Investigating Stochastic-threshold Incision Models accros a Climatic Gradient : example from the South-eastern Margin of the Massif Central, France

Abstract : Investigating landscapes evolution and their responses to tectonic or climatic perturbation requires accurate representations and models of surfaces processes. Rivers are keys agents of the transmission of external forcing through landscape and the main drivers of continental denudation. For these reasons the mechanisms of fluvial incision have been a key focus of geomorphological research over the last decades. Long-term river incision is usually modeled with the Stream Power Model (Howard et al. 1997; Whipple & Tucker 1999) based on the along-stream evolution of drainage area and channel elevation gradient. More complex development of the SPM (Tucker 2004; Lague et al. 2005) combine instantaneous river incision with the probability for discharge occurrence and take into account incision threshold and discharge variability. These models have been confronted with fields data in a limited number of cases (DiBiase et al. 2011, Scherler et al. 2017, Campforts et al. 2020). Our objective is to investigate the behavior of stochastic-threshold incision models across the south-eastern margin of the Massif Central (France) where a strong gradient in discharge variability is reported. We present a new dataset combining measurements of discharge variability from gauging stations, denudation rates on 34 basins from 10Be cosmogenic nuclides and morphometric parameters from river long-profile analysis. Our results show a complex relationship between denudation rates and steepness index which cannot be explain only by the regional discharge variability gradient. When compared with measured erosion rates, we observe that the predictions of a stochastic-threshold incision model are not better than simpler versions of the SPM and require to take into account spatial variations in incision thresholds.