Investigations of the recent decades have revealed a significant lowering of the river bed of the Danube in Hungary. This phenomenon can be observed on a long stretch of the Hungarian Danube and it can be presumed that similar changes occur on other parts of the river beyond the borders. There are several methods to reveal and to measure the lowering of the channel. Long records of the water level are available for the trend- and homogeneity analyses. Results show that erosion processes started even at the beginning of the 20^th century. In recent years results of bathymetry are available for the more detailed investigations of these changes. The main causes of the bed-lowering are river regulation activities, industrial dredging and decreasing of natural sediment load. Deepening of the river bed has unfavourable effects on the natural environment, on the navigation and also on the operation of man-made structures in the river. To avert these injurious effects a complex solution is needed, taking into consideration not just economical but ecological and social aspects as well.