Gravity currents are flows driven by buoyancy differences between two contacting fluids caused by differences in temperature, salinity, or by the presence of suspended particles. Such flows can reach high velocities near the bed, especially on the area behind the front of the current. As a result, rapid morphological changes may take place in river and estuarine beds due to the passage of these flows. Essential to determine the erosion induced by the current, are the spatial and temporal distributions of the bed shear stress. However, these are troublesome to measure in laboratory or in the field. To bridge this difficulty, the eddy-solving numerical simulations may be used. This study presents here the three-dimensional numerical simulations of lock-exchange salinity currents flowing over a mobile bed. It is aimed at the characterization of the sediment entrainment capacity of the current. The large eddy simulation technique is employed for analyzing the evolution and the structure of the current. For the sediment simulation, an Euler-Euler methodology based on a single phase approach is used. The main features of the current are compared with experimental data obtained in the laboratory. Velocity fields and bed shear stress distributions for different initial current densities are analyzed and linked to entrainment scenarios. The influence of small variations in particle size of the mobile bed is also discussed.