Rapid sub-daily flow fluctuations caused by hydroelectric power generation (hydropeaking) pose a significant challenge to aquatic ecosystems in Alpine rivers, particularly for young-of-the-year (YOY) fish. Hydropeaking reduces recruitment rates through reduced spawning success and habitat availability and persistence, as well as increased downstream drift and stranding rates. This study assessed the abundance of YOY brown trout (Salmo trutta) and European grayling (Thymallus thymallus) in multiple Austrian Alpine rivers sampled mainly in August and September. Hydropeaking was analysed alongside other factors, including land use, river morphology, hydrology and tributary influences. The results show that sub-daily flow fluctuations, characterised by event frequency and down-ramping rates, were the main predictor associated with lower YOY fish abundance. To capture their combined pressure, event frequency, down-ramping rate, and amplitude were integrated into a hydropeaking index. Along the gradient of the hydropeaking index, the abundance of YOY brown trout and European grayling declined from median values of 10.6 and 4.8 individuals per 100 m, respectively, to less than 1 individual per 100 m. Consistent with this pattern, the literature-based down-ramping threshold (0.2-0.3 cm/min) became more common with increasing hydropeaking index and coincided with higher event frequency. Suitable tributaries showed only limited mitigating effects on conditions in the main river channel. From a management perspective, the findings highlight the need to reduce down-ramping rates and event frequency, particularly during the critical emergence period of YOY fish, in order to mitigate the effects of hydropeaking. Furthermore, the findings show that YOY fish are a suitable indicator of hydropeaking.

ABSTRACT Photograph of the River Saane showing hydropeaking water-level fluctuations, with a visible low-flow waterline and a wetted gravel bank indicating the higher peak-flow stage. 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.

Hydropeaking, the intermittent operation of hydropower, generates rapid flow fluctuations that disrupt river ecosystems. However, the mechanisms driving stranding and displacement of young fish under varying hydropeaking conditions remain poorly understood. Using a nature-like experimental facility, we conducted ∼1,000 hydropeaking trials involving >120,000 larval and juvenile fish from four species to assess how hydropeaking intensity, environmental conditions, and biotic factors influence fish behavior. Stranding and downstream displacement increased with hydropeaking intensity, particularly at night, in smaller fish, and lower water temperatures. Flow peaks with cold-water releases (thermopeaking) further amplify displacement. Fish demonstrated behavioral adaptation, reducing displacement over successive peaks, suggesting learning effects. The patterns were largely consistent across species. By identifying critical ecological thresholds, our findings inform hydropower strategies that minimize ecological harm while supporting renewable energy production.

The extensive development of hydropower has significantly impacted Austria's river ecosystems over the last decades, leading to substantial habitat degradation for fish popula tions. This decline is exacerbated by a range of other human pressures including river channelization, climate change, pol lutants, and navigation. As a result, species once prevalent, such as the Danube salmon, are now at the brink of extirpa tion. Former mass species such as nase and barbel have also seen significant reductions in distribution area and stock sizes. Currently, about twothirds of native fish species in Austria are listed on the Red List of threatened species. To counteract these trends, it is imperative to preserve the remaining free flowing rivers. Furthermore, it is crucial to mitigate all other impacts as well as to implement all ecologically effective measures at hydropower plants to avoid extirpation of fish species in Austria.

Infolge des hohen Ausbaugrades der Wasserkraft in Österreich haben sich die Lebensraumbedingungen für Fische in den letzten Jahrzehnten drastisch verschlechtert. Im Zusammenwirken mit anderen anthropogenen Belastungsfaktoren (Flussregulierungen, Klimawandel, Schadstoffe, Schifffahrt etc.) führt dies dazu, dass früher weit verbreitete Fischarten, wie der Huchen, an den Rand des Aussterbens gedrängt wurden, Massenfischarten, wie Nase und Barbe, in ihrer Verbreitung und Bestandsgrößen drastisch reduziert wurden und ca. zwei Drittel der heimischen Fischarten auf der Roten Liste stehen. Es besteht daher dringender Bedarf, die wenigen verbliebenen ungestauten Fließstrecken vollständig zu erhalten und neben der Sanierung der anderen Belastungsfaktoren nicht nur ausgewählte, sondern alle ökologisch wirksamen Maßnahmen bei Wasserkraftanlagen umgehend umzusetzen, da ansonst Fischarten in Österreich in naher Zukunft unwiederbringlich verloren gehen würden.

