Riverine heatwaves and their dominant generation processes: a systematic typology.

The summer of 2025 in Europe provided another striking example of the severe consequences that riverine heatwaves, i.e. periods of extremely high river temperatures, can have on natural ecosystems and human societies. They led to a general decrease of water quality, massive fish die-offs, and in France and Switzerland, to the shutdown of nuclear power plants because of a lack of cooling capacity. Under continued climate change, river temperatures are expected to further increase, potentially leading to even more frequent and severe riverine heatwaves. Although the drivers of river temperatures have been widely studied, the factors causing riverine heatwaves remain largely unknown, in particular across large spatial scales. To address this research gap, we compiled the first large-sample dataset of river temperatures at the European scale and used it to assess the dominant hydro-climatic generation processes leading to riverine heatwaves over the period 1985-2020. For this assessment, we developed a systematic typology of riverine heatwaves, which classifies these events according to their associated antecedent hydro-climatic conditions. We used this typology to quantify the relative importance of each generation process for riverine heatwave development across 957 catchments, and describe the spatial and seasonal distribution of the different riverine heatwave types. We show that riverine heatwaves are mainly occurring during periods of anomalously warm air temperatures and that many severe summer events are occurring because of the cumulative effect of warm air temperatures and low discharge. Our results demonstrate that the importance of each generation process can significantly vary in space and time. They highlight the complex processes leading to riverine heatwaves, pointing towards the need to develop flexible and location-specific mitigation and adaptation measures