This is a numerical modelling study on the impact of dredging and disposal activities on the long-term morphological development of the Columbia River mouth (MCR). Dredging activities removed 120 Mm3 of sand from the MCR during the studied interval between 1958 and 1999. Only one-third of this volume was disposed in the active littoral zone. Therefore, dredging activities might be an important factor for recent erosion trend at the coasts directly adjacent to the MCR. The net influence of both dredging and disposal activities is analyzed by making use of a Delft3D model. Morphological acceleration techniques and wave climate reduction are used to bridge the gap between hydrodynamic and morphological time scales. The model performance is assessed with simulations for the 1926-1958 interval. These simulations indicate that the model captures the large-scale morphological changes that occurred in the study area. Simulations for the 1958-1999 interval are then used for the analysis of the impact of dredging and disposal activities. Based on the model results, dredging activities led to significant volume losses in the inlet itself and in the area west of the river entrance. On the Peacock Spit shoal directly north of the inlet, the volume losses due to dredging are almost completely reversed by disposal of dredged material on the shoal itself. Dredging and disposal activities did not have significant effects for the littoral drift at coastal stretches adjacent to the MCR. In the studied period, the Peacock Spit shoal did absorb the negative effects of dredging and the additional supply due to disposal activities, without reducing its sediment supply function to the adjacent coastal cell. Other effects of dredging and disposal activities were the formation of distinct sand mounds at the locations of some disposal sites and maintaining the southern orientation of the MCR channel at the estuary side of the inlet. Main implication of the model results for disposal strategies is that the sand volume of the Peacock Spit shoal should be maintained to avoid further erosion of the shoal and prevent undermining of the North Jetty and to maintain the sediment supply function of the shoal for the updrift coastal cell.