Fossa Carolina
Coordinates: 48°59′02″N 10°55′18″E / 48.9840°N 10.9216°E
The Fossa Carolina (or Karlsgraben in German) 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 (the Rhine basin to the Danube basin). 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.
Near Treuchtlingen and Weißenburg in Bayern the European Watershed between the rivers of the Rhine basin and those of the Danube basin is very narrow. Only around 2 km of fairly level terrain lie between the Swabian Rezat and the Altmühl.
Carolingean sources report that Franconian king Charlemagne gave orders to dig a 2 kilometers long canal from Treuchtlingen to Weißenburg in Bayern in 793. According to Einhard, writing around two decades later, Charlemagne was convinced by "supposed experts" that it would be possible to travel in comfort by ship from Rhine to Danube if the Rezat could be linked to the Altmühl. The king assembled a large number of workers and in person supervised construction in the fall of 793 of a 2,000 step long and 300 feet wide moat. Einhard reports that the project failed due to the marshy ground and continuous rainfall, which caused excavated material to slide back.
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