Dam reservoirs may play an important role in threatened freshwater mussel conservation. However,the restoration works on a dam and their consequences can threaten whole mussel populations.This study investigates the consequences of water release in the Czorsztyn Reservoir in order toimprove management of dam reservoirs. The analysis shows that the reservoir s water releasescaused massive death of freshwater mussels. The main threat for mussel survival was probably flatslopes that disturbed both their ability to perceive slope gradient and the possibility to escape. Inorder to decrease the losses of freshwater mussel populations, it is necessary to remodel a part ofthe reservoir bowl s shore and to create a refugium for the mussels. If terracing across the slopes isnecessary, the steps should not be entirely plain, but tilted towards the water.