Understanding the landscape evolution and potential geohazards of mountain landscapes requires quantifying the rates at which they uplift and erode, and the relative importance of different erosional processes. However, each of the available techniques to quantify rates of landscape change provides only a partial account of mountain erosion, given their inherent methodological biases and measuring timescales. Therefore, reconciling erosion and uplift rates estimated with different methods can be challenging, but can also provide insights into changes in erosion rates and dominant erosional processes.

Here, we present a combination of new and compiled data from the western Southern Alps of New Zealand (WSA), one of the fastest-eroding ranges on Earth, which is believed to be in steady state. We present new data on: (a) 20 in-situ ¹⁰Be catchment-averaged denudation rates, which mostly range between ~0.6-9 mm/yr and represent erosion integrated over the last 275 years on average; (b) 17 ¹⁰Be concentrations from recent landslide deposits, which together with drone photogrammetry of landslide scars, provide information about landslide recurrence intervals on millennial timescales; (c) the grain size distributions of sediment supplied from hillslopes and transported by rivers, which allows us to convert published suspended sediment load estimates (0.2-6.7 mm/yr, recorded over the 1960s-1990s) into total sediment flux estimates. Based on (b) and published landslide frequency-area data, we estimate landslide erosion rates on millennial timescales, and compare these to erosion rate estimates from (a), (c); and (d) modern published landslide erosion rates (1.8-18 mm/yr, 1948-1986), (e) published millennial soil erosion rates (up to 2.5 mm/yr), (f) compiled Late Quaternary fault throw rates (up to 12 mm/yr), and (g) recalculated thermochronological exhumation rates (up to ~6-9 mm/yr, Myr timescales).

This comprehensive data set allows us to examine: (i) the proportion of total erosion driven by landslides versus other erosional processes; (ii) how modern denudation rates and landslide erosion rates compare to long-term erosion rate, rock uplift, and exhumation rate estimates; and (iii) the potential fluctuations of erosion rates and processes over seismic cycles and Holocene climate change.

How to cite: Roda-Boluda, D., Schildgen, T., Prancevic, J., Tofelde, S., Bufe, A., Lupker, M., Wittmann, H., and Hovius, N.: Comparing erosion rates across timescales and processes: insights from the Western Southern Alps of New Zealand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6058, https://doi.org/10.5194/egusphere-egu23-6058, 2023.

The controls of drainage density remain a mystery, despite it being a fundamental attribute of landscape morphology. Studies have shown that dry climates with less vegetation or less-consolidated sediment often have higher drainage densities. It remains unclear, however, how lithology effects drainage density, and existing studies have mixed results. Here, we investigate a landscape in Central Chile where two adjacent granitoid plutons (a monzogranite and a granodiorite) have different drainage densities. Since the tectonic setting and climate are the same between the two lithologies, we hypothesized that lithological differences control drainage density through modulating groundwater infiltration and runoff. To test this, for each lithology, we quantified the density of streams, channel heads, and vegetation using a 1-m resolution LiDAR digital elevation model. In the field, we measured infiltration rates and surficial sediment grain size distributions. In the lab, we obtained major oxide compositions from bedrock samples using X-Ray Fluorescence, identified mineralogy within thin sections, measured the hydraulic conductivity of the bedrock, and obtained in-situ cosmogenic 10Be-derived denudation rates from hillslope and stream sediment samples. Our results show that the granodiorite has a lower drainage density, a more weatherable mineralogy (more abundant biotite, hornblende and plagioclase) and composition (lower levels of SiO2 and K2O and higher levels of Na2O and CaO), denser vegetation, and a larger average grain size. Infiltration rates and hydraulic conductivity are similar between the two rock types, and 10Be results suggest similar erosion rates between the two lithologies. Our results suggest that differences in grain size and/or vegetation cause the observed differences in drainage density. In addition, there may be differences in infiltration rates that were undetected by our measurements or were different in the past.

How to cite: Lodes, E., Scherler, D., Stammeier, J. A., Schleicher, A., and Loyola Lafuente, M. A.: Exploring lithological controls on drainage density in Santa Gracia, Central Chile, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11578, https://doi.org/10.5194/egusphere-egu23-11578, 2023.

