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

Abstract

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

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

“…Particularly, richness was lower than that recorded before the event, as well as diversity at the site closest to the reservoir (M1) and where relevant sediment deposition occurred (M3). This confirmed the expected spatial gradient of the perturbation (Tullos et al, 2016), and the relevance of permanent habitat alteration (Quadroni et al, 2016). Large effects shortly after siltation disturbance were documented in rivers affected by natural events (Miserendino et al, 2012) and anthropogenic activities (Doretto et al, 2019).…”

Tackling reservoir siltation by controlled sediment flushing: Impact on downstream fauna and related management issues

“…In fact, both perturbation and impact were expected to decrease in downstream direction as a result of deposition and dilution due to the freshwater input by tributary streams. In this paper, we present the results concerning the monitoring sites shown in Table 3, Fig 1 and S1 Fig, referring to further bibliography for more detailed information (e.g., [31] for CSFOs at VR; [30] for CSFOs at SR; [28] for CSFOs at CR; [32] for CSFO at MR). Except for fish sampling, authorized by the local authorities as requested, no further specific permission was necessary to performing the field activity presented in this study.…”

“…Trout monitoring was performed at V1, C2 and M2 (Table 3). Quantitative sampling was carried out by electrofishing (removal method with two passes), one time before and one time after the CSFO [32], being the lag-time between fish sampling and the related CSFO usually in the order of one month. The trout were counted, measured for total length and juveniles were differentiated from adults according to the threshold of 170 mm [28].…”

Drawdown flushing in a chain of reservoirs—Effects on grayling populations and implications for sediment management

“…Natural high flows may induce mortality or downstream displacement in stream fish populations (Carline & McCullough, 2003), especially in young salmonids (Ottaway & Clarke, 1981, Heggenes & Traaen, 1988, for a review see Warren, Dunbar, & Smith, 2015). During the flushing of Madesimo reservoir, parallel investigation in an affected reach and a second reach only affected by diluting flows showed that flow increases alone can disturb fish, suspended sediment increase had no additional effect on brown trout and bullhead (Quadroni et al, 2016).…”