The structure and abundance of brown trout populations varies greatly among years and stream reaches. Among the numerous environmental key factors explaining this variability, the effect of floods is predominant but rarely quantified. Here, we summarise the results of a PhD thesis that aimed to identify the demographic processes and the influence of biotic and abiotic drivers structuring the age-stages of trout populations in time and space. We used an extensive data set collected in 45 river reaches, including 22 located downstream a hydropower facility. Electrofishing surveys and detailed physical habitat characteristics (e.g. hydraulics, water temperature, cover) were available at all reaches. Firstly, an analysis of the determinants of synchrony in densities at a large scale, pointed out that the recruitment of close populations (up to 75 km apart) could be synchronized by large floods during emergence of fry and/or by the remobilization of spawning substratum. Secondly, the application of a deterministic trout population dynamic model, independently calibrated on nine reaches, revealed the influence of various local environmental drivers on population dynamics. Finally, a hierarchical model involving all reaches showed that density-dependent mortality among juveniles and adults was a key biotic process. The strength of the competition was greater in absence of shelters and varied with water temperature. The results of this work pointed out that trout populations share common processes that can be modelled as a function of quantitative environmental parameters. They provide a scientific basis to hydropower facility managers to reduce their influence on trout populations and respond to regulatory demands.