EIDER, a river of Prussia, in the province of Schleswig-Holstein. It rises to the south of Kiel, in Lake Redder, flows first north, then west (with wide-sweeping curves), and after a course of 117 m. enters the North Sea at Tönning. It is navigable up to Rendsburg, and is embanked through the marshes across which it runs in its lower course. Since the reign of Charlemagne, the Eider (originally Ägyr Dör—Neptune’s gate) was known as Romani terminus imperii and was recognized as the boundary of the Empire in 1027 by the emperor Conrad II., the founder of the Salian dynasty. In the controversy arising out of the Schleswig-Holstein Question, which culminated in the war of Austria and Prussia against Denmark in 1864, the Eider gave its name to the “Eider Danes,” the intransigeant Danish party which maintained that Schleswig (Sønderjylland, South Jutland) was by nature and historical tradition an integral part of Denmark. The Eider Canal (Eider-Kanal), which was constructed between 1777 and 1784, leaves the Eider at the point where the river turns to the west and enters the Bay of Kiel at Holtenau. It was hampered by six sluices, but was used annually by some 4000 vessels, and until its conversion in 1887–1895 into the Kaiser Wilhelm Canal afforded the only direct connexion between the North Sea and the Baltic.