Controls Over Particle Motion and Resting Times of Coarse Bed Load Transport in a Glacier‐Fed Mountain Stream

Abstract

Coarse bed load transport is a crucial process in river morphodynamics but is difficult to monitor in mountain streams. Here we present a new sediment transport data set obtained from 2 years of field-based monitoring (2014)(2015) at the Estero Morales, a high-gradient stream in the central Chilean Andes. This stream features step-pool bed geometry and a glacier-fed hydrologic regime characterized by abrupt daily fluctuations in discharge. Bed load was monitored directly using Bunte samplers and by surveying the mobility of passive integrated transponder (PIT) tags. We used the competence method to quantify the effective slope, which is the fraction of the topographical slope responsible for bed load transport. This accounts for only 10% of the topographical slope, confirming that most of the energy is dissipated on macroroughness elements. We used the displacement lengths of PIT tags to analyze displacement lengths and virtual velocity of a wide range of tracer sizes (38-415 mm). Bed load transport in the Estero Morales shown to be size-selective, and the distance between steps influences the displacement lengths of PIT tags. Displacement lengths were also used to derive the statistics of flight distances and resting times. Our results show that the average length of flight scales inversely to grain size. This contradicts Einstein's conjecture about the linear relationship between grain size and intervals between resting periods in a steep step-pool stream in ordinary flood conditions.

On How Episodic Sediment Supply Influences the Evolution of Channel Morphology, Bedload Transport and Channel Stability in an Experimental Step‐Pool Channel

“…The unreasonably high value of based on total shear stress (Figure 8) confirms that in steep streams with step‐pool morphology, only a small part of the flow energy contributes to bedload transport (Lamb et al., 2008; Schneider et al., 2015). The reduced energy slope obtained with the Rickenmann and Recking (2011) method was ∼18% of the reach‐averaged water surface slope throughout the experiments, indicating that ∼82% of the total energy was dissipated, in good agreement with previous results (Comiti et al., 2009; Wilcox et al., 2011; Mao et al., 2020). The mean value of (0.026) was within the range of 0.01–0.2 reported by Buffington and Montgomery (1997) and Lamb et al.…”

“…The complex hydraulics in steep streams cannot be fully captured by simplified reach‐averaged values. The reach‐averaged shear stress calculated with the depth‐slope product is derived under the assumption that the flow is steady and uniform, and has been commonly used for the prediction of bedload transport (Johnson, 2016; Schneider et al., 2016; Mao et al., 2020). While we kept the flow rate constant, the assumption of uniform flow requires that flow depth is constant along a river reach, or at least that the bed and water surface slopes are identical (Yager, Venditti, et al., 2018), which is rarely achieved in steep channels with a dynamic bed surface.…”

“…10.1029/2020WR029133 (Johnson, 2016;Schneider et al, 2016;Mao et al, 2020). While we kept the flow rate constant, the assumption of uniform flow requires that flow depth is constant along a river reach, or at least that the bed and water surface slopes are identical (Yager, Venditti, et al, 2018), which is rarely achieved in steep channels with a dynamic bed surface.…”

Anthropogenic contaminants in glacial environments I: Inputs and accumulation

“…The morphology of the ice can also influence the movement of contaminants and sediments. For instance, valley glaciers will move materials through the system much faster than large ice sheet outlet glaciers (Antoniazza and Lane, 2021; Choudhary et al, 2020; Lawson, 1982; Mao et al, 2020). Additionally, melting of the upper layers of snow, firn and ice will expose contaminants stored deeper within the glacier, increasing contaminant accrual and potentially accelerating melting processes (Gul et al, 2021).…”

Sediment Dynamics and Bed Stability in Step‐Pool Streams: Insights From 18 Years of Field Observations

“…These channels have high boundary roughness created by large clasts, wood, and rough material. They dissipate a substantial amount of energy (e.g., Mao et al., 2020) that might otherwise be available to mobilize bed material, resulting in small fraction of the total force exerted on the grains. In addition to flow characteristics, sediment mobilization in steep streams is impacted by the sediment supply (Whittaker, 1987).…”