Fossa Carolina

The Fossa Carolina was a canal named after Charlemagne in what is today the German state of Bavaria, intended to connect the Swabian Rezat river to the Altmühl river. It was created during the early Middle Ages, long before the Ludwig Canal and the Rhine-Main-Danube Canal. If it was indeed operational, this canal would have been the first to link the Rhine basin to the Danube basin, across the European Watershed. However, contemporary sources are contradictory as to whether it was ever finished or not.

Source: Wikipedia