What does it Happen to Mesohabitats of an Upland Stream after a Controlled Sediment Flushing Operation?

In the Alps, several controlled sediment flushing operations (CSFOs) have been carried out in the last decade to counteract the loss of storage of reservoirs due to siltation. These operations are characterized by the control of the suspended sediment concentration in the downstream watercourse, aimed to reducing the ecological impact. In this study, we assessed the effects of a CSFO from a small Alpine hydropower reservoir at the mesohabitat scale. Specifically, we repeatedly sampled three mesohabitats (one pool, one riffle, and one step-pool) in the final stretch of the stream subjected to the CSFO. In each mesohabitat, we measured both the amount of deposited fine sediment and the related effects on the lower levels of the food web (periphyton and benthic macroinvertebrates), before and up to two years after the CSFO. As expected, after the CSFO, sediment deposition was larger in the pool and a decreasing trend was observed in all mesohabitats. Both periphyton and benthic macroinvertebrates were negatively affected by the CSFO in the short-term. The lowest contraction of benthic macroinvertebrate communities, in terms of density and richness, was observed in the riffle. However, the invertebrate richness of this mesohabitat recovered slowly (more than one year after the CSFO). Differences in the impact and recovery patterns of the zoobenthic assemblages were clearly detected by the Siltation Index for LoTic EcoSystems (SILTES). An improved knowledge about the ecological impact of CSFOs and the related recovery patterns is evidently required to support sediment management practices in regulated river systems.