The Po di Adria, Frattesina and the Po Delta between the Middle-Recent Bronze Age and the Early Iron Age

The Southern Verona Lowlands /High-Polesine territories, in the second part of the Sub-Boreal period were subject together with a large part of the Po Plain to continental climatic conditions, interrupted by damp oscillations (Lobben oscillation), finally marked by sub-arid terminal peaks. At the same time, the anthropic impact derived from a widespread dissemination of settlements in the lower Po Valley became increasingly consistent. The interaction of these factors is debited from the first significant changes in the fluvial regime of the Po River, a principal padanian artery already present in the area from the first half of the III millennium B.C., travelled by a northern branch that I have called Po di Adria 1. The evidences of these first hydrographic changes are the formation of new flood-channels, of crevasse-splays and the diversions of various stretches of river courses, now less stable than in the preceding phases (post-Canar I and Pre-Canova phase: EBA-MBA: XVII-XVI century B.C.). These first changes were followed by a phase of relative stability, which coincides with a stage (MBA-RBA) of maximum settlement expansion of the Terramare sites in the central-eastern Po Plain, during which we assist, especially in the range between the South Verona plain and the Upper-Middle Polesine deltaic region, a flowering of a network of sites connected to the main chiefdom site of Fondo Paviani. To the end of this phase, the first evidences of a deterioration of the densely settled Po Plain territories, accentuated by the overexploitation of the soils and by the subsequent collapse of the Terramare settlement system, are seized. Between the end of the Terramare cycle and the beginning of the successive settlement cycle of the Frattesina site (between RBA2 and FBA1) some further paleohydrographic and paleoenvironmental transformations are documented, the effects of which from the hinterland Low Verona/High Polesine plains extended to affect the most eastern belts of the deltaic-coastal system. It now captures (between RBA2 and FBA1) the transition from an environment with very sinuous canals, anastomized, surrounded by basins dominated by peats-marsh sedimentation, to sub-rectilinear canals with wide sandy meanders, to which is associated the construction of first and most evident stripes of natural banks: these new morpho-fluvial patterns mark the transition from the Po of Adria 1 to the Po of Adria 1/2. These evident paleohydrographic transformations are caused in large part by the confluence of a new Po branch – The Po di Poggio Rusco-Dragoncello-Sermide, a result of a great diversion of the Po river flow from the hydraulic node of Brescello-Guastalla, in the Po of Adria 1. This new asset give a more efficient configuration to the Po course , named Po of Adria 1/2. At the same time, an increase is observed in the progradations of the coastal bodies (dunes and beach ridges) favoured by the confluence of the discharge contributions of Adige River in the Po di Adria River. These new dynamics, due to the cutting and obliteration of a series of very sinuous meanders inherited from the previous period (Ceneselli and Campestrin meanders), influenced the disorganization and participated in a rapid obliteration of pre-existing anthropic canals network having delivery in the F. Tartaro, at the same time placing serious difficulties to the previous connections between the central site of Fondo Paviani and the Po delta, now connected through the Po di Adria branch alone. After a phase of strong reduction and generalized abandonment of the Terramare sites (advanced RBA2), followed, starting from the FBA1, a phase of rapid recovery and resettlement in the regime of the "new" Po branch, the Po of Adria 1/2, favoured by the establishment of new average climatic conditions, more dry than the previous ones. This more stable situation determined the optimal conditions for the Frattesina site foundation, located at the apex of the delta system originated from the contributions of the main Po branch of the time, now disjointed from the connection with the Adige river, in following the clogging for overflood of the Saline-Cona branch. These new conditions, for the recovery of a stable river structure "in almost-balanced equilibrium" for the Po of Adria, determined by the confluence of the Po di Poggio Rusco-Dragoncello-Sermide in the ancient Po of Adria 1, favoured the rapid rise of the transformation and exchange pole of Frattesina, a role that lasted almost continuously for the three centuries of FBA. At the beginning of the Iron Age (EIA1: IX-VIII century B.C.) there were even more traumatic and extensive paleoenvironmental changes of the previous ones, determined by the establishment of a climatic phase of oceanic-cold type, with dryness peaks (Goschenen I fluctuation). This climate change reflected in the increase in the aggradation level of river courses, which now show accentuated phenomena of superelevation of the riverbeds, followed by repeated successions of floodings and river breaches. To these disastrous paleohydrographical processes, there are associated evident alterations in the coastal lines morphology, caused by deep erosions associated with disturbances of marine-weather origin. All these processes will quickly lead to the overflood of the entire river tract of Po di Adria 2, which is shortly to undergo a complete avulsion of its flow near Castelmassa, culminated in the Sermide breach-flood, the formative episode of new branch of the Po di Ferrara (Po di Spina).