Le rôle de l’hydrologie sur la destruction de la végétation dans le lit d’une rivière à galets aménagée : l’Isère en Combe de Savoie

The impacts of frequent or moderate floods (return interval <1 year to 10 years) on vegetated macroform turnover in river beds are interesting from a management perspective: artificial floods are an option often considered for managing vegetation in rivers. However their effects are not well documented. In this context, our study was aimed at characterizing the impact of high flow events of different magnitudes on sediment and vegetation dynamics in a heavily managed gravel bed river, from aerial photos and hydrological time series. Specifically, our aims were: (i) to characterize the evolution of vegetation surfaces at the reach scale and over decadal time scales, (ii) to analyze the link between different descriptors of the hydrological time series and vegetation removal, and (iii) to outline the main mechanisms of vegetation removal at these scales. Our study was conducted on the Isere River, a heavily managed gravel bed river flowing in the French Alps for the period 1996-2015. The hydrological parameters which best explained vegetation removal on the Isere bed were the volumes of water flowing between 350 m(3).s(-1) (return interval <1 year) and 600 m(3).s(-1) (return interval of approximately 5 years). Most vegetation removal was due to lateral erosion of the upstream end of the bars. The areas of vegetation removal were modest compared to those of newly established vegetation: 3.4% of vegetated area on average was removed annually, while vegetation cover increased by 6.2%. These results suggest that on this reach, artificial floods could potentially invigorate sediment dynamics and vegetation renewal, but are unlikely to be sufficient to maintain the bed free of vegetation.