Hydropower is an energy source that is currently considered to be both climate- and environmentally friendly. It is utilized on both large and small scales and has a low carbon footprint, which has led to increased attention in recent years. The growing influence of renewable energy sources such as solar and wind power has also brought the regulatory capabilities of hydropower within electrical grids into focus. Despite the significant climate benefits of hydropower, there remain substantial challenges in adapting it to minimize environmental impacts. The latest EU directive has highlighted how fish and other aquatic organisms are harmed by limitations on upstream and downstream migration caused by hydropower plants.

Given the importance of maximizing hydropower electricity production while minimizing environmental impacts, there is a need for more knowledge regarding how mitigation measures for aquatic organisms can be implemented. By combining knowledge of water flow patterns in hydropower areas with insights into the behavior of fish species in these environments, it is possible to facilitate migration both upstream and downstream while ensuring optimal use of hydropower.

The first section of this thesis focuses on downstream-migrating salmon smolts. In this study, telemetry data from tagged smolts were analyzed alongside a 3D flow model of an area in northern Sweden where a large hydropower plant affects one of the largest rivers for salmon reproduction. The study shows that the smolts follow the main channel and are influenced by the flow rate. Higher flow velocities tend to cause the smolts to concentrate more in the main channel, whereas lower flows result in a broader distribution across the riverbed. The smolts are also partially influenced by the boom installed to direct the flow towards the fish pass. By studying telemetry tracks and CFD in detail, one objective has been to develop a method for integrating these two types of data, thereby creating a behavioral model of how smolt navigate at different flow velocities.

The second work section addresses how climate change may impact flow events in a dammed river in northern Sweden. With anticipated climate change, more significant variation in precipitation is expected to affect the northern hemisphere, resulting in altered flow conditions. By studying historical data, an extreme flow event was identified and modeled using a 2D model. Flow variations were analyzed in relation to the preferred flow conditions for grayling. The study demonstrates that grayling are sensitive to large flow variations in the area. The most significant impact occurs when an extreme flow coincides with their spawning period. As the area is heavily regulated with hydropower plants placed closely together, downstream plants also have an impact on water levels upstream. The area is particularly sensitive if water levels are rapidly reduced, as this can lead to stranding. Since hydropeaking is a factor that affects the aquatic environment and is expected to become more pronounced in the future, it has also been studied in greater detail. By modeling how the river reach is influenced by different dewatering periods, it has been possible to identify specific types of zero-flow events that are particularly critical. Based on these findings, various mitigation measures have been proposed to guide hydropower operators in planning peak generation to reduce environmental impacts while accounting for potential energy production losses.

This thesis combines various methods to investigate flow and flow variations, utilizing both detailed 3D models and broader 2D models. It demonstrates the potential for flow models to interact with ecological studies to deepen our understanding of the ecological state of rivers affected by hydropower development.