Spatial variability of denudation rates and transient landscape adjustment in low‐deforming mountains (the Strengbach catchment, Vosges Mountains, France)

Abstract

A set of ¹⁰ Be‐derived catchment‐wide denudation (CWD) rates in a low mountain catchment of western Europe (Strengbach catchment, Vosges Mountains) are analysed to discuss landscape stability. Together with a morphometric study, the data highlight the heterogeneous behaviour of the main trunk and major tributaries. Overall, the rates range from 39 to 84 mm/kyr across the catchment, the main trunk shows increasing rates from 41 to 48 mm/kyr from source to outlet, and the tributaries have systematically higher rates (42 to 84 mm/kyr). To discuss the origin of these variations, we look for systematic characters related to both morphology and lithology. We apportion the landscape of each sub‐catchment (upper main trunk and tributaries) among four main lithologies (granitic, metamorphic and two sandstone units) and three main morphologies (steep hillslopes near riverbeds, gentle hillslopes and steep slopes) resulting in nine landscape types defined by their combined lithological and morphological characteristics. A Monte Carlo adjustment method based on a Quantum‐behaved Particle Swarm Optimization (QPSO) algorithm has been applied to retrieve the optimal denudations of each landscape type that contribute to each sub‐catchment CWD rate. Our results show that some zones erode up to 10 times faster, indicating out‐of‐disequilibrium relief evolution. We use the inversion results to quantify the pace of landscape adjustment, including cliff retreat in tributaries and retreat of major knickpoints within a stream‐power incision framework. This approach allows us to show that modest gradients of CWD at the catchment scale can coexist with an internally transient landscape with focused denudation near knickzones and cliffs. The horizontal retreat rates of knickpoints, which are up to one order of magnitude larger than sub‐catchment CWD, remain slow and may explain the persistence of strong relief contrasts in a low‐deformation setting. Although the ¹⁰ Be CWD rates vary only slightly, large variations in denudation rates can occur because of landscape and the lithological particularities. Our methodology makes it possible to discuss CWD variations in slowly evolving environments and their role in maintaining high relief in close to steady‐state mountain landscapes.