Hydropower is one of the main pillars of the industrial transition and a more sustainable future. A global reinforcement of renewable energy sources is required to face climate change and meet international goals and agreements. The phase-out of fossil fuels in the heavy industry and other sectors will result in a largely increased electricity demand. Among the intermittent renewable sources, hydropower can act as a regulation capacity, besides other energy storage solutions. In the deregulated electricity market hydropower reservoirs can be flexibly used to match production and demand. In the Nordic Countries, where hydropower provides 56 % of the electricity, this can already be seen on a daily basis.

While hydropower can be seen as a low-emission and relatively clean energy source it nevertheless has adverse effects on the ecological and social river system. The effects can be related to different phases of a hydropower project and different parts of the river system. The construction of the dam, filling of the reservoir, operation of the hydropower plant, and decommissioning all have various effects on the environment around, upstream and downstream of the dam and reservoir as well as the communities close to the river. 

The indicated increased flexibility in hydropower production leads to increasing and new pressures on the river system. The conflicting interests of renewable energy production and environmental protection are reflected in legislation and goals on different levels of governance. On the one hand, the EU biodiversity and water framework directives advocate the maintenance and restoration of healthy surface water bodies and the protection of ecological species. On the other hand, the EU renewable energy directive stresses the necessity of clean energy production. In Sweden, a national process for relicensing of the existing hydropower plants and assessment of their ecological impact was decided in 2020. The evaluation of effects and assessment of potential measures requires knowledge and a trade-off between energy production and environmental goals. 

This work aims to contribute to a more funded decision-making on sustainable hydropower. For this purpose, the ecological, social, and economic impacts of hydropower are investigated. The future need for more flexible energy production will further alter the natural flow of rivers. Therefore, the research focuses on the effects of river regulation and concentrates on the reaches downstream of hydropower plants. The possibilities of hydraulic models as predictive tools in impact assessment are investigated.

In a first step, the effects of river regulation on the downstream river system and the possibility of evaluating these effects with hydraulic models are investigated. The literature reveals numerous effects of flow alteration on the river ecosystem. Furthermore, socio-economic effects could be observed as a direct implication of the flow alteration or a consequence of ecological effects. Combining the evaluation of ecological effects with hydraulic modeling is state-of-the-art. However, a majority of ecohydraulic studies concentrate on fish habitat and only a limited number of hydraulic studies include social or economic effects. This reveals a necessity for further research on socio-hydraulic modeling, more diverse ecohydraulic studies, and the comparability of ecological and socio-economic interests to develop holistic evaluation approaches and inform founded trade-offs and decision-making.

In a second step, a restoration project was hydraulically analyzed as a case study. In 2021 the minimum flow into the 70-km-long bypassed reach of river Juktån (Västerbotten, Sweden) was seasonally adapted. The former discharge was mostly changing between constant winter and summer discharges. The new discharge scheme is imitating natural conditions with a significant spring peak while maintaining the annual discharge volume of 12 % of the natural flow. Additionally, morphological measures were taken. Both measures were intended to enhance and restore the state of the river ecosystem. The hydraulic effects of the measures were investigated with hydraulic 2d-simulations. The seasonal flow adaption led to more diverse hydraulic conditions with an increased water level variation and a larger maximum inundation area. Flow velocities relatively doubled for the highest discharge case but remained in a low range. These more varying conditions will benefit the ecosystem and especially the riparian components. The results show that seasonal flow alteration can be a useful measure to improve the hydraulic conditions in bypassed reaches at low costs. The morphological changes in a different river stretch were less effective according to the hydraulic models. 

In a further step, the effect of the seasonal flow restoration on the potential riparian vegetation in river Juktån was analyzed. The riparian ecosystem is seen as an important connection between terrestrial and aquatic ecosystems with naturally high biodiversity. Based on the results from the hydraulic simulations and ecological data on the inundation tolerance of riparian vegetation the states before and after restoration were compared. As a reference, the potential vegetation for larger annual discharge volumes up to natural flow was modeled. The results show that the seasonal flow alteration will diversify the riparian vegetation and strongly increase the extent of the riparian zone. When increasing the total annual discharge volume into the bypassed reach towards a natural flow scenario the total riparian vegetation area will grow steadily. The certain vegetation zones behave indeed diverse. While the willow shrubs region reaches its maximum extent at full natural flow, the graminoid zone and riparian forest peak at 50 % respectively 70 % of the natural flow.