Long-term drivers and impacts of abrupt river changes in managed lowlands of the Adige River and Northern Po Delta

We analysed the millennial-scale evolution of the alluvial systems of the Po and Adige rivers around the city of Adria, through the analysis of remote sensing images, digital terrain models, historical maps, and the execution of manual cores across the main alluvial ridges. Selected organic samples were radiocarbon dated. Results show that floodplain sedimentation dominated the area until 4.3–4.0 ka cal BP, followed by aggradation of a major alluvial ridge of the Po River (Adria ridge) until 2.6 ka cal BP. Due to a major upstream avulsion, the Po River shifted tens of kilometers to the south, and the minor Tartaro River started flowing through ancient Adria bringing stable geomorphic conditions. In the 10th century AD, the opening of two crevasse channels on the right bank of the Adige River, the Castagnaro and Malopera rivers, initiated the aggradation of the alluvial ridge of the Tartaro River at Adria. This happened during the Medieval Climate Anomaly, in a period of enhanced flooding recorded in Northern Italy, in the Alps and the Mediterranean basin. Extensive peat sedimentation followed since the 11th century, indicating a decrease of overbank flooding and silting by the Tartaro River due to concurrent early management of the crevasses, proximal sediment accumulation, and favorable regional climatic conditions. Fluvial sedimentation started again in the early 15th century, due to the reactivation of the Castagnaro and Malopera crevasses. This happened at the beginning of the Little Ice Age in a period of recurrent flooding of the Adige River, but human-induced modification of channel width and depth may have concurred. Overall aggradation along the Tartaro ridge led to the deposition of up to 4 m of sediment, and the complete burial of an early Medieval church in the city of Adria. The end of this sedimentary event predates the 17th century, and relates to efficient river management actuated by the Venetian Republic. Our investigation highlights how a major avulsion of a large river such as the Po can abruptly change the environmental conditions of large stretches of alluvial plain, allowing the development of an urban settlement on the former floodplain. It further shows that such favorable setting can be suddenly modified by the opening of crevasse channels tens of kilometers away, if water and sedimentary connectivity is allowed. Adria community was resilient to flooding and silting and always attempted to manage the crevasses, but its efforts were counterbalanced by fluctuating climatic conditions.