of Methedras, the southernmost peak of the Misty Mountains the growing stream flowed just south of Wellinghall, Treebeard's home on its way through the Fangorn Forest, leaving the southeast corner of the forest, emerging at the Entwash Fishing Camp, it continues southward, flowing through the Entwash Vale to join with the River Snowbourn flowing from across West Rohan at the town of Snowbourn. . The Snowbourn river is the Entwash's second tributary, as the Swiftwater flows into the Entwash just south of Eaworth.
Turning east, the Entwash flowed 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 the southernmost of the Entwashes' arms there. The influx of water was so great that across the Anduin the marshes of Nindalf (Wetwang) arose. 
At its origin the Entwash was a noisy stream that cascaded over rocks and then hurried down the sloped terrain of Fangorn. Upon leaving the forest the river was still swift-flowing and its banks were deeply carven.
However, as the river passed southward across Rohan and widened its flow became more sluggish and the banks became fens. At the confluence of the Snowbourn and the Entwash great thickets of willows grew. The delta where the Entwash widened and joined the Anduin was a land of marshes that travelers were advised to avoid.
- ↑ a b J.R.R. Tolkien, The Lord of the Rings, The Two Towers, "Treebeard"
- ↑ J.R.R. Tolkien, Christopher Tolkien (ed.), Unfinished Tales, "The West of Middle-earth at the End of the Third Age"
- ↑ J.R.R. Tolkien, The Lord of the Rings, The Two Towers, "The Uruk-hai"
- ↑ J.R.R. Tolkien, The Lord of the Rings, The Two Towers, "The Riders of Rohan"
The Entwash pours down from the Misty Mountains just south of Wellinghall in the Entwood.
The river clears the forest at the Eaves of Fangorn and enters Entwash Vale which is named after it.
Through its course, the Entwash marks the border between East Rohan and West Rohan.
The river passes the township of Oserley as it runs along the border of the Broadacres to the south.
At the Wódfen the current slows as the water seeps into stagnant pools. It is possible to ford the river here.
The Entwade Crossing is the most famous ford on the river, where a floating pontoon has been built to aid travelers across.
As the river passes the town of Snowbourn it is fed by the river of the same name.
Mist marks the marshy land as the Entwash becomes a border river north of the Beacon Hills of Gondor.
The river splits into even more branches named the Mouths of Entwash as it passes the East Wall with the mighty Falls of Rauros to the north,.
At the end of the Mouths north of Taur Drúadan in Far Anórien, the Entwash flows into the great Anduin on its long journey to the Bay of Belfalas.