The presence of small man-made barriers like small weirs, far more numerous than dams, have altered the river systems and negatively affected fish communities, mainly by disturbing fish movements. However, contrarily to the vast literature on dams, much less is known about the effects of small weirs on fish movements. This study aims to evaluate the upstream passage performance of a potamodromous cyprinid, the Iberian barbel (Luciobarbus bocagei) when encountering small weirs. According to literature, plunge pool depth and weir height are key variables in determining the success of upstream passage by fish. Thus, by combining different plunge pool depths (D = 10, 20, 30, 50 cm) with different waterfall heights (Δh = 5, 10, 15, 25 cm; distance from the plunge pool surface to the top of the weir crest) 16 experiments were carried out in an indoor flume. Flow patterns downstream of the weir, characterized with a 3D Acoustic Doppler Velocimeter, were used to understand weir negotiation behaviour of fish. Results demonstrated that both variables and their interaction term (D x Δh) were significantly correlated with the number of successful fish passages (PerMANOVA, p< 0.01). The highest number of passages (n=50) occurred for the experiment with D20Δh10, and the lowest (n = 1) for D10Δh25. Contrarily to what was expected, increased passage did not occur at higher plunge pool depths in association with lower waterfall height, demonstrating that passage success is a complex phenomenon where both variables interact to set the most effective hydraulic conditions for fish.