The Fossa Carolina (or Karlsgraben in German) was a navigable channel 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 channel would have been the first to link the Rhine basin to the Danube basin, across the European Watershed.