On artificially confined floodplains, various active processes influence the local peak flow level. However, these active processes are often neglected in flood management because calculations are based on static parameters. The aim of this study is to analyse the processes that contribute to rising local flood levels in a regulated channel and on an artificially confined floodplain of a lowland river. Our goals were to evaluate the role of cross-sectional channel changes, overbank floodplain aggradation, and riparian land-cover changes on local flood level increases since the late nineteenth century and early twentieth century and to assess the height changes of artificial levees. The research was performed in a low-gradient river (Lower Tisza, Hungary), which was regulated in the late nineteenth century; thus, the effects of century-long processes on flood levels could be evaluated. The results suggest that along the 92 km-long reach, the channel was narrowed by 9% and its cross-sectional area decreased by an average of 2%; however, the narrowing was over 30% in some locations. Because of these changes, flood levels increased by an average of 13 cm (maximum = 134 cm) since 1931. Because the artificial levee was constructed in the mid-nineteenth century, the mean overbank floodplain accumulation reached 1.2 m (maximum = 2.6 m), flood levels decreased by an average of 112 cm. During the same period, the land cover of the floodplain changed considerably, increasing the vegetation roughness (i.e., Manning's n) from 0.048 to 0.11. Based on our modelled data, the higher vegetation roughness increased flood levels by 42 cm (Scenario A) or 139 cm (Scenario B), on average, based on the increased vegetation roughness (by 10% or 30%, respectively). By overlapping these data, the results showed that since the river regulation work in the late nineteenth century, the actual flood level increased by an average of 175 cm (maximum = 350 cm) in the case of Scenario A and 272 cm (maximum = 443 cm) in the case of Scenario B. The latter is more consistent with the actual flood stage measurements. As these processes are still active, further increases in the flood level could be expected. In addition, the height of artificial levees decreased by an average of 23 cm (maximum = 75 cm); thus, some levee sections became more susceptible to overtopping during record high floods, especially along the eastern levee. Based on this approach, local hydrological managers can identify the processes that contribute more to peak flow level increases at a given location and determine the correct actions at the correct locations, which could lead to decreases in peak flow levels.