The year after an extraordinary sedimentation event in a regulated Alpine river: The impact on benthic macroinvertebrate communities

On August 2017, a massive rock slope failure triggered an extreme sedimentation event in a regulated Alpine river. Consequently, high sediment load affected the river for about 1 month, with sediment concentration peaking up to several hundred g/L, and overall sediment yield of 0.4–0.8 Mm3. The availability of sound pre-event data concerning benthic macroinvertebrate assemblages gave us the opportunity to assess the impact of this unusually severe sediment disturbance through before/after comparison. Within 1 month from the event, all the monitored communities displayed low density, richness and diversity, with minimum values never recorded before, particularly at the most upstream investigated reach, and where riverbed aggradation was comparatively larger. Persisting riverbed alteration was also identified as a major determinant justifying the slow recovery observed during the year after the event. Overall, the high resilience of some taxa, suited to a baseline unnatural condition, allowed the fast recolonization of the investigated river reaches. However, the recovery of other taxa, more sensitive to sediment pressure, was slow or did not occur during the monitored year.