This study examines downstream grain-size trends in gravel bars, a typical feature of natural gravel-bed rivers, from two neighbouring heavily channelized rivers: the Lubina River and the Ondřejnice River draining the Czech part of the Outer Western Carpathians. It aims to examine the effects of the grade control structures, significant tributaries, and lateral sediment inputs on the downstream fining trends. Additionally, the relationship between the channel width and the grain sizes in bars was analysed, as well as the depositional trends in frontal, central and distal parts of the examined gravel bars. The Ondřejnice River has, in most cases, higher D50 and median values of grain size of bar sediment and a higher downstream reduction coefficient (D50 = 0.033 mm km −1 ; D84 = 0.036 mm km −1 ) than the Lubina River (D50 = 0.026 mm km −1 ; D84 = 0.032 mm km −1 ). These intense reduction trends in the grain size are often observed in single and multiple-threaded rivers in the Western Carpathians. On both rivers, the predominant deposition of the highest D50 was detected in the central parts of the bars, and the wider channel widths often corresponded with finer sediment deposition in the Lubina River. The disruption of the downstream fining corresponded in some cases with the frequent grade control structures. However, in most cases, the downstream fining trends were not affected. A tributary and adjacent hillslope area could be possibly linked to the disruption of the downstream fining trend in the Ondřejnice River.

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<p>From a general point of view, river sediment is gradually fining the downstream direction, which is mainly a product of abrasion and selective transport. Downstream fining is not constant, and certain disturbances, such as sediment supply from tributaries or erosion connected with lateral and vertical shifts in the channel, can abruptly change sediment size. The main goal of this case study is to analyse discontinuities in the downstream fining of sediment size and link these variations with possible sources, i.e., lateral inputs. We focussed on the 75-km-long Odra (Oder) River reach in Czechia. The Odra is a relatively preserved meandering river with dominant gravel sediments. The supply of sediments is realised from two contrast geologically distinct mountain ranges, namely the Nízký Jeseník Mts. (generally offals and slates) in the Bohemian Massif and the Moravskoslezské Beskydy Mts. (flysch-based) in the Outer Western Carpathians. Analysis of the surface layer of 15 gravel bars was performed by the Wolman method, by which we randomly measured 100 clasts on each gravel bar. Additionally, we collected subsurface samples for sieving analysis. In the composition of the subsurface samples, there were dominant gravels and sandy gravels. Only four sites were muddy sandy gravels, which may be caused by the predominant slow flow in that part of the Odra River reach. The data of the coarse-grained material (≥ 2 mm) from the surface layer showed downstream fining only in the approx. first third of the river reach (27 km). The downstream part of the reach did not show significant fining, but rather a gradual increase in grain size of the surface bar layers. The most apparent fluctuation in grain size showed the 25th and 75th percentiles (14 - 29 mm and 25 - 42 mm, respectively), while the median indicated lesser variations. The discontinuity in fining can generally be linked to the influence of tributaries that carry coarse gravel from mountainous areas, but in this case the samples near tributaries do not show abrupt changes in sediment calibre. It is reasonable to assume that the coarse sediment was supplied to the river by lateral erosion of alluvial deposits in active meander bends, as indicated by the presence of coarse gravels in the architecture of the eroded alluvium.</p>

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<p>Since river regulations extensively changed the natural course and regime of many European gravel-bed rivers, most gravel bars tend to lose some of their natural dynamics. Gravel bars are often overgrown by vegetation when there is a lack of high flows for a long time. In many areas, they are objects of protection, as they are in some cases related to preserved parts of naturally braided rivers, and they represent a unique habitat for rare species of vegetation and fauna.</p><p>In our research, we analysed spatio-temporal gravel bar dynamics, with a focus on vegetation succession progress, in two rivers of the Outer Western Carpathians for twenty years (2000–2020): the Czech part of the Olše River (73 km) and the part of the Ostravice river downstream the valley dam (46 km). We used nine archive orthophotos (2000–2020) and hydrological data to analyse the effects of flood events, vegetation succession and river regulations on the gravel bar dynamics. The orthophotos were processed in ArcGIS Pro to analyse the coverage of vegetation and unvegetated area on gravel bars where the reference banks lines from the first two studied orthophotos (2000, 2003) were used to determine the edges of bars.</p><p>Two major floods in 1997 and 2010 greatly affected the vegetation cover of gravel bars in both rivers. After the 1997 and 1999 moderate floods, the total vegetated area on gravel bars in 2000 was the lowest in all studied years in the Olše (42 %) and the second lowest in the Ostravice (54 %). Following years, trends showed slow but progressive succession until the next major flood in 2010 and moderate floods in 2014. The total area of bar vegetation in the Olše decreased by 19,2 % in 2012 compared to the last studied year of 2009 (81,8 %), and by another 9 % in 2014 probably due to the time proximity of the flood event. Similarly, the total vegetated bar area in the Ostravice decreased by 24 % in 2012 (from 92 % in 2009) and by another 13,7 % in 2014. However, only two years later, the vegetated bar area significantly increased in both rivers: compared to the year 2014, it increased by 39,8 % in the Olše and by 42,1 % in the Ostravice. The remaining studied years showed a similarly high percentage of vegetation cover (2018=87,1–93,6 %, 2020=84,5–91 %). Although the Ostravice is regulated by a valley dam, the floods had a very similar effect on both rivers. The main difference is in the rate of vegetation coverage in the years not affected by floods. In the Ostravice almost all the bars were vegetated completely or for the most part, while in the Olše, there was more variability in vegetation coverage on gravel bars including only partially vegetated or unvegetated bars. Our results suggest an ongoing succession of vegetation on most gravel bars in the last twenty years and show a significant effect of major flood events and river regulations on their morphodynamics.</p>