The presence of small weirs, far more numerous than dams, has increased habitat fragmentation on rivers worldwide. This study aims to evaluate the upstream passage performance of a potamodromous cyprinid, the Iberian barbel (Luciobarbus bocagei), over an experimental broad-crested weir by varying key-hydraulic parameters. Fish passage success was studied for different combinations of waterfall height (∆h), plunge pool depth (D) and flow discharge (Q). The flow pattern downstream of the weir was characterized with a 3D Acoustic Doppler Velocimeter, to assess the effects of hydrodynamics on fish behaviour. Results showed that D, ∆h, and their