# Estimation of Changes in Sediment Transport along the Free-Flowing Middle Danube River Reach
## 6 Conclusions  
A comprehensive hydrological-morphological study of the 300 km long free-flowing Middle Danube River reach between rkm 1581 (Dunaiyairos), Hungary, and rkm 1255 (Novi Sad), Serbia, led to the following conclusions:  
1. The data series of minimum and average water levels are inhomogeneous and show a continuous decreasing trend. They are both the consequence of the permanent riverbed changes initiated by the rectification of the meandering river channel in the 19 th century, and they amplified with the consequent construction of groynes and longitudinal stone toes since 1876 and intensive dredging after the 1950s.
2. The rate and type of cross-sectional area change (an increase or a decrease) depend on the location of the cross-section along the river course. The slowest decrease is downstream of reaches with cut-offs and intensive dredging. More intensive changes in the cross-sectional area are at the upstream ends of rectified main-channel reaches.
3. The correlation between low water levels and the percentage increase in the main channel cross-sectional area at official gauging stations in the last 50 years is linear. Thus, the hypothesis about the continuous lowering of water levels caused by the incision of the main channel is correct. The large-scale longitudinal riverbed slope decreased in the last 25 years by 0.01%. A maximum water level decrease of approximately 1 m is on the upstream end of the study reach, and it reduces when approaching the Drava River confluence.
4. In the absence of systematic sediment transport monitoring, regularly surveyed cross-sections of the navigable river can be used to estimate the long-term averaged sediment transport, i.e., to infer the average rates of aggradation and degradation of the channel along the reach.
5. Along the sand bed reach of the free-flowing Middle Danube with no large tributaries, the maximum value of the positive sediment transport in the last 25 years is approximately 30% greater than the maximum absolute value of the negative transport.
6. The maximum absolute value of the negative transport is near Bezdan, at the upstream end of another rectified river sub-reach. The sediment transport ceases toward the confluence of the Drava River.
7. In 86% of the study reach, the long-term sediment transport for the studied period is negative. Thus, sediment balance changes also prove that decreasing water levels are a good indicator of an ongoing severe riverbed erosion process.  
Conceptualization, D.D. and E.A.T.; methodology, E.A.T., D.D. and B.K.; investigation, D.D., E.A.T., L.M., C.A., A.V. and B.K.; resources, D.D., E.A.T., L.M., C.A., A.V. and B.K.; data curation, A.V., C.A., L.M., E.A.T. and D.D.; writing--original draft preparation, D.D. and E.A.T.; writing--review and editing, E.A.T. and D.D.; visualization, D.D., E.A.T., L.M., C.A., A.V. and B.K.; supervision, D.D., E.A.T., L.M., C.A., A.V. and B.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability StatementData used for this research can be requested via email from the authors.  
The authors are grateful for the provision of data from the official institutions of both countries: the Hungarian Hydrological Forecasting Service; the Directorate for Inland Waterways; the Ministry of Construction, Transport, and Infrastructure--Plovput in Serbia; and the Lower-Danube-Valley Water Directorate (LDVWD) in Hungary.  
The authors declare no conflict of interest.