Restless rivers: impacts of frequent hydropeaking on habitat dynamics

Storage hydropower is central to the energy transition, yet its flexible mode of operation can generate frequent flow fluctuations, known as hydropeaking. This paper synthesises recent empirical and methodological advances to show that the ecological impacts of hydropeaking depend not only on the magnitude of individual flow fluctuations, but also on the frequency of these fluctuations. In Alpine hydropeaking rivers, flow fluctuations occur on average three times per day, with only minor seasonal differences. This frequency is 20–60 times higher than under natural flow conditions on an annual basis and 49–78 times higher during spring, an ecologically sensitive period. This high frequency of flow fluctuations drives cumulative ecological impacts, including repeated fish stranding and invertebrate drift. The underlying mechanisms operate at the patch scale, where recurrent shifts in habitat conditions lead to a pronounced increase in habitat dynamics. In practical terms, habitat persistence and temporal availability decline, as does the spatio-temporal connectivity of habitats. Three recently developed metrics (M1, M2, M3) quantify patch-scale habitat dynamics and provide a practical tool for assessing cumulative ecological impacts in a spatially explicit manner.