On the reintroduction of the endangered thick-shelled river mussel Unio crassus: The importance of the river's longitudinal profile.

•Freshwater mussels constitute one of the most endangered groups of organisms in the world.•The Biała River was studied with regard to existing and reintroduced populations of endangered mussel species.•Neither physicochemical water parameters nor fish hosts distribution correlated with the mussels distribution.•Hydromorphological variables were correlated with mussels' distribution, recruitment and success of the reintroduction.