Already within 10-20 years, the average temperature risks exceeding the 1.5 degree target limit and we can expect more extreme weather in the form of increased precipitation. Hydropower plants hinder the possibility of freshwater species to migrate between for example their spawning grounds and rearing habitat. This effect is a contributing factor to the fact that populations of migrating freshwater species have decreased by 93% in Europe. The purpose of this study is to investigate overwinter growth and survival of juvenile brown trout and Atlantic salmon in a hydropeaking river which is affected by a future climate change. In this study, juvenile brown trout and Atlantic salmon in the river Gullspång were investigated using simulations in an agent-based model (inSTREAM 7-SD). An existing model parameterized in another study was used, and I manipulated two parameters: water temperature and flow, to investigate their effects on salmonid growth and survival. Increased flow did not have a statistically significant effect on the growth or survival of juvenile salmonids during the winter, but an increased temperature had a positive effect on growth. An elevated temperature also had a negative effect on the survival of 0+ salmonids. The salmonid populations in river Gullspång may be negatively affected by future climate change and the hydropower operation may need to be adapted to preserve the endangered Gullspång salmon and Gullspång trout.