Charlemagne's summit canal: an early medieval hydro-engineering project for passing the Central European Watershed.

Christoph Zielhofer,Sven Linzen,Eva Leitholdt,Jens Bussmann,Michael Schneider,Stefanie Berg-Hobohm,Peter Ettel,Andreas Stele,Lukas Werther,Cornelius Meyer,John P Hart

doi:10.1371/journal.pone.0108194

Abstract

The Central European Watershed divides the Rhine-Main 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 connecting the Schwabian Rezat and the Altmühl rivers. The artificial waterway would provide a continuous inland navigation route from the North Sea to the Black Sea. The shortcut is known as Fossa Carolina and represents one of the most important Early Medieval engineering achievements in Europe. Despite the important geostrategic relevance of the construction it is not clarified whether the canal was actually used as a navigation waterway. We present new geophysical data and in situ findings from the trench fills that prove for the first time a total length of the constructed Carolingian canal of at least 2300 metres. We have evidence for a conceptual width of the artificial water course between 5 and 6 metres and a water depth of at least 60 to 80 cm. This allows a crossing way passage of Carolingian cargo scows with a payload of several tons. There is strong evidence for clayey to silty layers in the trench fills which reveal suspension load limited stillwater deposition and, therefore, the evidence of former Carolingian and post-Carolingian ponds. These findings are strongly supported by numerous sapropel layers within the trench fills. Our results presented in this study indicate an extraordinarily advanced construction level of the known course of the canal. Here, the excavated levels of Carolingian trench bottoms were generally sufficient for the efficient construction of stepped ponds and prove a final concept for 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.

Highlights

European Dimension of a Carolingian canal Central Europe is covered by a dense network of navigable rivers
There is one extraordinary piece of engineering which highlights the enormous economic and human resources which have been mobilised for the building of navigable waterways: the Fossa Carolina which is the major focus of our geoarchaeological study presented here
A successful bridging of the Central European watershed would be a performance of engineering of European dimension in the scope of Early Medieval infrastructure and economies [1], [19]

Summary

Introduction

European Dimension of a Carolingian canal Central Europe is covered by a dense network of navigable rivers. Freund [9] highlights the important role of Central European river valleys for the communication networks of these groups. The canal mirrors the first attempt to bridge the Central European watershed during the Early Medieval period [15]–[18]. A successful bridging of the Central European watershed would be a performance of engineering of European dimension in the scope of Early Medieval infrastructure and economies [1], [19]. The demand on economic resources for the canal construction was enormous, but even bigger was the potential advantage of controlling this pivotal point in the inland navigation network of Europe, connecting such tremendous powers as the Carolingian and the Byzantine Empires [1]. Political control was a prerequisite for the canal construction and the outstanding man who gathered this control in a long series of conflicts and wars during the late 8th century AD was Charlemagne [8], [20], [21]

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