Flow variability is recognised to be a major driving force shaping in-stream ecosystem patterns. Large area and regional analyses often results in significant but poor correlation levels between species traits and indices of flow variability. One reason would be the scarcity of biological data on large areas but also flow variability indices must be developed considering the role of the spatial scale on the overall flow variability. Using 150 hydrological stations from a large French basin it is shown the parasitic role of basin scale in the definition of ecohydrological relevant indices. In the same idea it is verified the influence of flow management on the temporal succession of frequent flows.