Eider (Īˈdər) [key], river, 117 mi (188 km) long, rising S of Kiel, N Germany, and flowing N to the Kiel Canal before turning west and meandering to the North Sea at Tönning. It is navigable for most of its length. The Eider's lower course is regulated by locks; part of its upper course is followed by the Kiel Canal. The river separates Schleswig from Holstein.
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