A river's ghost in the landscape
The Arno River flows through Florence, a feature of the city. However, the river's present course is a relatively recent development. Geological and historical analyses reveal that for much of its history, the main channel of the Arno flowed through the wide Florence-Prato-Pistoia plain, a 45-kilometer-long and 10-kilometer-wide intermontane basin. This ancient river, or paleo-Arno, was not a single channel but a braided system, with multiple, shifting watercourses spread across a bed that was up to 1000 meters wide in places. Today, the river is confined to a channel just 150 meters wide at its broadest point near Florence.
Evidence for this former course is buried beneath the modern landscape. Scientists use sediment cores and stratigraphic analysis to map the old riverbeds. These cores reveal deposits of coarse river sands and gravels in locations far from the current channel, buried under layers of fine-grained alluvial silt. These sandy deposits trace the path of the powerful paleo-Arno. Analysis of historical maps and aerial photography further corroborates the data, showing faint traces of paleo-beds that are invisible from the ground. Before extensive human intervention, the Arno's natural tendency was to occupy this wide plain to the north.