The influence of stream discharge and habitat suitability history was investigated over 12 years on three natural brown trout (Salmo trutta) population dynamics. Discharge and habitat (described by Weighted Usable Area, WUA) variability during three "bottleneck" periods of population dynamics (spawning, fry, and summer) were used to explain variability of trout age-class densities (young of the year, juveniles, and adults). Discharge and WUA variability for each period was described with mean value over the period, standard deviation value over the period, and Continuous durations Under ( or upper) a given Threshold (CUT curves analysis). Thresholds used for CUT curves analysis were defined from existing knowledge as (1) the median discharge during spawning period, (2) five times the median discharge of the fry period (high flows); and (3) half of the median discharge of the summer period (low flows). Young of the year trout density decreased when the duration of high flows during the fry period increased. Adult trout density decreased when the duration of low flows during summer increased. Beside these key periods, trout population dynamics seemed also regulated by biotic factors. The number of juveniles at year n was directly related to the number of young of the year at year n-1, and number of adults at year n was directly related to the number of juveniles at year n-1.