The classification of a hydropower scheme as run-of-the-river (or run-of-river; ROR) evokes an image of a low-impact installation; however, examination of eight case studies worldwide shows that substantial negative societal and ecological impacts are tied to them, albeit in somewhat different ways. We conclude that ROR dams not only potentially displace communities, disrupt livelihoods, and degrade environments in surrounding areas, but they also divert water from areas of need, impact aquatic ecology through habitat destruction and disruption of fish migrations, emit non-trivial amounts of greenhouse gases over the lifespan of the project, and disrupt streamflow in downstream river sections. While these negative impacts vary on a case-by-case basis, medium and large ROR dams consistently have multiple and cumulative impacts, even when not having appreciable reservoirs. We contend that many impactful dams do not qualify as low-impact ROR projects, despite being defined as such. Such mislabeling is facilitated in part by the ambiguous definition of the term, which risks the ROR concept being used by proponents of impactful structures to downplay their negative effects and thus mislead the public or gain status, including within the Clean Development Mechanism in relation to mitigating climate change.

Research and management of hydropeaked rivers largely overlook the ecological impacts of recurring flow fluctuations, such as fish stranding, on ecosystem health. This article synthesizes scientific and grey literature, field studies, and experiments to assess the effects of frequent hydropeaking on fish. Findings show that hydropeaking frequency significantly affects the ecological integrity of alpine rivers, with an average of three daily down-ramping events. Despite some evidence of behavioral adaptation of fish to recurrent flow fluctuations, this adaptation appears insufficient to counter the cumulative effect of a series of single hydropeaking events. Larval and juvenile fish are particularly vulnerable, with stranding impacts extending to the population and community levels. Effective mitigation should prioritize reducing the cumulative impact of recurring hydropeaks while ensuring single-event ramping rates and flow amplitudes remain within ecological limits. To effectively safeguard sensitive habitats, targeted mitigation efforts informed by an understanding of habitat dynamics are critical. Furthermore, maintaining lateral connectivity within river systems is essential for supporting resilient fish populations, especially where hydropeaking mitigation possibilities are limited. Finally, this study identifies future research directions on hydropeaking frequency and its ecological effects.

This chapter provides a holistic overview of the science and management of rapid artificial flow fluctuations caused by peak-operating hydropower. Using a process-based framework, this chapter illustrates the links between hydropeaking drivers and associated pressures on river ecosystem structure, function and integrity, and summarizes the role and effects of mitigation measures. In detail, this chapter highlights how hydropeaking causes abiotic (physical) alterations, which facilitate direct and indirect adverse ecological processes and ecosystem changes. As a response to these effects, different hydrological, morphological and emerging or complementary measures are presented and discussed from a river- and energy-grid-specific perspective and exemplified by case studies. Overall, this chapter provides a concise insight into this controversial topic, and presents possible solutions towards a more sustainable management of hydropeaked rivers.

Hydropeaking is known for its adverse impacts on river ecosystems. However, the implementation of mitigation measures is still largely pending due to conflicting priorities of ecology and economics, which require scenario building to assess trade-offs. Therefore, widely applicable and standardized tools are needed to analyze hydropeaking hydrology in affected rivers to expedite mitigation efforts. Here, we present a novel empirical approach-PeakTrace-that can (a) detect and follow source-specific hydropeaking waves in the downstream direction by using multiple hydrographs and (b) describe how to flow metrics of hydropeaking waves change along a river's course. In detail, PeakTrace first identifies associated flow events and then models translation and retention processes between neighboring hydrographs. Finally, the models can be combined to establish a non-linear hydropower plant-specific model. We demonstrate the PeakTrace method's usability in 16 Austrian case studies. The results underline the high performance of PeakTrace, describing the longitudinal development of flow metrics with high model accuracy up to 25 km or more. Ecologically-relevant metrics, such as rate of change or amplitude, decrease with distance from the hydropower outlet regarding down-ramping events; the same pattern can be observed for up-ramping events too, except for the rate of change for which an intensity increase may be observed, probably due to slope and the roughness difference between base flow and peak flow. Overall, this paper underlines the usability of PeakTrace as a basis to assess hydropower plant-specific hydro-ecological impacts and evaluate hydropeaking mitigation measures, especially by incorporating critical flow thresholds of river biota and life stages.

Rivers of the large Alpine valleys constitute iconic ecosystems that are highly threatened by multiple anthropogenic stressors. This stressor mix, however, makes it difficult to develop and refine conservation and restoration strategies. It is, therefore, urgent to acquire more detailed knowledge on the consequences and interactions of prevalent stressors on fish populations, in particular, on indicator species such as the European grayling Thymallus thymallus. Here, we conducted a multi-river, multi-stressor investigation to analyze the population status of grayling. Using explorative decision-tree approaches, we disentangled the main and interaction effects of four prevalent stressor groups: flow modification (i.e., hydropeaking), channelization, fragmentation, and water quality alteration. Moreover, using a modified variant of the bootstrapping method, pooled bootstrapping, we determined the optimal number of characteristics that adequately describe fish population status. In our dataset, hydropeaking had the strongest single effect on grayling populations. Grayling biomass at hydrological control sites was around eight times higher than at sites affected by hydropeaking. The primary parameters for predicting population status were downramping rate and peak amplitude, with critical ranges of 0.2-0.4 cm min-1 and 10-25 cm. In hydropeaking rivers, river morphology and connectivity were the preceding subordinated parameters. Repeating the procedure with pooled bootstrapping datasets strengthened the hypothesis that the identified parameters are most relevant in predicting grayling population status. Hence, hydropeaking mitigation based on ecological thresholds is key to protect and restore already threatened grayling populations. In hydropeaking rivers, high river network connectivity and heterogenous habitat features can dampen the adverse effects of pulsed-flow releases by offering shelter and habitats for all life cycle stages of fish. The presented approach of explorative tree analysis followed by post-hoc tests of identified effects, as well as the pooled bootstrapping method, offers a simple framework for researchers and managers to analyze multi-factorial datasets and draw solid management conclusions.

