Alluvial channel response to environmental perturbations: Fill-terrace formation and sediment-signal disruption

Abstract

Abstract. The sensitivity of fluvial fill terraces to tectonic and climatic boundary conditions make them potentially useful archives of past climatic and tectonic conditions. However, we currently lack a systematic understanding of the impacts of base-level, water discharge, and sediment discharge changes on terrace formation and associated sediment storage and release. This knowledge gap precludes a quantitative inversion of past environmental changes from terraces. Here we use a set of seven physical experiments to explore terrace formation and sediment export from a braided channel system that is perturbed by changes in upstream water discharge and sediment supply, or downstream base-level fall. Each perturbation differently affects (1) the geometry of terraces and channels, (2) the timing of terrace formation, and (3) the transient response of sediment discharge. In general, an increase in water discharge leads to near-instantaneous channel incision across the entire fluvial system and consequent local terrace cutting, preservation of the initial channel profile on terrace surfaces, and a transient increase in sediment export from the system that eventually returns to its pre-perturbation rate. In contrast, changes in the upstream sediment supply rate may result in longer lag-times before terrace cutting, leading to a less well-preserved pre-perturbation channel profile, and may also produce a gradual change in sediment output towards a new steady-state value. Finally, downstream base-level fall triggers the upstream migration of a knickzone, forming terraces with upstream-decreasing ages. The gradient of terraces triggered by base-level fall mimicks that of the newly-adjusted active channel, whereas gradients of terraces triggered by variability in upstream sediment or water discharge are steeper compared to the new equilibrium channel. Our findings provide guidelines for distinguishing between different types of perturbations when interpreting fill terraces and sediment export from fluvial systems.

Alluvial channel response to environmental perturbations: fill-terrace formation and sediment-signal disruption

“…Manual slope and Q s,out measurements can be found in the Supplement. Topographic scans, overhead photos, and time-lapse movies of the experimental runs as well as experiment documentation are available through the SEAD Internal Repository (Tofelde et al, 2019).…”

“…Data availability. Topographic data from laser scans, overhead photographs, time-lapse movies, and experiment documentation have been published through the Sediment Experimentalists Network Project Space to the SEAD Internal Repos-itory (Tofelde et al, 2019) and can be accessed through https://doi.org/10.26009/s0ZYPUYN.…”

Times associated with source-to-sink propagation of environmental signals during landscape transience

“…erosion of underlying rock or remobilization of sediment ( Fig. 3e1; Allen and Densmore, 2000;van den Berg van Saparoea and Postma, 2008;Armitage et al, 2011Armitage et al, , 2013Armitage et al, , 2018bTofelde et al, 2019;Zhang et al, 2020). This pattern in the Qs response can be observed at individual locations along the channel (Fig.…”

“…Environmental signals are typically defined as changes in the amount of produced, transported and deposited sediment (Qs [m 3 s -1 or kg s -1 ]) in response to a change in boundary conditions (Romans et al, 2016 and references therein). Therefore, many analog-material and numerical modelling studies investigating the effects of changing boundary conditions on signal propagation focus on changes in Qs (e.g., Allen and Densmore, 2000;van den Berg van Saparoea and Postma, 2008;Simpson and Castelltort, 2012;Armitage et al, 2013;Coulthard and Van De Wiel, 2013;Li et al, 2018a;Moussirou and Bonnet, 2018;Tofelde et al, 2019). However, changes in boundary conditions do not only affect the amount of transported sediment, but can also alter the sediment grain size distribution (Armitage et al, 2011;Parsons et al, 2012;D'Arcy et al, 2016D'Arcy et al, , 2017Bataille et al, 2019) or its geochemical composition and detrital geochronological signature (Sharman et al, 2019;Lenard et al, 2020).…”

“…A landscape can either be in steady state (synonym: equilibrium) or in transient state, depending on whether the landscape is adjusted to the prevailing boundary conditions or not (e.g., Mackin, 1948;Howard, 1982;Allen, 2008). The adjustment of river profiles exerts a major control on the state of a landscape and, hence, the evolution of longitudinal river profiles following a change in boundary conditions is often studied in analog-material experiments (van den Berg van Saparoea and Postma, 2008;Rohais et al, 2012;Grimaud et al, 2016;Baynes et al, 2018;Tofelde et al, 2019;Savi et al, 2020), by using numerical models (Davy and Lague, 2009;Armitage et al, 2011Armitage et al, , 2013Armitage et al, , 2018bSimpson and Castelltort, 2012;Goren et al, 2014;Braun et al, 2015;Nie et al, 2018), and sometimes in the field (Whittaker et al, 2008). Figure 3 summarizes schematically the general response of a river following a step increase in water discharge (Qw [m 3 s -1 ]) (Fig.…”