The river Kongeå (in German Königs Au) defines the border between North and South Jutland in Jutland in Denmark.[1]
In 1864-1920 it was (except for near the North Sea) the border between Denmark and Germany.[1]
In the Middle Ages it was called Skodborg Å after the royal castle Skodborghus that was at a crossing south of Vejen.<http://arosleo.dk/skodborg.htm>
There was for centuries a customs border near the Kongeå, that separated the Kingdom of Denmark from the duchy of Schleswig.
The Kongeå rises southeast of Vejen and Vamdrup and after about 50 km flows into the North Sea north of Ribe.
The Kongeå is mentioned (as "Skotborg river") in the Heimskringla [2] in a description of when King Magnus I of Norway and Denmark defeated at Hlyrskog Heath in 1043 a big invasion of Slavs from what is now Mecklenburg and around, who had invaded south Denmark in retaliation for a Viking attack on Jomsborg, which at the time was a Slav kingdom's main town on Wolin island.