Parameterization of river incision models requires accounting for environmental heterogeneity: insights from the tropical Andes

Abstract

Abstract. Landscape evolution models can be used to assess the impact of rainfall variability on bedrock river incision over millennial timescales. However, isolating the role of rainfall variability remains difficult in natural environments, in part because environmental controls on river incision such as lithological heterogeneity are poorly constrained. In this study, we explore spatial differences in the rate of bedrock river incision in the Ecuadorian Andes using three different stream power models. A pronounced rainfall gradient due to orographic precipitation and high lithological heterogeneity enable us to explore the relative roles of these controls. First, we use an area-based stream power model to scrutinize the role of lithological heterogeneity in river incision rates. We show that lithological heterogeneity is key to predicting the spatial patterns of incision rates. Accounting for lithological heterogeneity reveals a nonlinear relationship between river steepness, a proxy for river incision, and denudation rates derived from cosmogenic radionuclide (CRNs). Second, we explore this nonlinearity using runoff-based and stochastic-threshold stream power models, combined with a hydrological dataset, to calculate spatial and temporal runoff variability. Statistical modeling suggests that the nonlinear relationship between river steepness and denudation rates can be attributed to a spatial runoff gradient and incision thresholds. Our findings have two main implications for the overall interpretation of CRN-derived denudation rates and the use of river incision models: (i) applying sophisticated stream power models to explain denudation rates at the landscape scale is only relevant when accounting for the confounding role of environmental factors such as lithology, and (ii) spatial patterns in runoff due to orographic precipitation in combination with incision thresholds explain part of the nonlinearity between river steepness and CRN-derived denudation rates. Our methodology can be used as a framework to study the coupling between river incision, lithological heterogeneity and climate at regional to continental scales.

Growing topography due to contrasting rock types in a tectonically dead landscape

“…Yet such relationships are consistent with the stream-power model if one accounts for the magnitude of variations in the fluvial erosion efficiency coefficient (K) estimated for the study area. Our findings are in agreement with studies that demonstrated that the link between denudation rates and channel steepness is obscure in settings where lateral variations in rock strength are important (e.g., Cyr et al, 2014;Campforts et al, 2020), and that a modified version of the stream-power model including variations in rock strength should be adopted for better predicting spatial patterns of channel incision (Campforts et al, 2020). Our results imply that modelling studies attempting to reconstruct uplift histories from river profile morphology assuming uniform fluvial erosion efficiency over large areas (e.g., Roberts and White, 2010) are likely to lead to flawed results, at least in post-orogenic settings.…”

“…We estimated how θ varies over the study area based on the disorder method (Hergarten et al, 2016), which was carried out using routines developed by Mudd et al (2018), to derive an optimal θ value for the entire landscape. The drainage network was extracted using an area threshold of 0.5 km 2 (e.g., Beeson et al, 2017;Campforts et al, 2020), which is lower than the minimum drainage area among the catchments where we collected fluvial sediments (0.86 km 2 ), and is a reasonable critical threshold for https://doi.org/10.5194/esurf-2020-68 Preprint. Discussion started: 25 August 2020 c Author(s) 2020.…”

“…We have a limited understanding of how K varies in different geomorphic conditions and what controls its variability due to the few studies that derived absolute constraints on K and confounding between the multiple controls encoded in K (Snyder et al, 2000;Whipple et al, 2000a;Duvall et al, 2004;Harel et al, 2016;Zondervan et al, 2019). More sophisticated versions of the stream-power model may explicitly account for different controls in K (e.g., DiBiase and Whipple, 2011; Zondervan et al, 2019;Campforts et al, 2020), yet such approach requires specific data on an adequate resolution such as river hydraulic geometry, rock strength measurements, and hydrological data to resolve spatial and temporal runoff variability, which are not readily available.…”

Climate Change Impacts on Sediment Yield and Debris‐Flow Activity in an Alpine Catchment

“…Modeling sediment transport and storage is challenging because of complex relationships between climatic forcing, hydrological connectivity, sediment production, and the different geomorphic thresholds involved (e.g., Campforts et al., 2020; Coulthard & Van De Wiel, 2013; Lancaster & Casebeer, 2007; Peizhen et al., 2001; Pelletier, 2015; Phillips, 2003; Temme et al., 2009). Modeling experiments examining the sensitivity of basin sediment yield to climate change cover a large range of process scales and environments, particularly in relation to landscape evolution (e.g., Coulthard et al., 2012; Istanbulluoglu, 2009; Perron, 2017; Tucker & Slingerland, 1997).…”

Interactions between deforestation, landscape rejuvenation, and shallow landslides in the North Tanganyika – Kivu Rift region, Africa

“…In relict landscapes, on the other hand, hillslopes will slowly become less steep and landslides occur more sporadically, potentially allowing for a thick regolith cover to develop (Schoenbohm et al, 2004;Bennett et al, 2016). Climatic variations can also drive differences in landscape response to deforestation (Crozier, 2010), and in the context of lithologically diverse landscapes, the effect of rock strength on both knickpoint retreat and landsliding must also be acknowledged (Parker et al, 2016;Baynes et al, 2018;Campforts et al, 2020).…”