Study of the impacts of the Little Ice Age period on the Loire River floodplain: contributions of the morpho-sedimentary approach.

The Loire River is France's largest river, with a watershed covering 1/5 of the country's total area. However, we know very little about the reajustment of the Loire River fluvial forms to climate change. To better predict future consequences, we study the Loire River system in the past, and more specifically during the Little Ice Age (LIA) (14^th-19^th c.). In the light of studies on other European rivers, we assume that the LIA intensified the Loire River hydrological activity and sediment transport, and consequently modified its fluvial pattern. The use of documentary archives enabled us to characterize phases of intense hydrological activity during the LIA (Mesmin et al., accepted¹). The aim here is to study the impacts of the LIA on the readjustment of fluvial forms and on the sedimentary construction of the floodplain using a morpho-sedimentary approach.

This study is essentially based on the analysis of historical Loire River paleochannels, combining two approaches. The first involves studying the evolution of channel geometry using Lidar images, old maps (since first half of the 18^th century) and geophysical measurements (ERT). This approach enables us to precisely characterize the fluvial paleo-forms and calculate the discharges associated with the major floods of the LIA. The second approach focuses on the study of the sedimentary filling of paleochannels in order to precisely characterize the deposits of major floods. Over thirty boreholes were drilled in the Loire River paleochannels.  Grainsize measurements (every one cm) were carried out, coupled with magnetic susceptibility and XRF measurements, to determine variations in the type of sedimentary deposits and assess sedimentation rates as a function of fluvial unit type. Deposits has been dated by C14 and OSL dating. The complementary nature of the method helps to reconstruct the evolution of the Loire River floodplain over the last millennium. The results show that LIA discharges were much larger than current flood discharge, explaining the construction of very large paleo-channels. The paleochannels filling revealed the importance of sandy deposits in paleochenal formation during the LIA. However and surprinsingly, silty overbank deposits on the floodplain are relatively thin. Finally, while fluvial metamorphosis of the Loire downstream has been documented during the LIA, in our study area further upstream, fluvial metamorphosis may not have occurred.

¹ Mesmin, E., Gautier, E., Arnaud-Fassetta, G., Foucher, M., Martins, G., Gob, F., accepted. Characterization of periods of high and low hydrological activity in the Loire River, France, between the 14th and mid-19th centuries. Journal of Hydrology.