Geomorphic effects of a run-of-the-river dam in a multi-driver context: The case of the Upper Garonne (Central Pyrenees)

In this paper, we evaluate morphological changes related to the Plan d'Arem dam (1970), a run-of-river (RoR) dam located on the Upper Garonne (central Pyrenees), and disentangle its morphological effects from other drivers (post-Little Ice Age [LIA] climate change, changes in agricultural practice, catchment afforestation, up stream damming, and bypassing). The work is based on a before-after-control-impact approach, a space-time framework that allowed the stating of four hypotheses distinguishing the effects of the considered dam from other pressures. We first examined the potential reduction to the flow regime (QL) and bedload transport (QS) from these pressures, then assessed planimetric changes (1942–2019), vertical evolution (1922–2014), and sediment size within the channel. The results show the river completed adjustments related to post-LIA climate change and catchment afforestation at the beginning of the study period, with channel narrowing affecting the whole study reach and ranging from 0.6% to 1.2% yr− 1 . Upstream dams and catchment afforestation reduced both the frequency and magnitude of peak flows and sediment supply, resulting in an increase in the channel narrowing rate on the upstream sub-reach (− 1.2% yr− 1 ). However, downstream tributaries buffered these changes, and no downstream propagation was found. The effects of the Plan d'Arem started around 15 yr after its construction, with channel narrowing at a rate of 0.9% yr− 1 until the 2010s. The exceptional flood of June 2013 resulted in important channel widening followed by a new period of narrowing upstream of the Plan d'Arem dam, combined with channel stability downstream caused by a new dam management regime (flushing actions). We conclude that the before-after-control-impact approach is effective for isolating the effects of an RoR dam from those of other pressures, and that flushing actions mitigated the effects of the dam.Electricité de France, the Occitanie Region and the Adour - Garonne Water Agency are gratefully acknowledged for providing financial support to our research. We also specially thank the Catalan Meteorological Institute and Jordi Tuset for sharing annual precipitation data, as well as the Ebro Water Authorities (CHE) for gravel mining data. We acknowledge Mélodie David and Pierre Hédon for preliminary data collection and processing, and María Béjar for her support on the analyses of land use changes in the Val d'Aran Region. Finally, we acknowledge Carl Embletton for its work which greatly improved the paper grammar. This work was performed within the framework of the EUR H2O'Lyon (ANR-17-EURE-0018) of Université de Lyon, within the programme “Investissements d'Avenir” operated by the French National Research Agency (ANR)