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Sustainable development of hydropower demands a holistic view of potential impacts of water level regulation (WLR) on reservoir ecosystems. Most environmental studies of hydropower have focused on rivers, whereas environmental effects of hydropower operations on reservoirs are less well understood. Here, we synthesize knowledge on how WLR from hydropower affects alpine lake ecosystems and highlight the fundamental factors that shape the environmental impacts of WLR. Our analysis of these impacts ranges from abiotic conditions to lower trophic levels and ultimately to fish. We conclude that the environmental effects are complex and case-specific and thus considering the operational regime of WLR (i.e., amplitude, timing, frequency, and rate of change) as well as the reservoir's morphometry, geology and biotic community are prerequisites for any reliable predictions. Finally, we indicate promising avenues for future research and argue that recording and sharing of data, views and demands among different stakeholders, including operators, researchers and the public, is necessary for the sustainable development of hydropower in alpine lakes.

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Hydropeaking negatively affects fish assemblages, but knowledge gaps still constrain our ability to rank and mitigate the impacts of different hydropower operation regimes at particular power plants. This is especially relevant for species and rivers for which the effects of hydropeaking are less investigated, such as the Iberian cyprinids and Mediterranean rivers. Therefore, a recently developed hydropeaking tool to systematically assess hydropeaking impacts on salmonids has been adapted for Iberian cyprinids within the research project FIThydro. The general tool framework developed for the salmonids was kept for the cyprinids, with the combined use of factors describing the physical effects and indicators of fish vulnerability to assess hydropeaking impact. An initial set of effect and vulnerability factors was developed for Iberian cyprinids. In addition, preliminary thresholds were established for each indicator to account for different levels of impact of hydropeaking on the focus taxa. The proposed factors/indicators and thresholds were critically reviewed by experts on Iberian cyprinids ecology and Mediterranean rivers functioning, and a final set of effect and vulnerability factors was established. The final factors retained most of the ones proposed for salmon, but included new ones, particularly for vulnerability.

The production of electricity from hydropower is exclusively determined by the availability of water. Upstream water use such as irrigation and drinking water supply, downstream constraints and climate change are just some of the factors that can pose a risk to the hydropower producer. The relationships between these factors can in many river basins be very complex, introducing large uncertainties to future revenues. Tools to analyze the wider understanding of the hydrological risks in river basins with multiple and geographically distributed water uses have to a limited extent been applied in the long-term planning of hydropower projects. The use of such tools will reduce the financial risk of a project, as well as providing a basis for a dialogue between stakeholders. We have reviewed a set of different tools/approaches to assess the hydrological risks of hydropower projects, which include; i) hydrological models with functions to run scenarios with climate change and different allocation and priorities between sectors, ii) integration of model simulation and expert judgement using Bayesian network methodology and iii) other risk assessment approaches, including the decision-tree framework, as proposed by the World Bank.