Debris‐flow and anthropogenic disturbance on 10Be concentration in mountain streams with contrasting structural geomorphic connectivity

Abstract

<p>This study examines the sensitivity of 10Be concentrations to debris‐flow and anthropogenic perturbations in steep mountain streams of the Eastern Alps, characterized by contrasting structural geomorphic connectivity. Using cosmogenic 10Be as a tracer for functional geomorphic connectivity, we conduct sampling replications across four seasons in Gadria, Strimm and Allitz Creek. Sampling sites encompass a range of structural connectivity configurations, including the conditioning of a sackung, all assessed through a geomorphometric index (IC). By combining information on contemporary depth of erosion and sediment yield, disturbance history and post‐LGM (Last Glacial Maximum) sedimentation rates at the Gadria fan, we constrain the effects of debris‐flow disturbance on 10Be concentrations along Gadria Creek. Here, we argue that bedrock weakening imparted by the sackung promotes high depth of erosion. Consequently, we show that debris flows recruit sediment beyond the critical depth of spallogenic production (e.g., >3m), which in turn, episodically, due to predominantly muogenic production pathways, lowers 10Be concentration by a factor of 4, for over 2 years. In contrast, steady erosion in Strimm Creek yields very stable 10Be concentrations through time. In Allitz Creek, we observe two‐ to fourfold seasonal fluctuations in 10Be concentrations, which we explain as the combined effects of water diversion and hydraulic structures on sediment mixing. We further show that 10Be concentration correlates inversely with the IC index, where sub‐basins characterized by high concentrations (long residence times) exhibit low IC values (structurally disconnected) and vice versa, implying that, over millennial time scales a direct relation exists between functional and structural connectivity, and that the IC index performed as a suitable metric. In terms of landscape evolution, we argue that the sackung, by favouring intense debris‐flow activity across the Holocene, as recorded by the paraglacial sedimentary wave associated with the formation of the Gadria fan, has aided rapid postglacial reshaping of the Gadria source basin, which currently exhibits a topographic signature characteristic of unglaciated debris flow-dominated systems.</p>