Climate change is expected to affect precipitation, streamflow, and sediment transport. These changes are particularly relevant in mountainous environments that play a crucial role in water resources and sediment supply for downstream reaches. We investigated the impact of climate change on hydrology and geomorphology in the upper Emme catchment (127 km²) in the Swiss pre-Alps by simulating its hydromorphological response to present climate and three climate scenarios at the end of the century using the distributed CAESAR-Lisflood landscape evolution model. The mean seasonal changes, intensification of short-duration rainfall extremes, and snow processes were explicitly modeled. The results highlight the importance of the intensity of rainfall events to predict sediment transport at the outlet, while changes to snow processes are predominant to understand the seasonal hydrological shift. For the highest emission scenario (RCP8.5), the sediment yield at the outlet increased by 6% despite a reduction in precipitation by 7% compared to the present climate, as a result of heavy precipitation intensification. On a seasonal scale, discharge increased in winter while it decreased in spring in all scenarios due to changes in snow accumulation and melting. Furthermore, we found that erosion and deposition will change spatially by the end of the century, with a shift from erosion- to deposition-dominated valleys.

How to cite: Cache, T., Ramirez, J. A., Molnar, P., Ruiz-Villanueva, V., and Peleg, N.: Climate change impacts on precipitation and future erosion rates in a pre-Alpine region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1124, https://doi.org/10.5194/egusphere-egu23-1124, 2023.

Land is the principal support for human livelihoods as it supplies food, freshwater and multiple other ecosystem services. In the current context of global anthropization and climate change, soil erosion is becoming a threat for human societies and one of the question that most deserve the attention of the entire world & scientific community. Given the large uncertainties underlying the drivers of land erosion, it is crucial to assess the impact of human activities and climate fluctuations over erosion, especially in mountain areas, where erosion is the highest. Only studies combining large spatial and temporal approaches allow to assess the effect of the different forcing factors on soil erosion rates. Here, we apply a retrospective approach based on lake sediments to reconstruct the long-term evolution of erosion in alpine landscapes. Lake Bourget, located in the northern French, acts as a natural sink for a fraction of the erosion products from its large catchment. We combined a multiproxy study of lacustrine sediment sections covering the Holocene with a source-to-sink method, using isotopic geochemistry (εNd). The applied methodology allows us to disentangle the role of climate and land use as erosion forcing factors through their differential impact on the various rock types present in the catchment. Indeed, high-altitude areas of the study site, where the erosion is dominated by precipitation and glacier advances, present isotopic signature different from those of the sedimentary rocks located in the rest of the catchment, where both human activities and precipitations impact erosion through time. To understand the effect of human activities, erosion signals from high-altitude and the rest of the catchment were compared to local and regional indexes of human activities, obtained from pollen and environmental DNA studies conducted on lake sediment sequences. For the last 3.8 kyr, climate fluctuations alone cannot explain measured erosion trends. Between the Late Bronze Age and the modern times, human activities are at the origin of a two-fold increase of the erosion rates in the Alps. Human activities, by modifying the soil erodibility through land-use (agriculture, grazing, ore extraction and deforestation) is the dominant forcing factor of the physical erosion in mountainous environment of the European Alps at least for the last 3800 years.

How to cite: Rapuc, W., Giguet-Covex, C., Bouchez, J., Genuite, K., Jacq, K., Sabatier, P., Messager, E., Poulenard, J., Gaillardet, J., and Arnaud, F.: Human activities : main drivers of the erosion in the northern French Alps over the last 3800 years, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8397, https://doi.org/10.5194/egusphere-egu23-8397, 2023.

Erosion rates in cold, bedrock hillslopes where temperatures are below freezing for a considerable portion of the year are believed to be set by frost-cracking. Nevertheless, numerous studies from the European Alps have shown that permafrost thaw-induced rockfalls can contribute non-trivially to long-term erosion rates.

Here, we report 27 new bedrock hillslope erosion rates from across the European Alps estimated using in-situ cosmogenic ¹⁰Be. Samples were collected from bedrock hillslopes as well as talus slopes with identifiable (steep, bedrock) source areas using amalgamated sampling techniques. Our sites range in elevation from 2700 m to 4040 m, and consist of a range of lithologies, bedrock temperature conditions, and deglaciation histories. Furthermore, several of our selected sites include hillslopes wherein erosion rates have been previously estimated by others using methods other than in-situ cosmogenic nuclides. We explore how our rates vary against these previously derived rates, which may integrate over shorter (or, in exceptional cases, longer) timescales.

