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. 
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.
- 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"