Extreme hydroclimates impact sediment fluxes from mountainous catchments to the oceans. A challenge is to reconstruct paleo-sedimentary fluxes to assess the sensitivity of erosion in mountainous catchments to environmental perturbations such as climate and tectonics. Here, we study the response of catchments to the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma) using sedimentary archives and numerical modeling. In the Tremp Basin (Southern Pyrenees, Spain), our results demonstrate that depositional volumetric rates of siliciclastic sediments increased two-fold during the PETM. The BQART and Stream Power Law models indicate that changes in mean annual temperature and precipitation explain only 20% of the flux increase. This comparison between field data and model predictions suggests that other conditions, such as extreme rainfall events and landslides, may have been crucial sediment generation processes during the PETM. This is consistent with predictions of enhanced climate variability in a warmer world, leading to significant sediment flushing.