Abstract
AbstractSediment supply (Qs) is often overlooked in modelling studies of landscape evolution, despite sediment playing a key role in the physical processes that drive erosion and sedimentation in river channels. Here, we show the direct impact of the supply of coarse‐grained, hard sediment on the geometry of bedrock channels from the Rangitikei River, New Zealand. Channels receiving a coarse bedload sediment supply are systematically (up to an order of magnitude) wider than channels with no bedload sediment input for a given discharge. We also present physical model experiments of a bedrock river channel with a fixed water discharge (1.5 l min−1) under different Qs (between 0 and 20 g l−1) that allow the quantification of the role of sediment in setting the width and slope of channels and the distribution of shear stress within channels. The addition of bedload sediment increases the width, slope and width‐to‐depth ratio of the channels, and increasing sediment loads promote emerging complexity in channel morphology and shear stress distributions. Channels with low Qs are characterized by simple in‐channel morphologies with a uniform distribution of shear stress within the channel while channels with high Qs are characterized by dynamic channels with multiple active threads and a non‐uniform distribution of shear stress. We compare bedrock channel geometries from the Rangitikei and the experiments to alluvial channels and demonstrate that the behaviour is similar, with a transition from single‐thread and uniform channels to multiple threads occurring when bedload sediment is present. In the experimental bedrock channels, this threshold Qs is when the input sediment supply exceeds the transport capacity of the channel. Caution is required when using the channel geometry to reconstruct past environmental conditions or to invert for tectonic uplift rates, because multiple configurations of channel geometry can exist for a given discharge, solely due to input Qs. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd
Highlights
The understanding of the processes that set the morphology of bedrock river channels, and the feedbacks and interactions that drive their evolution, are crucial to develop a detailed understanding of rapidly evolving landscapes (Whipple, 2001; Turowski et al, 2006; DiBiase et al, 2015)
Despite the lack of the ‘tools effect’ in the experiments, we suggest that the important process for studying the effect of sediment supply of bedrock channel geometry is the cover effect, with channel widening occurring when the bed is protected from vertical erosion and flow is directed towards the channel walls (e.g. Turowski, 2018)
Sediment is a critical component of bedrock channel systems, with the supply of hard, coarse‐grained material as bedload having a direct impact on the width of channels in the Rangitikei River, New Zealand
Summary
The understanding of the processes that set the morphology of bedrock river channels, and the feedbacks and interactions that drive their evolution, are crucial to develop a detailed understanding of rapidly evolving landscapes (Whipple, 2001; Turowski et al, 2006; DiBiase et al, 2015). In response to variations in climate or tectonic uplift rate, it has been shown that bedrock channels evolve dynamically through adjustments to both width and slope. This was observed across contrasting settings, including Nepal (Lavé and Avouac, 2001), California (Duvall et al, 2004), Italy (Whittaker et al, 2007), Taiwan (Yanites et al, 2010) and Scotland (Whitbread et al, 2015), as well as in experiments (Turowski et al, 2006) and predicted by mechanistic numerical modelling approaches This restricts the applicability of these models to account for SEDIMENT FLUX‐DRIVEN CHANNEL GEOMETRY ADJUSTMENT changes in channel geometry due to climate or tectonic forcing that are important for setting river sediment transport capacity (Qsc) (Croissant et al, 2017), bank erosion (e.g. Turowski et al, 2008), channel sinuosity (Stark, 2006; Turowski, 2018) and the undermining or stabilization, in the case of channel narrowing, of adjoining hillslopes (e.g. Golly et al, 2017)
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