The Sauer (German, Luxembourgish) or Sûre (French) is a river in Belgium, Luxembourg and Germany. A left tributary of the river Moselle, its total length is 173 kilometres (107 mi).Rising near Vaux-sur-Sûre in the Ardennes in southeastern Belgium, the Sauer flows eastwards and crosses the border with Luxembourg near Martelange. It forms the border between Belgium and Luxembourg for 13 kilometres (8 mi) north of Martelange.