[2] 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.

[3] Today it is the border between Schleswig, Holstein and Eiderland, the northern and southern parts, respectively, of the modern German state of Schleswig-Holstein.

The mouth of the river is crossed by a closeable storm surge barrier, the Eider Barrage.

The Eider therefore provides an alternative route from the North Sea to the Kiel Canal, avoiding the tides of the estuary of the Elbe.