Hydropeaking impairs upstream salmonid spawning habitats in a restored Danube tributary

Abstract Hydropower is considered an important form of renewable energy, often involving hydropeaking. While the effects hydropeaking on aquatic communities in areas downstream dam are well understood, there a lack studies investigating potential impacts tributaries located further upstream. In this study, we tested operations upstream restored area Danube River, with focus periods backlog and release water (up‐ramping down‐ramping, respectively) during filling reservoir. We used brown trout egg larval mortality, linked to hydraulic, sedimentary physiochemical changes spawning grounds as indicator. compared hydropeaking‐affected versus non‐affected sites using HydroEcoSedimentary Tools (HESTs) loaded clean gravels eggs. Egg mortalities were significantly higher site more than 80% mortality almost 100% values 55–63% 80–85%, respectively, sites. Spawning ground quality was altered site, where highest observed. Overall, duration time flow velocities close zero key variable, potentially decreasing oxygen supply for eggs larvae. Such driven by reservoir, revealing that such events can severely impair ecological integrity dams slowing flows tributaries. processes reduce fish population recruitment need be future restoration projects.