Downstream fining of fluvial gravels along the eastern Tibetan Plateau rivers

Abstract

The downstream grain size variation along active river channels has been a long-standing topic for fluvial geomorphologists. However, identifying the influence and controls on downstream change in bed sediment texture is not straightforward. In this study, we investigated the lithological composition of bedrock and riverbed gravels, rock strength and grain size distribution of fluvial gravels within three active rivers flowing out of the Longmenshan Mountain in the eastern Tibetan Plateau where human disturbances are locally occurring, to examine the spatial pattern of grain size distributions and the controlling factors. The field investigations and geological mapping showed that granite lithology contributed more coarse clasts to the river bed substrate than its areal proportion. Downstream fining trends were detected across the entire river systems of all three rivers, while distinct patterns emerged when comparing upstream and downstream segments from the mountain front. The D₅₀ of granitic gravels in the Yazi River (YZR) and Mianyuan River (MYR) have the same fining rate of about −0.013 km⁻¹, while the sandstone gravels in the YZR and limestone gravels in the MYR have a similar fining rate of −0.010 km⁻¹. In contrast, the granite and plagioamphibolite gravels in the Shiting River (STR) have higher fining rates, with values of −0.022 and −0.029 km⁻¹, respectively. For coarser grain size D₈₄, the granitic gravels in the YZR and MYR have similar fining rates between −0.018 and −0.019 km⁻¹, while sandstone gravels and limestone gravels have lower fining rates between −0.012 and −0.014 km⁻¹. Although the D₈₄ fraction for granite and plagioamphibolite gravels has a higher fining rate of −0.034 and −0.022 km⁻¹ in the STR, the D₅₀ and D₈₄ in STR have the same fining rate (−0.022 km⁻¹) for plagioamphibolite gravels. We interpret our data to indicate that selective deposition is the main control on downstream fining of fluvial gravels, as the mobility of gravel is strongly size-dependent. In addition, the higher downstream fining rates of both granite and plagioamphibolite gravels in the STR than those in the YZR and MYR also reveal the river channel slope control on the downstream change pattern of fluvial sediments. The significant difference in downstream fining rates between granite and plagioamphibolite gravels in STR indicates that the downstream fining in some reaches would be lithology-dependent. Although the weathering phenomenon on the surface of granitic gravels in river channels could not explicitly account for its contribution to the downstream fining of granitic gravels, its significance on gravel abrasion deserves further work in future investigations. Our field investigations show that human activities, such as gravel mining and dam construction, had temporal impacts on downstream grain size changes. Our findings highlight that the sediment transport in the Longmenshan Mountain region is strongly influenced by selective deposition, river channel slope and lithology. The new findings provide new insights for understanding the links of grain size patterns to sediment supply, fluvial topography and bedrock lithology in the eastern Tibetan Plateau.