Quantifying Decadal‐Scale Impacts of Flow and Sediment Regulation on an Alpine Gravel‐Bed River and Consequences for Fish Habitat: The Buëch River, France

Predicting long-term effects of dam operation and restoration strategies on channel morphology remains a complex but essential challenge for maintaining fish habitat through effective reservoir management. We address this challenge by integrating a one-dimensional morphodynamic model and a generalized fish habitat model using one-dimensional data, to assess multi-decade habitat changes on the gravel bed Buëch River, southeastern France. Morphodynamic results indicate that flow regulation and sediment reduction lead to bed incision, coarsening, and decline in sediment transport over several kilometers. In contrast, flow regulation alone (i.e., no change in sediment supply) causes rapid aggradation immediately downstream of the dam with minimal morphological change further downstream. Simulations of a one-time reinjection indicate that sediments travel through the study reach within 20 years, and do not significantly impact long-term morphodynamics. However, repeated reinjections are effective at mitigating bed degradation, fining of bed surface sediments, and sustaining higher transport volumes. While hydrological alterations due to dam construction have a pronounced impact on habitat quality, morphological changes alone do not substantially alter habitat suitability. This suggests that sediment management on its own is insufficient for habitat restoration and must be combined with modifications to flow regulation to maintain ecological integrity. These findings underscore the importance of an integrated approach to sediment and flow management in regulated river systems. By providing insights into optimizing sediment augmentation practices and reservoir operation policies, this study contributes valuable knowledge on how results of morphodynamic and habitat models can be integrated to address real world problems over several decades.