Fossa Carolina: The link between the harbour networks at the rivers Danube and Rhine. Studies on bridging the central European watershed in the Middle Ages.

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Between the Altmühl and Rezat rivers and close to the village of Graben impressive remnants of the Fossa Carolina are conserved. The canal was constructed by Charlemagne in 793 for bridging the European Watershed and is one of the major hydro-engineering projects of the Early Medieval Period. The canal mirrors an early and for a long period solely attempt to connect the harbour networks of the Rhine catchment with those of the Danube catchment with an artificial waterway and highlights the important role of Early Medieval inland navigation in central Europe. In our interdisciplinary project proposal we aim to analyse the canal on its construction level but also its local, regional and supra-regional role for inland navigation. The canal, appendant Altmühl and Rezat harbours, local settlement structure and environmental history will be reconstructed in a diachronic approach using a full set of archaeological, historical, geoarchaeological and geophysical methods. At the onset of the first project phase no significant findings about the hydro-engineering concept of the canal and its history of use were available. We verified the initial Carolingian age of building and proved the concept of a summit canal. The major aims of the second project phases are to prove the final hydro-engineering concept and the potential use of the canal and to synthesize all available (geo-)achaeological, geophysical, topographical and historical data. We will use a hydrological modelling approach based on new high-resolution dendrochonological data sets from the region. Reconstructed findings about the canal construction style will be integrated in the hydrological model to get information in Early Medieval harbour construction techniques. On local scale the project focuses on the prospection and investigation of the still unknown Carolingian infrastructure of the building site. Additionally, we have preliminary evidence for a collapsed trench in the southern part of the canal but also for a surprising extension of the course in northern direction. This might lead to significant shifts in the local distribution of potential hythes and harbours. Here, four potential sites will be reconstructed and interpreted in a synthetic context. On regional and supra-regional scale we compare the Rhine-Fossa Carolina-Danube corridor with other networks of inland navigation and watershed passages between Saône, Seine, Moselle and Rhine. Here, watersheds are interpreted as stable discontinuities in enduring and diachronic networks of inland harbours and waterways.