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

Remains of the Fossa Carolina are still visible near the small town of Graben in Middle Franconia (Bavaria/Germany). Documentary sources report that the canal was constructed in 793 on the initiative of Charlemagne to bridge the major European watershed. It was one of the most important large-scale waterway construction projects of the Early Middle Ages. Within the Special Research Project (SPP) represents the Fossa Carolina the intersection of the inland navigation systems between the catchments of the rivers Danube and Rhine. An interdisciplinary team from the universities of Jena and Leipzig and from the Bayerisches Landesamt für Denkmalpflege (Bavarian State Agency for Heritage Protection) will employ historical, archaeological and geoarchaeological methods to investigate the canal, its associated harbors on the Altmühl and Rezat rivers, as well as past settlement in the surroundings and the environmental conditions.

Image in the header: Illustration of constructing the Fossa Carolina published in the Fries-Chronic of bishops from Würzburg (16^th century), source: franconica.uni-wuerzburg.de/ub/fries/fries_miniaturen.html

There are multiple questions and problems gaining from previous researches of the construction ensemble of the Fossa Carolina. Thus, a systematic source-critical analysis of the historic lore fragments is missing. Extent and course of the canal structure northern and southern of the central section are not yet known, nor the connections to the rivers Altmühl and Rezat.  Cross- and longitudinal sections of the canal are also unknown. And finally, there are only few findings about the trench bottom. Therefore the question of whether the construction was planned as continuous canal or as chain of ponds remains unanswered. Previous findings on sediment stratigraphy within the refills of the canal are not absolutely reliable: a thick peat layer within the central section of the canal was defined as early medieval peat. Today we can indicate by new radiocarbon ages that the peat layer was built in the early medieval but its development proceeded up to the high and late medieval. We have found pieces of wood within the trench bottom layers that prove by radiocarbon dating the Carolingian construction period for the first time. Besides the Carolingian period there are younger stages of development which also have to be taken into consideration, especially with focus on possible damming constructions: new radiocarbon ages indicate that a former Carolingian-called bank was actually built some hundred years later during the high medieval. Therewith the previous model concept of water retention within the Fossa Carolina has to be corrected by new elaborated theories of its functionality.

The development of the settlements, landscape/environment and traffic in the surroundings of the canal were not in the research focus so far. The available written sources have been regarded very selectively. The archaeological sources within the closer surrounding of the Fossa Carolina are hardly accessed. The overland passage with its associated ports in the north as well as in the south of the watershed has neither been considered yet. Findings of changes in topography and hydrology within the potential hythe zones are still missing, too. These problems are perpetuated at the regional level: navigable waterways inclusive its associated ports reaching close to the watershed plus roadways connecting each other are still poorly investigated. Thereby it raises the question which streams were actually navigable in early medieval times. Connected with this the Carolingian land extension and its outcome on transport system and ecosystem is only selectively comprehensible up to now. The Fossa Carolina should be discussed in its function as the joint of port networks and sub-systems of inland navigation between the rivers Rhine and Danube from supra-regional point of view. Finally, a comparative study of different passages across the central European watershed and its significance in the framework of the inland navigation systems is still missing.

Our project is pursuing the following objectives based on the above mentioned problems: written sources about the construction ensemble should be assembled within a source book and being re-assessed. In addition, the economic and traffic relevance of the canal as an element of regional and supra-regional traffic and communication networks has to be broadly analyzed. The ground monument with all its more recent modifications will be recorded in a GIS cadastre that will form the basis to clarify total length and course of the canal.

Stratified features from the Carolingian trench bottom give information about the longitudinal section of the canal. Regarding the cross section there is research needed concerning potential bank fixations, the navigable width and depth of the canal as well as terrain levels before the canal construction.

The high-resolute chronological stratification of sedimentological specifiable aggradation sequences of the canal is a requirement to answer those questions above. We plan to develop alternative models of ponding and functionality of the canal based on new findings about palaeo-hydrology and history of holdup devices. Especially the post-Carolingian usage history of the construction ensemble is in the focus of our research. Therewith, a discussion can be carried out about total dimensions, construction, dating, and chronological stratification of single events as well as the completion of the construction. Our results have to be compared with other medieval large-scale hydraulic and geological engineering projects. 

Archaeological assemblages in the surroundings have to be inspected systematically to reconstruct local settlement structures and facilities of construction sites and traffic. We plan to develop a diachronic model of the development of cultural landscape. Additionally, we are looking for former overland routes and hythes at the rivers Altmühl and Rezat that enable the connection between the Fossa Carolina and the supra-regional inland navigation network.  

Closely connected is the question of navigability of Fossa Carolina tributaries and further waterways as alternative passages across the major watershed. Finally, we want to re-assess at supra-regional level the watershed passage at the Fossa Carolina as an intersection of harbor networks and inland navigation systems between the river catchments of Rhine and Danube. The comparative analysis of the highly alterable inland harbor network with view on watersheds as steady straits throughout many epochs provides a specific information potential for the SPP.