The Lesse (very rarely also called the Lesder) is a tributary of the Meuse. It rises at an altitude of 480 meters in Ochamps, a sub-municipality of Libin in the Belgian province of Luxembourg on the Ardennes plateau. Part of the river is part of the Parc National Lesse et Lomme.
It has a fairly large decline, especially in the upper reaches, before flowing into the Meuse near Anseremme at a height of approximately 100 meters. At the "Gouffre de Belvaux" the Lesse goes underground (a 'karst river') to resurface via the Caves of Han, a few kilometers further in Han-sur-Lesse, municipality of Rochefort. The Lhomme, as the main tributary, flows into the Lesse at Éprave. From Wanlin the Lesse de Famenne leaves depression.
The last 20 kilometers of its course, from Houyet to the mouth of the Meuse, are very suitable for canoeing. Along the way you pass Walzin Castle, located high on a rocky outcrop above the river, and the Furfooz national park.
The precise course of the Lesse in the Caves of Han was only discovered at the end of the 20th century.
The lowest temperature ever measured in Belgium was also recorded in the Lesse valley: –30.1 °C on January 20, 1940.
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