ALT The two left-hand plots show the reported frequency of responses in water quality on the basis of the compilation of literature case studies (Supplementary Data): during droughts, heatwaves and compound events (part a) and under rainstorms and floods (part b). An increase in a water quality constituent is shown in orange, a decrease is shown in blue and no marked change, mixed or unclear responses are in grey. Focus is on case studies that reported concentration responses (except for water temperature). The schematic diagrams show a selection of key processes and main driving mechanisms that impact river water quality during hydroclimatic extremes.