Terrestrial water storage is the primary source of river flow. We introduce storage sensitivity of streamflow (Ïμ S), which for a given flow rate indicates the relative change in streamflow per change in catchment water storage. Ïμ S can be directly derived from streamflow observations. Analysis of 725 catchments in Europe reveals that Ïμ S is high in, e.g., parts of Spain, England, Germany, and Denmark, whereas flow regimes in parts of the Alps are more resilient (that is, less sensitive) to storage changes. A regional comparison of Ïμ S with observations indicates that Ïμ S is significantly correlated with variability of low (R2 = 0.41), median (R2 = 0.27), and high flow conditions (R2 = 0.35). Streamflow sensitivity provides new guidance for a changing hydrosphere where groundwater abstraction and climatic changes are altering water storage and flow regimes.