The Eider 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.

The Kiel Canal is a 98 km (61 mi) long freshwater canal in the German state of Schleswig-Holstein. The canal was finished in 1895, but later widened, and links the North Sea at Brunsbüttel to the Baltic Sea at Kiel-Holtenau. An average of 460 km (290 mi) is saved by using the Kiel Canal instead of going around the Jutland Peninsula. This not only saves time but also avoids storm-prone seas and having to pass through the Danish straits.