There is emerging concern over the impact of extreme events such as heavy rainfall on the quality of water entering the drinking water supply from aboveground sources, as such events are expected to increase in magnitude and frequency in response to climate change. We compared the impact of rainfall events on streamwater quality in four contrasting upland (peatland and mineral soil) and lowland agricultural catchments used to supply drinking water in France (Brittany) and the United Kingdom (North Wales) by analyzing water samples collected before, during, and after specific events. At all four streams, heavy rainfall led to a considerable rise in organic matter concentration ranging from 48 to 158%. Dissolved organic carbon (DOC) quality, as determined using specific ultraviolet absorbance, changed consistently at all sites during rainfall events, with a greater proportion of aromatic and higher molecular weight compounds following the onset of rainfall. However, the change in DOC quality and quantity did not significantly alter the trihalomethane formation potential. We observed small increases in trihalomethane (THM) generation only at the Welsh peatland and agricultural sites and a small decrease at the Brittany agricultural site. The proportion of brominated THMs in chlorinated waters was positively correlated with bromide/DOC ratio in raw waters for all sites and hydrological conditions. These results provide a first indication of the potential implications for surface-based drinking water resources resulting from expected future increases in rainfall event intensity and extension of dry periods with climate changes.