The Grimbrook (Rûthhir in Sindarin) is a river that arises in the White Mountains of Rohan and flows down into the Westfold through the town of Grimslade. It passes north, running parallel with the road to Brockbridge, and is crossed in a ford by the Great East-West road. The Grimbrook then angles northwest through a few bends, before it flows into the Swiftwater coming in from the southwest as it enters the Stonedeans.
The Grimbrook pours down from the White Mountains in the Westfold, south of Grimslade.
The flow crosses the mountain path between Underharrow and Helm's Deep, before entering a canyon.
As it continues, it enters the town of Grimslade where it loses much of its fury.
The Grimbrook is a source of fresh and clean water for the townfolk of Grimslade, and is considered a vital part of the location's identity.
The Grimbrook then continues on through the wooded lands of Westfold.
The river enters the open plains of Rohan, giving herd animals access to its fresh waters.
As it approaches the Stonedeans, the Grimbrook starts to bend northwest.
The river veers towards the foothills of the Misty Mountains, and its steep river gorges.
Finally, the Grimbrook flows into the longer Swiftwater as they head together towards the great Entwash.