Lys (lēs) [key], Du. Leie, river, c.135 mi (220 km) long, rising in the hills of Artois, N France, and flowing northeast, forming the Franco-Belgian border between Armentières and Menen. It continues into Belgium past Kortrijk to empty into the Scheldt (Schelde) River at Ghent (Gent). The Lys is canalized from Aire to Ghent. The Lys valley is known for flax spinning and weaving. It was the scene of severe fighting in World War I.