Late Holocene alluvial architecture of Po and Adige rivers revealed by high resolution stratigraphical and geomorphological data.

The sediments of Po and Adige rivers interfingered, during late Holocene, in the area behind the southern Venice lagoon. New high-resolution sedimentological and geochronological data from cross sections allow the definition of an architectural model of the alluvial plain after the sedimentary hiatus recorded across the entire Venetian–Friulian Plain, between 14.5 and 8.0 kyr BP. Alluvial plain aggradation started as a consequence of the post-glacial sea level rise: the result is a sequence of silty-clay and peats, characterized by the accumulation of a laterally-continuous peat bed (up to 1.5 m thick and dated 5595-5754 to 3158-3383 cal BP in Santa Margherita cross-section) that interests the whole area behind the coastline. After peat deposition, a phase of major aggradation occurred in the area, which resulted in the formation of alluvial ridges by both Po and Adige paleochannels, as seen in Santa Margherita and Conselve cross-sections. Around 3000 BP the Po River northernmost branch was no more active; aggradation continued until Roman times due to sedimentation by the Adige alluvial system. The Po and Adige sediment supply, unbalanced by sea-level rise, seems to have caused the progradation of a delta system in the southern Venice lagoon after 6 kyr BP, which was documented by Zecchin et al. (2009). Geomorphological evidences from high-resolution digital terrain model and remote sensing, coupled with stratigraphic data, show prominent Po and Adige palaeochannels which run behind the southern Venice Lagoon and fed the growing delta. The vertical aggradation in the alluvial plain and the progradation of the delta is synchronous to deforestation in the catchment, especially since the Bronze Age and during the Roman Age. This timing suggests a possible influence of man-induced soil erosion on the increase of the rivers sedimentary load.