Downstream fining trends of gravel bar sediments: a case study of Czech Carpathian rivers

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.