Extended Data Fig. 2: Evaluation of the newly reconstructed Lake Bourget palaeoflood record.

Evaluation of the newly reconstructed Lake Bourget palaeoflood record over the last 350 years (a) and the last two millennia (b). Over the last 350 years, the new record (b.) is compared to the record from Evin et al. (2019)²³(a.), which is an update of the dataset from Jenny et al. (2014)⁵⁶ that combined reconstructed palaeoflood discharges from 1650 to 1852 (black squares) and annual flood discharges from 2010 to 1852 (black and red dots). Red dots denote the floods recorded in the sedimentary sequence among the gauged, annual floods (black dots). Our new record includes 27 on these 32 floods and the five extreme floods correspond well to the largest flood discharges. The lacking floods in our dataset can be related to the lower number of sediment cores compared to Jenny et al. (2014)⁵⁶ and Evin et al. (2019)²³. Over the last 2000 years, the new record (d) is compared to the record from Arnaud et al. (2016)⁶⁰, which correspond to detrital inputs brought by the Rhône River floods to the deepest part of Lake Bourget. These detrital inputs were very low in the oldest part of the record and largely increased during the Little Ice Age. This period of increased detrital inputs corresponds well to higher occurrence of flood events. Therefore, these comparisons support the robustness of our extended Rhône River palaeoflood record.