Lithologic Controls on Landscape Evolution: Modeling how the Cover Effect Influences Effective Erodibility

Lithology influences river channel morphology in two main ways: by setting the substrate erodibility and by producing sediment. Recent landscape evolution modeling work has explored the dynamics of rivers incising through layered strata with contrasting erodibilities using detachment-limited incision models, which offer a predictable relationship between channel steepness and erodibility, allowing us to back-calculate erodibility based on channel slope and erosion rates. In layered settings, lithologic contrasts create spatial and temporal variations in erosion rates that complicate this relationship. Furthermore, the detachment-limited model does not account for the role of sediment, which can inhibit erosion by armoring the channel bed (the cover effect), complicating the relationship between steepness and erodibility. This work explores how feedbacks between erosion and sediment production in landscapes with lithologic contrasts influence channel morphology. The recently developed Stream Power with Alluvium Conservation and Entrainment (SPACE) model allows for the simultaneous treatment of bedrock, fully alluvial, and mixed bedrock-alluvial channels and transitions smoothly between detachment- and transport-limited behaviors. Here, we use the SPACE model to explore how sediment load influences effective erodibility in layered landscapes with lithologic contrasts. We use the Landlab Toolkit, a python-based library for modeling earth surface processes, to simulate fluvial incision through horizontally layered strata using the SPACE model. We compare models of detachment-limited and mixed bedrock-alluvial incision through alternating horizontal layers of hard and soft rock, motivated by the lithologic variability found in the Guadalupe Mountains of Texas and New Mexico. In particular, we compare channel profiles in chi space and channel steepness for a given bedrock erodibility across model runs in order to assess how the cover effect influences the effective erodibility of a given rock type. We also explore how variations in erodibility in a given rock layer influence channel slope in the surrounding layers, particularly where a layer of hard rock overlies soft rock.