Peak-operating hydropower plants are usually the energy grid's backbone by providing flexible energy production. At the same time, hydropeaking operations are considered one of the most adverse impacts on rivers, whereby aquatic organisms and their life-history stages can be affected in many ways. Therefore, we propose specific seasonal regulations to protect ecologically sensitive life cycle stages. By reviewing hydropeaking literature, we establish a framework for hydrological mitigation based on life-history stages of salmonid fish and their relationship with key parameters of the hydrograph. During migration and spawning, flows should be kept relatively stable, and a flow cap should be implemented to prevent the dewatering of spawning grounds during intragravel life stages. While eggs may be comparably tolerant to dewatering, post-hatch stages are very vulnerable, which calls for minimizing or eliminating the duration of drawdown situations and providing adequate minimum flows. Especially emerging fry are extremely sensitive to flow fluctuations. As fish then grow in size, they become less vulnerable. Therefore, an emergence window', where stringent thresholds on ramping rates are enforced, is proposed. Furthermore, time of day, morphology, and temperature changes must be considered as they may interact with hydropeaking. We conclude that the presented mitigation framework can aid the environmental enhancement of hydropeaking rivers while maintaining flexible energy production.

FRANZ GREIMEL1, JÜRGEN NEUBARTH2, BERNHARD ZEIRINGER1, DANIEL S. HAYES1, MELANIE HASLAUER1, SIMON FÜHRER1, STEFAN AUER1, NORBERT HÖLLER3, CHRISTOPH HAUER4, PATRICK HOLZAPFEL4, MARTIN FUHRMANN4, MARKUS PFLEGER5, PETER MATT6, VERONIKA KOLLER-KREIMEL7, STEFAN SCHMUTZ1 1Institute of Hydrobiology and Aquatic Ecosystem Management, University of Natural Resources and Life Sciences, Vienna, Austria 2e3 consult GmbH, Innsbruck, Austria 3Center for IT Services, BOKU University, Vienna, Austria 4Christian Doppler Laboratory for Sediment Research and Management, Institute of Water Management, Hydrology and Hydraulic Engineering, University of Natural Resources and Life Sciences, Vienna, Austria 5VERBUND Hydropower GmbH, Vienna, Austria 6Vorarlberger Illwerke AG, Bregenz, Austria 7Austrian Ministry for Agriculture, Forestry, Environment and Water Management, Vienna, Austria

Hydropeaking has negative effects on aquatic biota, but the causal relationships have not been studied extensively, especially when hydropeaking occurs in combination with other environmental pressures. The available evidence comes mainly from case studies demonstrating river-specific effects of hydropeaking that result in modified microhabitat conditions and lead to declines in fish populations. We used multiple lines of evidence to attempt to strengthen the evidence base for models of ecological response to flow alteration from hydropeaking. First, we synthesized evidence of ecological responses from relevant studies published in the scientific literature. We found considerable evidence of the ecological effects of hydropeaking, but many causal pathways are poorly understood, and we found very little research on the interactive effects of hydropeaking and other pressures. As a 2nd line of evidence, we used results from analyses of large-scale data sets. These results demonstrated the extent to which hydropeaking occurs with other pressures, but did not elucidate individual or interactive effects further. Thus, the multiple lines of evidence complemented each other, but the main result was to identify knowledge gaps regarding hydropeaking and a consequent pressing need for novel approaches, new questions, and new ways of thinking that can fill them.

Hydropeaking is one of the main environmental impacts on running water ecosystems in Austria. Many affected rivers have a poor ecological status. Habitats for fish and invertebrates can be reduced, abundances can be diminished and organisms are affected by drift and stranding during the increase and decrease phases of peaks. To assess the ecological impact of hydro peaking, intensity thresholds according to different species and life stages can be applied. However, due to retention effects, the hydrological impact is not constant in a longitudinal view. A model to assess hydrological impacts considering distances to the power plant outlets is needed. For this purpose, we developed a model by detecting fluctuation intensities out of multiple hydrographs along the affected river reaches. This forms the basis for describing potential effects of hydro peaking by contrasting the varying hydrological impact intensity to threshold values of river organisms. Furthermore, mitigation scenarios can be evaluated, both ecologically and economically, by modelling modified hydro peaking intensities.