Hydropower Flushing Events Cause Severe Loss of Macrozoobenthos in Alpine Streams

Alpine hydroelectric power exploitation often aims to increase the volume of water stored behind impoundments, which may be achieved through flow ion and lateral transfer to storage. Intakes are designed to separate water from sediment which accumulates in settling basins and may be flushed sometimes at subdaily frequencies in glaciated basins. In some countries (e.g., Switzerland) intakes drain a greater basin area than impoundments yet legislation designed to improve instream ecosystems impacted by hydropower has almost entirely ignored them. Some research suggests that such streams have exceptionally low abundance and diversity of macroinvertebrates for some kilometers downstream of the intake flushing at high frequency in summer, but that populations can recover rapidly as soon as flushing frequency decreases in early autumn. However, such patterns could also result from natural flow variability, sediment transport, and morphological change in glacier‐fed streams. We combine field measurements with habitat modeling to assess the impacts of sediment flushing on macrozoobenthos as compared to what might be expected in a natural, hydromorphologically dynamic Alpine stream. We show that water ion in itself could improve habitat conditions because it increases the relative contribution of less turbid and groundwater/unregulated sources. However, intake flushing leads to short duration, sediment‐laden flows that can destabilize substantial areas of the stream bed and cause rates of lateral displacement of habitat much greater than the possible response by macroinvertebrates. Our results challenge current emphasis on minimum flows in such streams and argue that much more emphasis needs to be placed on sediment management. Key Points Flow ion at hydropower intakes in glaciated streams improves downstream low flow habitat conditions compared to natural conditions Intake sediment flushing degrades habitat due to rapid discharge rise, turbidity increase, euphotic depth reduction, and bed instability Sensitivity to flushing relates to the rate of discharge increase as zones of suitable habitat move faster than possible benthos migration