The Eau Rouge is a small, 15-kilometre (9 mi) long stream in the Belgian province of Liège, right tributary of the Amblève. It starts in the Hautes Fagnes ("High Fens") and ends in Challes, near Stavelot in the river Amblève. The river gets its name from the coloration of the stones and riverbed due to reddish iron oxide deposits. The Eau Rouge is particularly geomorphologically interesting, as it appears to be using the old Warche river stream bed. The Eau Rouge has been a border river for several periods in its existence, including an administrative boundary under the Roman Empire between Cologne and Tongeren, and the state border between the Netherlands and Prussia from 1815 to 1839 and then between Belgium and Prussia from 1839 to 1919.