Quantifying the controls on potential soil production rates: a case study of the San Gabriel Mountains, California

Abstract

Abstract. The potential soil production rate, i.e., the upper limit at which bedrock can be converted into transportable material, limits how fast erosion can occur in mountain ranges in the absence of widespread landsliding in bedrock or intact regolith. Traditionally, the potential soil production rate has been considered to be solely dependent on climate and rock characteristics. Data from the San Gabriel Mountains of California, however, suggest that topographic steepness may also influence potential soil production rates. In this paper I test the hypothesis that topographically induced stress opening of preexisting fractures in the bedrock or intact regolith beneath hillslopes of the San Gabriel Mountains increases potential soil production rates in steep portions of the range. A mathematical model for this process predicts a relationship between potential soil production rates and average slope consistent with published data. Once the effects of average slope are accounted for, a small subset of the data suggests that cold temperatures may limit soil production rates at the highest elevations of the range due to the influence of temperature on vegetation growth. These results suggest that climate and rock characteristics may be the sole controls on potential soil production rates as traditionally assumed but that the porosity of bedrock or intact regolith may evolve with topographic steepness in a way that enhances the persistence of soil cover in compressive-stress environments. I develop an empirical equation that relates potential soil production rates in the San Gabriel Mountains to the average slope and a climatic index that accounts for temperature limitations on soil production rates at high elevations. Assuming a balance between soil production and erosion rates on the hillslope scale, I illustrate the interrelationships among potential soil production rates, soil thickness, erosion rates, and topographic steepness that result from the feedbacks among geomorphic, geophysical, and pedogenic processes in the San Gabriel Mountains.

Landslides after wildfire: initiation, magnitude, and mobility

“…In the Colby fire, landslides are concentrated in the leucocratic and granitic rocks, and the slides in the Morris fire reburned by the SGC fire are in the Pelona Schist and a gneissic granodiorite (Morton and Miller 2003). This corresponds to the prior observations in the San Gabriel Mountains that rock type may not strongly influence erosion and that erosion rates may be more strongly tied to fracturing due to tectonic deformation (Lifton and Chase 1992;Pelletier 2017;Spotila et al 2002).…”

Hilltop Curvature Increases With the Square Root of Erosion Rate

“…However, the absence of any climatic influence in our results suggests that these bedrock processes are not associated with the typical chemical and physical weathering processes; instead, they are likely related to a more universal mechanism. Recent work has begun investigating how, even in soil‐mantled landscapes, the generation of fractures in bedrock by topographic stresses may exert an important influence on landform shape (e.g., Clair et al., 2015; Pelletier, 2017; Slim et al., 2015). However, whereas the regional tectonic stress is an important contributor to topographic stresses (e.g., Clair et al., 2015; Miller & Dunne, 1996), the tectonic stress regime varies widely between our sites.…”

Hilltop Curvature Increases with the Square Root of Erosion Rate

“…However, the absence of any climatic influence in our results suggests that these bedrock processes are not associated with the typical chemical and physical weathering processes; instead, they are likely related to a more universal mechanism. Recent work has begun investigating how, even in soil-mantled landscapes, the generation of fractures in bedrock by topographic stresses may exert an important influence on landform shape [e.g., Clair et al, 2015;Pelletier, 2017;Slim et al, 2015]. However, whereas the regional tectonic stress is an important contributor to topographic stresses [e.g., Clair et al, 2015;Miller and Dunne, 1996], the tectonic stress regime varies widely between our sites.…”