TY - JOUR A1 - Stolle, Amelie A1 - Schwanghart, Wolfgang A1 - Andermann, Christoff A1 - Bernhardt, Anne A1 - Fort, Monique A1 - Jansen, John D. A1 - Wittmann, Hella A1 - Merchel, Silke A1 - Rugel, Georg A1 - Adhikari, Basanta Raj A1 - Korup, Oliver T1 - Protracted river response to medieval earthquakes JF - Earth surface processes and landforms : the journal of the British Geomorphological Research Group N2 - Mountain rivers respond to strong earthquakes by rapidly aggrading to accommodate excess sediment delivered by co-seismic landslides. Detailed sediment budgets indicate that rivers need several years to decades to recover from seismic disturbances, depending on how recovery is defined. We examine three principal proxies of river recovery after earthquake-induced sediment pulses around Pokhara, Nepal's second largest city. Freshly exhumed cohorts of floodplain trees in growth position indicate rapid and pulsed sedimentation that formed a fan covering 150 km2 in a Lesser Himalayan basin with tens of metres of debris between the 11th and 15th centuries AD. Radiocarbon dates of buried trees are consistent with those of nearby valley deposits linked to major medieval earthquakes, such that we can estimate average rates of re-incision since. We combine high-resolution digital elevation data, geodetic field surveys, aerial photos, and dated tree trunks to reconstruct geomorphic marker surfaces. The volumes of sediment relative to these surfaces require average net sediment yields of up to 4200 t km–2 yr–1 for the 650 years since the last inferred earthquake-triggered sediment pulse. The lithological composition of channel bedload differs from that of local bedrock, confirming that rivers are still mostly evacuating medieval valley fills, locally incising at rates of up to 0.2 m yr–1. Pronounced knickpoints and epigenetic gorges at tributary junctions further illustrate the protracted fluvial response; only the distal portions of the earthquake-derived sediment wedges have been cut to near their base. Our results challenge the notion that mountain rivers recover speedily from earthquakes within years to decades. The valley fills around Pokhara show that even highly erosive Himalayan rivers may need more than several centuries to adjust to catastrophic perturbations. Our results motivate some rethinking of post-seismic hazard appraisals and infrastructural planning in active mountain regions. KW - fluvial response KW - sediment yield KW - earthquakes KW - Nepal KW - Himalaya Y1 - 2018 U6 - https://doi.org/10.1002/esp.4517 SN - 0197-9337 SN - 1096-9837 VL - 44 IS - 1 SP - 331 EP - 341 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Tofelde, Stefanie A1 - Duesing, Walter A1 - Schildgen, Taylor F. A1 - Wickert, Andrew D. A1 - Wittmann, Hella A1 - Alonso, Ricardo N. A1 - Strecker, Manfred T1 - Effects of deep-seated versus shallow hillslope processes on cosmogenic Be-10 concentrations in fluvial sand and gravel JF - Earth surface processes and landforms : the journal of the British Geomorphological Research Group N2 - Terrestrial cosmogenic nuclide (TCN) concentrations in fluvial sediment, from which denudation rates are commonly inferred, can be affected by hillslope processes. TCN concentrations in gravel and sand may differ if localized, deep-excavation processes (e.g. landslides, debris flows) affect the contributing catchment, whereas the TCN concentrations of sand and gravel tend to be more similar when diffusional processes like soil creep and sheetwash are dominant. To date, however, no study has systematically compared TCN concentrations in different detrital grain-size fractions with a detailed inventory of hillslope processes from the entire catchment. Here we compare concentrations of the TCN Be-10 in 20 detrital sand samples from the Quebrada del Toro (southern Central Andes, Argentina) to a hillslope-process inventory from each contributing catchment. Our comparison reveals a shift from low-slope gullying and scree production in slowly denuding, low-slope areas to steep-slope gullying and landsliding in fast-denuding, steep areas. To investigate whether the nature of hillslope processes (locally excavating or more uniformly denuding) may be reflected in a comparison of the Be-10 concentrations of sand and gravel, we define the normalized sand-gravel index (NSGI) as the Be-10-concentration difference between sand and gravel divided by their summed concentrations. We find a positive, linear relationship between the NSGI and median slope, such that our NSGI values broadly reflect the shift in hillslope processes from low-slope gullying and scree production to steep-slope gullying and landsliding. Higher NSGI values characterize regions affected by steep-slope gullying or landsliding. We relate the large scatter in the relationship, which is exhibited particularly in low-slope areas, to reduced hillslope-channel connectivity and associated transient sediment storage within those catchments. While high NSGI values in well-connected catchments are a reliable signal of deep-excavation processes, hillslope excavation processes may not be reliably recorded by NSGI values where sediment experiences transient storage. (c) 2018 John Wiley & Sons, Ltd. Y1 - 2018 U6 - https://doi.org/10.1002/esp.4471 SN - 0197-9337 SN - 1096-9837 VL - 43 IS - 15 SP - 3086 EP - 3098 PB - Wiley CY - Hoboken ER -