Microplastic particles are found globally in all kinds of water bodies posing a serious threat to aquaticorganisms and ecosystems. However, plastic concentrations are not homogeneously distributed and can decreasealong a river course. Thus, potential sinks need to exist. Dams and reservoirs are proposed to be such potentialsinks for microplastics. We investigated plastic concentrations at the water surface upstream and downstream of areservoir inundated by a wastewater treatment plant effluent by filtering water with a net with 200 μm mesh size.We sampled at flow velocities ranging from 6 cm s–1 to 35 cm s–1. Upstream of the reservoir significantly higherconcentrations of microplastics (19.9 ± 7.3 particles per m– 3) than downstream (3.4 ± 2.2 particles per m– 3) wereidentified. Furthermore, the plastic concentrations increased with increasing flow velocities. Hence, reservoirs canbe sinks of microplastics as decreased flow velocities may lead to an increased sedimentation or decreased erosionpotential. Consequently, flow velocities are critical when identifying plastic loads. Single time or single spot measurementsmay not well represent plastic loads if flow velocities vary considerably in time or space.