The Instream Flow Incrementai Methodology (IFIM) was developed to determine flows that must be maintained downstream of hydropower plants to preserve aquatic populations. It is based on the hypothesis that the characteristics of the habitat in periods of low flow have a crucial impact on the dynamics of these populations. Other parameters that may also affect populations include: water quality, temperature, flood episodes, strategies for stocking, fishing, etc. Dynamic models of fish populations are now being developed in an attempt to integrate ail these factors. The model presented here was applied on 3 populations (on Oir River, Neste d'Oueil River, and Roizonne River). The study has highlighted the fundamental role of the temporal variability of environmental parameters (particularly temperature, discharge and Weighted Surface Area) in structuring trout populations. Particularly, the flow-related habitat role simulated in the model - through a phenomenon of compensation during the first months of life and through displacement resulting in mortality, and mortality among fry when discharge is high - was illustrated by these examples.