Effects of sediment flushing from a small Alpine reservoir on downstream aquatic fauna

Sediment flushing is currently performed to recover the storage capacity of small-sized to medium-sized reservoirs. However, its environmental impacts are not yet adequately quantified. This work aimed to evaluate the effects of a 3-day sediment flushing from a small reservoir in the Italian Alps on downstream freshwater fauna. Biomonitoring was carried out in two streams. In the impounded stream, benthic macroinvertebrates were surveyed immediately below the flushed reservoir. In the receiving stream, where diluting flows were released to reduce the sediment concentration and deposition, fish and macroinvertebrates were monitored at two sites, one above and one below the stream junction. Above the stream junction, the only disturbance was the increase in streamflow, while the reach below the stream junction was perturbed by the increase in both streamflow and sediment load. At the site closer to the flushed reservoir, the benthic community was almost completely impaired after the operations, and its recovery was still incomplete after 1 year. In the receiving stream, the average sediment concentration of approximately 6 g l(-1) and deposition of 30 kg m(-2) were estimated at the site subjected to the sediment increase. A density reduction of 80% and a change in composition affected the macroinvertebrate assemblage, which recovered in approximately 9 months. At the same site, the fish survey showed that brown trout juveniles were affected by the sediment load from the flushing operations. Only minor effects were detected on macroinvertebrates and fish subjected only to the increase in streamflow, i.e. above the stream junction