The Eider is the longest river in the German state of Schleswig-Holstein with a length of 188 kilometres. It has an average flow of 6.5 m³/s and a catchment area of 3,275 km².
It rises near the town of Bordesholm, from where it flows northwards almost to the edge of the regional capital Kiel on the Baltic Sea coast. Here, however, it turns sharply westwards and flows towards the North Sea, where it also empties. In the upper reaches near Kiel it flows through several larger lakes such as the Schulensee and the Westensee. The middle part of its course has been developed as part of the Kiel Canal. Its largest tributary is the Treene. The Eider flows into the North Sea at the town of Tönning. Its estuary is characterised by tidal tidal phenomena extending up to 6 km upstream. The water in the estuary is brackish. The mouth of the river is dammed by a flood barrier.
In the early Middle Ages, the river was probably the boundary between the Germanic settlement of the Jutes and the English. In the High Middle Ages it separated the territories of the Saxons and Danes and formed the border between Denmark and the Holy Roman Empire for centuries. Today, it forms the border between the historical countries of Schleswig and Holstein.
From its mouth to the town of Rendsburg, the river is navigable thanks to several locks, which gives ships alternative access to the Kiel Canal.
Freight traffic on the Eider has lost importance with the opening of the Kiel Canal. Several ferries and bridges cross the Eider.
