The Central European Watershed divides the Rhine catchment and the Danube catchment. In the Early Medieval period, when ships were important means of transportation, Charlemagne decided to link both catchments by the construction of a canal. The Fossa Carolina would have provided a continuous inland navigation route from the North Sea to the Black Sea. Despite the important geostrategic relevance of the construction it is not clarified whether the canal was actually used as a navigation waterway. ¹⁴C dating and sapropel layers reveal evidence of Carolingian and post-Carolingian ponds. Geoarchaeological drillings, high-resolution direct-push-sensing equipment and archaeological excavations indicate a conceptual size of the canal, which allows a crossing passage of Carolingian cargo scows with a payload of several tons. Our results indicate an extraordinarily advanced construction level of a summit canal. We have evidence for the artificial Carolingian dislocation of the watershed and assume a sophisticated Early Medieval hydrological engineering concept for supplying the summit of the canal with adequate water.