Downstream fining in a megaclast-dominated fluvial system: The Sabeto River of western Viti Levu, Fiji

Abstract

Megaclast-dominated fluvial systems have been largely ignored by researchers. Yet megaclasts exert an important control on channel geomorphology, flow behaviour and sediment transport, and such systems are probably far more common than this neglect would suggest. Despite the explicit identification of megaclasts as the products of cataclysmic flow, observation of modern fluvial systems confirms that megaclasts can be entrained by flows that lie within the normal flow regime. The pattern of fining along the megaclast-dominated Sabeto River of western Viti Levu, Fiji reveals a strong negative exponential relationship between particle size and distance downstream, and a rate of downstream fining comparable with that of other gravel-bed rivers. The downstream fining pattern is straightforward and step-like discontinuities in the fining gradient are absent. This is suggestive of a sedimentary system only minimally disrupted by the supply of sediment from tributaries or the reworking of material from sources such as hillslopes and alluvial terraces. There is evidence, however, of anomalously coarse sub-populations at two sites. These coarser components must predate the other deposits at the sites. We speculate that they represent the products of earlier and higher-magnitude events. The bed sediments may thus be composite features, made up of individual components each transported by a specific event of particular magnitude. Our work confirms the relationship between the rate of downstream fining and particle size. This may arise because the distance over which individual particles are transported is likely to be size dependent. It is also possible that abrasion may be more effective in the case of larger particles than smaller ones, with the result that the coarser fraction of the river's bed sediment fines downstream more rapidly than the finer fraction. There is a long-standing debate about the relative roles of abrasion and sorting in downstream fining. Our data reveal little evidence for a downstream increase in the roundness of the river's gravels, implying that abrasion is likely to play a minor role in the fining process in this system.

The shape of fluvial bedload gravels: A large, high-quality dataset of active-channel deposits

“…1 ). The Sabeto River is an active, bedload transporting, gravel-bed system [3] . The lithology of the river gravels reflects the make-up of the rocks exposed in the catchment.…”

“…1 The geology of the catchment of the Sabeto River of western Viti Levu, Fiji, showing the location of the sampling stations. Modified from Gale et al [3] .

Fig.

…”

New Evidence of Megaclasts from the Russian South: The First Report of Three Localities

“…Particularly, geomorphologists deal with modern sediments and depositional environments, and huge detrital rock particles often constitute either small-scale landforms or elements of bigger landforms. Surprisingly, the term "megaclast" and the other relevant terms (e.g., the term "block") are not used in the high-class geomorphological studies as frequently, as one would expect (e.g., [54]). Many influential articles mention boulders, although their figures illustrate true megaclasts.…”

The nature and controls on downstream change of channel sediment along the Shiyang River, Northwest China

“…River sediment grain size is one of the characterizing indicators of river hydrodynamic conditions and transport environment (Knighton, 1999), and its size and distribution characteristics are related to many factors such as regional geology, geomorphology, climate and hydrology (Allen, 2008;Duller et al, 2010;Blom et al, 2017). Fluvial abrasion and sorting of the reconstituted population, in the absence of further inputs, often produces relatively systematic downstream fining trends (Sternberg, 1875;Yatsu, 1955;Gale et al, 2019;Ivan and Tomáš, 2021a). Sternberg (1875) suggested that abrasion is the main cause of sediment decomposition and grain size refinement.…”

“…For example, affected by the characteristics of the tributary sediments, the inflow of the tributary can cause the coarsening or refinement of the main stream channel sediment (Blom et al, 2017;Ivan and Tomáš, 2021b). Abrasion and sorting also have different effects on the refinement of river sediments in different river sections (Gale et al, 2019). The Shiyang River Basin (SYRB), which is an inland river system in Northwestern China, provides a good case for comprehensive comparative analysis of the impact of the above factors on the channel sediments.…”