Fluvial dynamics changeover at the Middle to Late Holocene transition in Europe: insights from the Upper Seine River

Alluvial rivers have long acted as focal zones of human activity, providing suitable settlement locations and serving as major corridors for transportation and communication. Their dynamics are closely linked to Holocene paleoenvironmental changes, which are partly influenced by human activities. To better understand this feedback, Holocene river dynamics evolutions thus need to be thoroughly documented. In this study, we document how channel size evolved during the Holocene along the Upper Seine River in the Bassée alluvial plain (France). Channel geometry is used as a proxy for paleo–river discharge and lateral migration rates. Eleven paleochannels were mapped, described, and dated. These channels fall into two distinct size groups. We show that Early to Mid-Holocene channels were two to five times wider than Late Holocene channels, implying a long-term decrease in both mean annual discharge and channel migration rates. A sharp reduction in channel size occurred at the Mid-to Late Holocene transition. This 4.2 ka BP shift coincides with the Neolithic–Bronze Age transition and with a major wetland forest maximum in pollen records. The associated geomorphic changes likely reflect a transition from a predominantly single-thread fluvial style to a multithread (anabranching) configuration. Two non-exclusive drivers are proposed: (i) a rise in groundwater levels that favored alluvial vegetation development, and (ii) increased terrigenous fluxes from uplands due to intensified human-induced erosion. Comparison with other European river systems indicates that similar synchronous transitions occurred elsewhere, although their geomorphic manifestations varied regionally.