Multiple abiotic and biotic processes structure trout population dynamics at different space and time scales [1], [2]. The key role of hydrological variability in structuring brown trout populations is well-established [3-5]; [6]; [7]. All age-stages were considered as potentially influenced by high spring floods because spates have a major impact on 0+ fish [8] [9] [10] and may be strong enough to influence the survival and dispersal of older stages [11]. Nevertheless, authors often failed to identify constraining abiotic conditions for juveniles and adults (e.g.,[4]) except after exceptional events (e.g., a 50-year flood in [11]). For instance, discharge thresholds (e.g.,maximum mean daily flood) have often been used, although they correspond to very different hydraulic constraints depending on rivers. Describing high flow based on standardized quantitative variables for the hydraulic habitat of brown trout (e.g., depth, velocity) might reduce these inconsistencies. In this communication, we pointed out the results quantifying the effect of floods on trout densities obtained through three different approaches taking into account abiotic and biotic parameters [12, 13]. We used an extensive data set collected in 45 river reaches, including 22 reaches located downstream a hydropower facility. Electrofishing surveys and detailed physical habitat characteristics (e.g. hydraulics, water temperature, and cover) were available at all reaches.