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The morphodynamics of alluvial rivers is controlled by the mobilization of bed material. However, the details of mobilization of mixed‐texture bed materials at low flows, increasingly common due to climate change, are still unclear. The 161‐km‐long Hungarian alluvial reach of the Drava River, downstream of sections where flow characteristics have been heavily modified by human interference, was investigated in 2019. A monitoring campaign at cross‐sections, on average 5.55 km apart, was launched to study channel morphology, bedload entrainment dynamics with regard to texture. For the survey, a sonar, an ADCP and a Helley–Smith bedload sampler mounted on a double‐hull vessel was used. Our research pointed out an abrupt fining between river kms (hereafter: rkm) 175 and 170 (distance from the mouth), probably due to reduced armouring. The d60 fraction was found to be finer than in 2003 and 2012 for the upstream stations of Botovo and Bélavár, and showed a good correspondence with the records of the Barcs and Drávaszabolcs stations. Temporal fining and higher entrainment rate are due to (a) changing climate of the catchment, that is, diminishing flow between the monitoring dates (2003, 2012 and 2019); (b) reduced armouring, (c) variability of cross‐sectional position of sampling points and (d) the different mesh size of the bedload samplers employed. Calculations of shear velocity, Reynolds and Shields numbers indicate more dynamic sediment motion than observed by previous studies. Our reach‐scale results may be relevant for the alluvial sections of other alpine and subalpine, partially channelized rivers of similar size, flow dynamics and mixed bedload. [ABSTRACT FROM AUTHOR]

Sonar survey of shallow water bodies has challenged scientists for a long time. Although these water courses are small, still they have an increasing ecological, touristic and economical role. As maritime sonars are non-ideal tools for shallow waters, the bathymetric survey of these rivers has been taken with cross-sectional methods. Due to recent developments, interferometric surveying technology have also burst into the market of recreational-grade fish-finders. The objective of the current study was the development of a novel, complex and integrated surveying technique which is affordable, robust and applicable even at low water levels. A recreational-grade sonar system was assembled and mounted on a double-hull vessel and connected with a geodetic Global Navigation Satellite System (GNSS) device. We have developed a novel software which enables the bridging between a closed sonar file format and the commonly used Geographic Information System (GIS) datasets. As a result, the several month-long conventional bathymetric survey of the 146 km-long reach of the Drava River was reduced to 20 days and provided channel bathymetry of many orders of magnitude higher than the classical methods. Additionally, a large number of spatial derivatives were generated which enables the analysis of channel morphology, textural variation of channel sediments and the accurate delineation of navigational routes. [ABSTRACT FROM AUTHOR]