The Eider (German: Eider [ˈaɪdɐ] ; Danish: Ejderen; Latin: Egdor or Eidora) is the longest river in the German state of Schleswig-Holstein. The river starts near Bordesholm and reaches the southwestern outskirts of Kiel on the shores of the Baltic Sea, but flows to the west, ending in the North Sea. The lower part of the Eider was used as part of the Eider Canal until that canal was replaced by the modern Kiel Canal.
In the Early Middle Ages the river is believed to have been the border between the related Germanic tribes, the Jutes and the Angles, who along with the neighboring Saxons crossed the North Sea from this region during this period and settled in England. During the High Middle Ages the Eider was the border between the Saxons and the Danes, as reported by Adam of Bremen in 1076. For centuries it divided Denmark and the Holy Roman Empire. Today it is the border between Schleswig, Holstein and Eiderland, the northern and southern parts, respectively, of the modern German state of Schleswig-Holstein.
