Author(s): Kevin Merl; Bernhard Zeiringer; Daniel Hayes; Patrick Holzapfel; Christoph Hauer; Martin Schletterer; Stefan Schmutz; Franz Greimel

Linked Author(s): Daniel Hayes, Christoph Hauer

Keywords: Pulsed flow; Juvenile fish; Flow ramping; River widening; Habitat restoration

Abstract: Hydropeaking, caused by the rapid release of water from storage hydropower plants, disrupts riverine ecosystems and particularly impacts young-of-the-year (YOY) fish. These YOY fish are highly vulnerable to hydropeaking due to their dependence on shallow, slow-flowing near-bank habitats, which are frequently exposed to rapid dewatering. This study evaluates the effectiveness of the GKI hydropeaking diversion power plant on the Inn River, Austria, in mitigating fish ecological impacts. Pre- and post -implementation monitoring was conducted, focusing on YOY brown trout (Salmo trutta) and European grayling (Thymallus thymallus) two years before (2021–2022) and two years after (2024–2025) the plant began operations. Preliminary results show that amplitude was reduced, particularly during low-flow periods. YOY brown trout abundance increased in more heterogenous river sections with better structural habitat heterogeneity. In contrast, YOY grayling abundance remained unchanged following the implementation of the measure. The conditions could improve in the following years as the ecosystem might need more time to adapt to these changes. These preliminary findings emphasize the need to integrate hydropeaking management with habitat restoration measures, such as river widening, to achieve meaningful ecological improvements.

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.213

Year: 2026