The Río Llancahue is a whitewater river in the Panguipulli commune of Los Ríos Region, draining Pellaifa Lake and flowing westward into Calafquén Lake. It originates from the southern flanks of Volcán Villarrica (2,840 meters) and Volcán Quetrupillán (2,360 meters), and the river's name is pronounced "Yan-Cow-Weh." The river is located directly south of Pucon on the opposite side of the volcano, roughly an hour's drive from Pucon. Despite its proximity to the Río Palguin as the crow flies, the volcano and rough four-wheel-drive roads between them mean access typically requires going the long way around the volcano.

The upper section is characterized by a notable waterfall near the put-in — a clean 18-to-20-foot drop rated Class IV- to IV+. This feature, known as Llancahue Falls, has an auto-boof line left of center with a pool below, and offers good safety positions both above and below. Paddlers are advised to scout from the left eddy at the top. Below the falls, a couple of easy rapids lead into tighter terrain featuring multi-move rapids in narrow, walled mini-gorges and several small vertical drops. The final rapid, approximately 100 yards above the take-out, is considered a significant hazard — one documented run resulted in a swimmer recirculating three times in the hydraulic. Paddlers should scout this feature carefully before committing.

The lower section carries a reputation as arguably the most demanding steep creek run in Chile, requiring Class 5+ skills with exceptional eddy-catching ability. Flow assessment at the put-in can be made by observing the lip of a feature called Gerd's Drop: if water is spilling over a right channel at the lip, the river is likely too high to run safely.

The upper run is considered a classic by visiting paddlers and has been used as a warm-up run alongside the Río Fuy. The scenery is shaped by the volcanic landscape, and paddlers on the upper section have noted the opportunity to paddle behind the main waterfall. The river is part of the broader Valdivia River Basin.