# Estimation of Changes in Sediment Transport along the Free-Flowing Middle Danube River Reach
## 5 Discussion  
The minima and means of water levels are inhomogeneous along the entire reach except at Backa Palanka and Novi Sad, where the Danube approaches and enters the Iron Gate Reservoir and where the effects of the backwaters are or could be felt (Table 1). The non-homogeneity of water levels results from their strong relationship with riverbed changes, which are the consequence of intensive river training work along this stretch of the river. On the other hand, the yearly maxima are homogeneous, as the shape of the main-channel riverbed at high flows merely influences the water level, and it is supposed that the riverbed incision mainly affects the riverbed of the main channel and not the floodplains. Unlike water level data, the yearly characteristic discharges at the investigated gauging stations are homogeneous for all three characteristics--minima, means, and maxima. The homogeneity of the discharge data (Figure 3d-f) can be explained by the fact that the shape of the riverbed does not affect water quantity, which arrives from upstream, and the fact that there are no large tributaries downstream of Szob (rkm 1700). The trend analysis of a 70-year long time series for the annual characteristic water level values of the entire free-flowing Middle Danube reach [5] confirmed the decreasing  
Figure 11: Twenty-five-year long-term changes in sediment transport with sediment deposition and erosion patterns. Figure 10: Longitudinal profiles of the main-channel cross-sectional area in 1996/1997 and 2021/2022. t trends in all three characteristics (minima, means, and maxima) found in [4; 17] along the Hungarian stretch until 1992 and 2005, respectively. Inspecting the variation in the rates of change of \(Z_{\text{min}}\) and \(Z_{\text{av}}\) along the studied reach reveals that rates along the gravel bed reach are up to three times larger than those along the sand bed reach. Between Mohacs and Bogojevo, the \(\Delta Z/\Delta t\) is almost the same for \(Z_{\text{min}}\), with an average value of 1.1 cm/year. Downstream of the Drava River confluence (in Backa Palanka) and at the entrance to the Iron Gate I reservoir (in Novi Sad), the decreasing rate rapidly reduces because of the deposition of sediments brought by the Drava River and the backwater effects of the Iron Gate I Dam. Moreover, under the influence of Iron Gate I backwaters, the incision terminates and changes into aggradation, an usual physical process where a river enters a reservoir, lake, or sea. Thus, this is a positive answer to the first research question. The second research question about the correlation between the observed minimum water levels and the recorded incision of the riverbed resulted from the fact that the water level decrease is usually a consequence of a channel incision. The riverbed incision is a continuous process in rivers with numerous cut-offs. Rectification of the river course by cutting-off meanders and disconnection of narrower side channels along anabranching river reaches was a river engineering technique applied in the 19 th century aiming at facilitating navigation, flood control and agricultural production along European rivers that played a role in demographic, economic, cultural and urban development, such as the Rhine and Danube Rivers. Dredging and groyne construction might amplify the incision process, both of which happened in the second half of the 20 th century along the Middle Danube. The comparison between the longitudinal profiles of the sand bed free-flowing Middle Danube River reach in 1996/1997 and 2021/2022 (Figure 12) shows that the riverbed incision continued after 2013 when a report on long-term morphological development was published [21] (this was approximately 110 years after the start of cutting-off work on the Middle Danube). Long-term analysis of longitudinal riverbed profiles of the Upper Rhine River has shown, at one location, a drop in thalweg as large as 7 m 120 years after [PERSON]'s work [22].