The Eau Rouge is a small, 15 km long stream in the Belgian province of Liège, right tributary of the Amblève. It starts in the Hautes Fagnes and ends in Challes, near Stavelot in the river Amblève.
The river gets it's name from the colloration of the stones and river bed which are rich in iron oxide which are red in color.
The river has lent its name to the Eau Rouge corner, one of the best-known corners in Formula One racing circuits in the Circuit de Spa-Francorchamps at the point where the track crosses it for the first time.
The river has particularly geomorphological interest, as it appears to be the old position of the Warche river. The river used to be a frontier several times, including an administrative limit under the Roman Empire between Köln and Tongeren, and between Belgium and Prussia from 1830 to 1919.