This paper investigates the effect of density differences on the forces acting on a moored vessel during lock operations, focusing on the effect of the position of the moored vessel in the lock chamber in the presence of density currents. The extensive scale-model research performed for the new sea lock in IJmuiden, The Netherlands has shown that the combination of density differences and an asymmetric layout of the moored vessel in the lock chamber may lead to high forces on the vessel that can largely exceed the allowable force limit. In particular, as a result of the density currents, forces in the transverse direction build up, pushing the vessel away from the chamber wall, during leveling and after opening the lock gate, leading to higher loads on the mooring lines. The forces caused by the density difference are the dominant forces and the performance of the leveling system cannot be assessed without taking this into account. Based on the results of the performed tests, criteria for achievable leveling times and allowable hydrodynamic forces during leveling are determined for the new sea lock of IJmuiden. Furthermore, the obtained results can be used for calibration or validation of numerical models that are valuable tools for the design of future locks.