How does the streamwater quality change during storm events, and how to characterize it? The purpose of this study is to investigate the evolution of major ions in streamwater from base ow to storm ux. Regular manual sampling and automated sampling campaigns have been performed on the rivers Sorge and Mebre in the Lausanne area, Switzerland. Discharge and quality parameters such as EC, Na+, K+, Mg2+, Ca2+, F-, Cl-, NO-2, NO-3 ,SO-24 were analyzed for the development of relationships between runo and water quality, and between EC and major ions concentration. From the hydrographs and concentration graphs analysis, a dilution eect is observed for all major ions during storm events with dilution factors up to 2.5 compared to base ow conditions. This result suggests that contribution from groundwater signicantly decreases at storm ux. The study has shown that EC is a reliable proxy to assess the variations in water quality of the streamwater from base ow to storm ux. EC allows a much higher resolution in the measurements compared to sampling and chemical analysis.