Geological evidence and written sources of the medieval “diluvium”: a strong natural phase after the Roman anthropogenic period?

In the Mediterranean area and especially in Italy the Roman Age corresponds to a period of heavy and evident human impact on the environment, which in many places coincides with a general geomorphological stability. In the period following the collapse of the Roman Empire (namely the 5th-8th centuries AD), strong alluvial processes occurred in Northern Italy, triggering a fast and strong sedimentary phase that led many large Alpine rivers to avulse. Major changes are clearly documented in the alluvial systems of Adige, Brenta, Piave, Livenza, Tagliamento and Isonzo rivers; the major episodes are represented by the so-called “Rotta della Cucca” (Adige R.) and the floods which buried Concordia Sagittaria (Tagliamento R.). An important phase of reactivation also affected several of the alluvial cones in the valleys of Trentino and Alto Adige, as well the cones fed by the Apennine streams which prograde towards the Po Plain (e.g. near Modena). In the same period an important alluvial deposition is recorded also in other areas of the Italian peninsula (e.g. Florence and Rome). Since early studies, many Authors considered this Early Medieval period both for its archaeological and geomorphic importance, trying to sort out the natural from the humaninduced component. Several papers stressed the importance of the coincidence between an important natural event/events and the lack of territorial management, particularly when compared to the strong anthropogenic control occurred in the centuries before. New geomorphologic and stratigraphic evidence from NE Italy support the assessment of the magnitude of the natural component (mainly driven by climate) of the alluvial processes occurred in the early Middle Ages; moreover, the chronological information allow to constrain the main alluvial phase between the second half of the 6th and the first part of the 7th century thus, confirming the importance of so-called “Diluvium” described by the Lombard historian Paolo Diacono and dated 589 AD.