Climatic controls on steady state erosion using the relationship between channel steepness and cosmogenic 10Be-derived catchment averaged erosion rates

To understand landscape response to climate change, baseline controls on erosion rates must be established for given climate conditions. Theory suggests a number of climate metrics should be important to erosion (i.e. precipitation, temperature, storminess, seasonality, snow fraction). Nevertheless, definitive field evidence quantifying how climate affects erosion rate has proven difficult to obtain. This is at least partly due to the difficulty of isolating climatic influences on erosion rates from topographic and rock strength influences. We circumvent this problem by evaluating how climate influences the relationship between erosion rate and topography in settings with similar rock types. At steady state, tectonic uplift dictates erosion rate, and climate and rock strength are manifest as changes in erosional efficiency - the topographic relief necessary to maintain the tectonically imposed erosion rate. In fluvial landscapes, bedrock rivers set the relevant scale of topographic relief, which can be described by the channel steepness index. A number of recent studies have shown that the relationship between channel steepness and millennial scale erosion rates is non-linear, implying that erosional efficiency increases with relief. Work in the San Gabriel Mountains suggests this relationship is due to erosion thresholds that limit incision of channels in low relief landscapes. By using a fluvial incision model that incorporates a range of daily discharge events coupled with an erosion threshold (Lague et al., 2005), the influence of flood frequency on the relationship between channel steepness and erosion rate can be explored. We apply this same modeling approach to five other landscapes that exhibit a range of channel steepness, have similar rock types (granitoids), but that are in dramatically different climate regimes ranging from desert to rainforest (annual rainfall, P, from 0.25 to 3 m/yr). Specifically, we present new cosmogenic ¹⁰Be erosion rate data from the San Jacinto Mountains, CA (P = ~0.25 m/yr); Sierra San Pedro Martir, MX (P = ~0.25 m/yr); Sangre de Cristo Mountains, NM (P = ~1 m/yr); North Sierra Nevada, CA (P = ~2 m/yr); Sierra Nombre de Dios, HN (P = ~3 m/yr). These landscapes exhibit large differences in mean annual precipitation, mean annual temperature, mean daily runoff, and runoff variability. Using long time-series hydrological and meteorological data, we assess the capabilities and limitations of stochastic-threshold incision models for predicting the relationship between channel steepness and erosion rate. Further, we document quantifiable changes in erosional efficiency for differing climate conditions that can be used as a baseline for interpreting the effects of climate change in steep mountain catchments.