Extreme Flood Events (EFE) and related disasters are becoming an increased economic and social concern in the context of global environmental change. The current and expected impacts of EFEs at regional scale are highly variable, as our knowledge of climate and land cover/use dynamic interactions is hampered by the limited availability of long time series. We investigate such relationships during the last millennia in the northern Iberian Peninsula where both Atlantic and Mediterranean climate patterns interact, and the landscapes have been heavily modified since Medieval times. Three lacustrine sedimentary archives located along an W-E transect in Northern Spain (Sanabria, Arreo and Montcortès) and with robust age models provide high-resolution reconstructions to assess the effects of climate variability and land use change on flood frequency and intensity. Preliminary results show a large spatio-temporal flood variability at decadal to centennial scales. The northwestern (Lake Sanabria) and central (Lake Arreo) sites show an increase in extreme events since the 17th century. On the other hand, more extreme floods occurred in Lake Montcortès (NE Iberia) during the Late Middle Ages and the 19th century. A see-saw pattern in flood frequency seems to occur during the Medieval Climate Anomaly/Little Ice Age transition with more (less) flood events occurring during cold (warm) phases in Atlantic and Mediterranean areas respectively. The occurrence of a large flooding event on January 9th, 1959 caused by the Vega de Tera dam failure in the Sanabria watershed provides a unique opportunity to compare its depositional signatures with previous paleofloods. Detailed geomorphologic mapping and sedimentological analyses of a transect of cores in the lake underline the uniqueness of the 1959 flood in terms of sediment transport, lake-watershed connectivity and hydraulic properties. The depopulation of the mountains since the mid 1950s have changed the land covers and consequently the hydrological behavior of the watersheds. Although at a regional scale, climate variability remains the main flood control, land-use changes have strongly modulated the magnitude of run-off events and ultimately affecting soil erosion and land degradation in northern Iberian Peninsula during the last millennium.

AGU Fall Meeting 2019 in San Francisco, 9-13 december 2019