# Estimation of Changes in Sediment Transport along the Free-Flowing Middle Danube River Reach
## 5 Discussion
Thus, a recorded maximum thalweg drop of approximately 6 m in a bend upstream of the Drava River confluence (Figure 12) agrees with observations in other rectified rivers. In addition to actual longitudinal riverbed profiles, large-scale bed slopes at the beginning and end of the considered time interval are drawn in Figure 12. These readily demonstrate that the large-scale bed slope of the study reach is decreasing, i.e., that the river channel is still responding to changes from 150 and 60 years ago. The average bed slope reduced from 0.094% to 0.083%. To the best of the authors' knowledge, there is no information on the correlation between the minimum water level and the incision of the channel at GSs on the Danube River. Figure 12: Actual and averaged longitudinal profiles of the Danube River with corresponding water surface profiles in 1996/1997 and 2021/2022. In this study, a linear relationship between the \(Z_{\text{min}}\) and the corresponding percentage of the cross-sectional area increase due to a channel incision after the reference year of observation was found at three out of four considered gauging stations. The exception is the Apatin GS, where the channel dynamically changes with the growth and washing of a large sand bar. This location is one of the critical sectors for navigation along the Serbian/Croatian border reach. The rates of \(Z_{\text{min}}\) lowering decrease in the downstream direction, which is consistent with the results of the trend analysis in Figures 5 and 6 and in [6]. Since the Danube River channel on the investigated reach was predominantly of the meandering type in the 19 th century, it was rectified with numerous cut-offs (Figure 4.1.12 in [21] (p. 42)). Only on the Hungarian side were there more than 20 cut-offs between Dunafoldvar and the Hungarian-Serbian border [23]. Thus, an alternating degradation and aggradation pattern could be expected. This pattern is further amplified by the construction of river-training structures starting in 1876 and dredging in the second half of the 20 th century. The answer to the third research question, i.e., what are the long-term sediment transport characteristics along the investigated reach, is sought through the indirect estimation of the sediment transport rate between Baja and the Drava River confluence, as there are no large tributaries along the reach that could affect riverbed morphology. Both figures--Figure 10 (with the raw data) and Figure 11 (with the results of the sediment transport estimation)--show an expected pattern. The stretch between Baja and a location some distance upstream of Mohacs agradates (the main-channel cross-sectional area reduces--Figure 10 and positive values of \(Q_{s}\) in Figure 11, which are more than four times greater than the negative ones along the reach exposed to degradation). It is expected since it is located downstream of a stretch with numerous cut-offs and intensive dredging in the second half of the 20 th century (the area close to Dunafoldvar). The short river reach around Baja can be considered stable (Figures 7a and 8a), possibly owing to the very wide floodplain (\(\sim\)6 km), where large discharges with large velocities (high energy level) can spread. This area is known as Gemenc from \(\sim\)1503 to \(\sim\)1469 rkm. Downstream of Gemenc, the river channel is narrow until it reaches Mohacs. Close to Mohacs, a reach with numerous cut-offs and river-training structures built to aid navigation, it starts again (see the map on the right in Figure 4.1.12 in [21] (p. 42)). As such, it continually degrades with a mean 25-year long-term sediment transport of approximately 10 t/year. The most intensive washing of the riverbed material during the analyzed period (around 50 t/year) is throughout the length of 5 km downstream of Bezdan (even the translation of the name of this small town--bottomless--suggests that there is a permanent deepening of the riverbed at this location). This is logical, as Bezdan is at the upstream end of the 25 km rectified river channel.