Hydropeaking is a widely applied management practice in the generation of hydropower. When the demand in electricity is high, the operator of a hydropower plant rapidly increases released discharges to meet this demand. Vice versa, when the demand is low, no or less water is released. However, while river regulation practices offer valuable resources, they can also inflict adverse environmental consequences on downstream river segments. More specifically, hydropeaking introduces high sub-daily variance in downstream reaches of the hydropower plant. This large sub-daily variance is harmful to the river regime and ecosystems. In Finland, hydropeaking has the highest negative environmental impact of all river regulation practices. The most prominent, known negative impacts due to high flow variations are a direct impact on aquatic biota, such as trout, and a compromised recreational use of the river corridor. The main objective of the thesis research is to investigate the impact of hydropeaking on rapids in a complex riverine system. More precisely, a system that includes multiple subsequent weirs, vegetation, large riverine boulders, a rather flat topography and a small-scale anabranch. Additionally, related to the modelling approach, two state-of-the-art calibration methods and their benefits and limitations are discussed. The formulation of operational and morphological mitigation measures to counter impacts is the secondary objective. A case study for the downstream region of Hamari Hydropower Plant at Ylivieska (Finland), the site Juurikoski, provides insight on the different aspects related to these queries. During a series of simulation runs, including the original situation and three hypothetical mitigation scenarios, the impact of hydropeaking is quantified and studied. Results include post-processed water level, velocity and shear stress data. Furthermore, conclusions and a discussion are given regarding the potential of calibration and validation for a 2D hydrodynamical model by state-of-the-art methods. These methods refer to a hot spot analysis comparison based on LSPIV-data and velocity profile comparisons based on ADCP-data. On a local scale, mitigation recommendations are suggested or excluded from a practical point of view. From a more global perspective, the report provides a modelling approach to tailor mitigation measures according to riverine lay-out, despite limited bathymetry data and increased riverine complexity.