Abstract
Downstream fining of riverbed gravels is generally linked with the processes of hydraulic sorting and abrasion. Hydraulic sorting is when larger gravel clasts stop moving in response to decreasing flow energy, whereas, finer grains will continue to be carried downstream. Furthermore, transportation of gravel clasts causes abrasion, bringing about a gradual decrease in grain size. Hydraulic sorting and abrasion have different dominant effects on the downstream fining of clasts in rivers with different climatic and tectonic backgrounds. At present, most studies focus on humid areas, and relatively few studies have explored this issue for the northeastern margin of the Tibetan Plateau in arid and semi-arid areas. Detailed investigations of the grain size, lithology, and roundness of riverbed gravels have been performed here along the Taolai, Hongshuiba, and Fengle Rivers, which flow across the northeastern margin of the Tibetan Plateau and debouch into the arid inland of North China. The obtained data were subsequently employed in a hydraulic pattern of grain-size distribution of riverbed gravels in this area, which is characterized by the combined influences of tectonic activity and climatic aridity. Analysis reveals that there is no new rock type appearing in the lithological compositions of riverbed gravels along these rivers, only showing fluctuations in proportions of lithology even though they are adjacent to uplifting mountains. Fresh gravel material from these mountains does indeed mix into the fluvial bedload, inducing a notable decrease in roundness in the Taolai and Hongshuiba bedloads downstream from here. The downstream fining of gravel along the three rivers, with median grain sizes above 128 mm and falling into the range from 20 to 128 mm, can probably be attributed to hydraulic sorting and abrasion. Further analysis suggests that the former presents a high correlation with channel gradient, which may be sustained by fault activity at the northeastern margin of the Tibetan Plateau. The grain-size distribution in these riverbed gravels thus provides insights into the evaluation of fluvial responses to active tectonic uplift.
Highlights
Apart from twelve sampling sites located on the watersheds between the tributaries of the Beida River, a total of thirteen, eleven, and seven sites has been investigated along its tributaries: the Taolai, Hongshuiba, and Fengle Rivers, respectively (Figure 3A)
Along the Fengle River, granite decreases from ~74 to 40%, while sandstone increases to reach a relatively steady proportion of ~20% (Figure 3D) and other lithologies tend towards stability
Despite a notable fluctuation in lithological percentage due to this intrusion, the lithological composition of the bedload gravels presents a stable pattern along the three rivers (Figure 3), implying that the fluvial clasts transported through the northeastern margin of the Tibetan Plateau, characterized by active tectonism and dry climate, appear to represent material that has been recycled from earlier deposits (Zhang et al, 2020)
Summary
The interaction between climate and tectonic activity, and their relative roles as driving mechanisms for fluvial processes, have long been topics for debate (e.g., Molnar and England 1990; Molnar 2003), attracting wide attention from geoscientists (e.g., Bridgland 2000; Westaway 2009; Wang et al, 2014). In coarse sediments (>2 mm), downstream clast-size decrease can result from abrasion leading to actual size reduction (Kodama 1994; Humphrey 1997), or from selective transportation and deposition, which is the above-mentioned sorting process (Ferguson et al, 1996), or from their combined action. Evidence from bed-load trap measurements and the dispersion of magnetic tracer pebbles indicate that rapid downstream fining of clasts along a small river can probably be attributed to sorting rather than abrasion (Ferguson et al, 1996) This suggestion is in good agreement with the combined study of bed-load transport rates, grain-size distribution, shear stresses, and tracer pebble movements along three high-power gravel-bed rivers, in Scotland and Norway (Ashworth and Ferguson 1989)
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