Thirty years ago, the importance of natural stream flow variability (magnitude, frequency, duration) in maintaining healthy aquatic ecosystems (biodiversity) was virtually ignored in a management context (Poff et al. 1997). Now, the objective of population dynamic models integrating cause and effect in fish habitat relationships is to define flow management strategies satisfying stream and human water needs (Cardwell et al. 1996, Whittaker and Shelby 2000). This paper is an overview of how population dynamics has become indispensable simultaneously in research and management contexts.