| Summary: | Abstract. The lowermost 45 kms of River Kalajoki in northern Finland experiences fluctuations in flow rate due to hydropeaking practise of Hamari hydropower plant (Hamari HPP). This has decreased good ecological status in the lower part of Kalajoki and affecting fish habitat condition. The main objective of this study is to investigate the effect of hydropeaking of hydropower power plants on fish habitat through field measurements and modeling, and suggest possible mitigation measures. The study specifically aims to (1) evaluate the level of current hydropeaking practice on lower part of Kalajoki, (2) investigate with 1D HEC-RAS model the extent to which hydropeaking practice affect the fluctuations in water surface elevation (WSE) downstream of the Hamari HPP, (3) investigate the impact of current hydropeaking regulatory practice on the morphological structure of two rapids on the Kalajoki namely; Juurikoski (located just below the Hamari HPP in Ylivieska) and Hihnalankoski (located about 32 km below Hamari HPP in Tynkä), and (4) investigate the with 2D fish habitat modeling. The effect of hydropeaking practice on the quality and quantity of fish habitat and possibilities for mitigation. In general, it was found that significant hydropeaking induced WSE fluctuations could impact all the way to the mouth of the Kalaojoki depending on the magnitude of discharges from the Hamari HPP. The state of hydropeaking below Hamari HPP was found to be ‘high impact’ and therefore needs improvement in the ecohydraulic state of the river. The current hydropeaking practise had a more negative effect on the quantity and quality of brown trout habitat at Juurikoski than Hihnalankoski partly due to the nearness of Juurikoski to the Hamari HPP and also partly due to its poor river construction compared to Hihnalankoski. This study explored the morphological restoration of Juurikoski with Hihnalankoski river structure and found a significant increase in fish habitat quantity in terms of weighted usable area (WUA) in excess of 200 % ...
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