The present study considers long-term morphologic development of the inlets forming the Western Dutch Wadden Sea. This concerns analysis of field measurements carried out since closure of the Zuider Sea (1932) as well as an assessment of possible imporvements of long-term morphological prodictions made for this area using ASMITA. From field measurements basin characteristics can be derived. In former studies this was done using fixed water levels and fixed basin boundaries. Comparison with characteristics determined using actual water levels and/or actual boundaries leads to the conclusion that both the movement of the basin boundaries and the variation of water levels have a significant influence on observed trends in the development of basin characteristics. Basin boundaries in this study are defined as lines between basins, where deviation of current velocities is minimal. Current velocities are determined using a 2DH tidal flow model. A shift of this hydrodynamic boundary between Marsdiep and Vlie basins towards the east can be observed in the period considered. The considered adjustments to ASMITA deal with adaptation of the element surface areas in time, reassessment of the equilibrium coefficients, sediment exchange over boundaries between different basins and movement of these basin boundaries. Regarding sediment exchange over the boundaries it is concluded that the present formulation to express sediment exchange between elements does not model the driving forces of sediment movement over the boundaries correctly. Prescribing the movement of basin boundaries during ASMITA simulations leads to the conclusion that in addition to a direct change of basin characteristics due to the movement of these boundaries also the changes of the tidal prism have a significant effect on calculated developments of element volumes and areas.