Eider River at English => English (The Britannica Concise) Of Explained:
River, Schleswig-Holstein state, N Germany. It rises east of Rendsburg and flows west for 117 mi (188 km) to the North Sea. It is navigable up to Rendsburg. It formed the N limit of the Roman empire from the reign of Charlemagne (768-814), was recognized as the boundary of the Holy Roman Empire in 1027, and formed the traditional frontier between Schleswig and Holstein.