Estimating river flood risks under global warming is challenging, largely because of the compounding nature of various drivers1,2. Yet to date, the interplay of multiple drivers and how they affect river floods are not well understood. Here we use explainable machine learning to disentangle the interactions between flood drivers and identify the compounding drivers of river floods in thousands of catchments around the world. We find that the majority of river floods worldwide over the past 40 years were attributable to compounding drivers, which often amplified river flood magnitude. Furthermore, the role of compounding drivers becomes more important with increasing flood magnitude in nearly all of the studied catchments, with the strength of this relationship generally depending on the catchment physio-climatic conditions. Based on these findings, we demonstrate that traditional statistical methods using flood frequency analysis underestimate the magnitude of extreme floods because compounding drivers are not properly taken into account3. Overall, our results highlight the need for careful incorporation of compounding drivers in flood risk assessment to improve estimates of extreme floods, in particular in the face of climate change4,5.