Modeling feedbacks between lithologic contrasts, erosion, and sediment production in layered landscapes

This work explores how feedbacks between erodibility contrasts and sediment production influence the evolution of mountain rivers incising through layered rocks. Mountain rivers are key drivers of landscape response to climatic and tectonic forcing. Transient perturbations from changes in climate or tectonics are reflected in the profiles of bedrock rivers, but these signals can be challenging to unravel in lithologically complex settings. In layered rocks in particular, contrasts in erodibility cause erosion rates to vary through space and time, complicating landscape response to external forcing. Recent studies have used the detachment-limited stream power incision model to illustrate the complex variations in erosion rates that arise in layered rocks (Forte et al. 2016, Perne et al. 2017). However, these studies do not capture the effect of sediment cover on channel evolution. We use the Stream Power with Alluvium Conservation and Entrainment (SPACE) model (Shobe et al. 2017) to explore how sediment cover influences landscape evolution and modulates the topographic expression of erodibility contrasts in mixed bedrock-alluvial rivers incising through horizontally layered rocks.

Lithology influences channel evolution by setting the substrate erodibility and by producing coarse sediment. Motivated by the layered strata of the Guadalupe Mountains in Texas and New Mexico, USA, this study uses the SPACE model to simulate how relative differences in sediment grain size influence long-term landscape evolution. The influence of sediment particle size is tested by varying both the particle settling velocity and the erodibility of the alluvial layer across model runs. While the SPACE model does not resolve individual grains, varying these two parameters allows us to test the relative effects of finer vs. coarser sediment. We simulate river evolution through alternating layers of hard and soft rock over million-year timescales, with a constant uplift rate of 1 mm/year. We find that compared to the detachment-limited case, model runs with sediment cover have systematically higher channel steepness values in soft rock layers and lower channel steepness values in hard rock layers. As the relative sediment particle size is increased, the contrast in steepness between the two rock types decreases. We also find that model runs with relatively coarser sediment reach a steady average elevation more quickly than model runs with finer sediment, suggesting that sediment size exerts a strong control on landscape adjustment times. We also analyze how erosion rates across the entire model grid and sediment flux at the watershed outlet vary through time as hard and soft layers are exposed in different proportions. This work illustrates how strongly sediment cover influences channel evolution in mountains with layered stratigraphy, and demonstrates the importance of taking lithology and sediment cover into account when interpreting channel profiles.