Preliminary calculations yield ¹⁰Be-based erosion rates ranging from 0.1 mm yr^-1 to 2.7 mm yr^-1 and show no apparent correlation with elevation, aspect, or bedrock thermal conditions. In addition to erosion rates, we likewise calculate site-specific frost-cracking intensities using modern ground surface temperatures and modelled paleoclimatic conditions. Comparing the calculated frost-cracking intensities against our erosion rates inferred using ¹⁰Be suggests that frost-cracking alone is likely not the rate-limiting erosion rates in cold, high-Alpine bedrock hillslopes.

How to cite: Dennis, D. and Scherler, D.: Controls on bedrock hillslope erosion in the European Alps, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16464, https://doi.org/10.5194/egusphere-egu23-16464, 2023.

Alpine topography of many high- and mid-latitude mountain ranges gives the qualitative impression that glaciers have been highly efficient erosive agents during the Quaternary. Glacial retreat leave slopes in an unstable state, highly susceptible to modification as new geomorphic processes take over. The glacially induced transience of the topography may however last considerably longer than the redistribution and clearance of most of the glacigenic sediments. Response times to fully erase the glacial signature of an orogen must exceed an interglacial and rather is on the order of 10⁵ to 10⁶ years, depending on climate conditions, rock type, and tectonic uplift. Considering multiple cycles of glacial erosion during the Quaternary, the glacial-topographic signature will tend to become more dominant in low to medium uplifting mountain ranges. It is generally accepted that changes in hillslope relief have a first order impact on the type and magnitude of denudation, but the glacial history of a mountain range might be critical in setting the long-term pace on the magnitude of denudation rates. In order to investigate the long-term effects of the glacial-topographic overprint on non-glacial (postglacial) erosion of watersheds we analyze catchment-wide denudation rates (CWDR) inferred from the in-situ produced cosmogenic nuclide¹⁰Be of basins showing a high variation in glacial modification. We found the High Tatra Mts. (Western Carpathians, Slovakia) a suitable site to evaluate our hypothesis given i) its uniform, granitic (quartz bearing) lithology, ii) the presence of basins with a varying degree of glacial perturbation iii) the well-known glacial history, with the absence of glaciers at least for the Holocene, iv) the similar hillslope steepness distribution across analyzed catchments and v) a spatially close distribution of basins where strong climatic gradients (acting on postglacial erosion) can largely be ruled out. In this context, our study represents the first attempt to derive ¹⁰Be-inferred CWDR in the entire Carpathian Mountain range.

Analyzed catchments involve basins that show intense, slight, and no glacial impact on topography (Klapytia and Zazdni 2018; Salcher et al., 2021). Those with a high glacial impact were occupied by ice during glacial maxima (i.e. LGM and equivalents) but also during less cold stadials (i.e. younger dryas and equivalents). Those with medium perturbation, at lower elevation, experienced glacial erosion likely only during glacial maxima, while the latter have never been glaciated. We follow a nested sampling strategy to assess whether denudation rates scale with the degree of glacial imprint on topography. Derived rates range from around 60 mm/kyrs (no impact) to more than 300 mm/ka (high impact).

Even tough investigations remain at a local scale, our results point to a positive relationship between the degree of glacial perturbation and the magnitude of CWDR. To better understand the geodynamic impact of glaciation on mountain range dynamics the knowledge of such dependences is highly relevant and especially critical during the Late Cenozoic cooling climate.

Zasadni,J. Kłapyta,P., 2014. The Tatra Mountains during the Last Glacial Maximum. Journal of Maps, 10, 3. https://doi.org/10.1080/17445647.2014.885854.

Salcher,B., Prasicek,G., Baumann,S., Kober,F., 2021. Alpine relief limited by glacial occupation time. Geology, 49, 10. https://doi.org/10.1130/G48639.1.

How to cite: Salcher, B., Neuhuber, S., Delunel, R., Prekopova, M., and Zasadni, J.: The degree of glacial modification controls non-glacial erosion in alpine landscapes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8531, https://doi.org/10.5194/egusphere-egu23-8531, 2023.

Glaciers are well known for their erosive power and the large quantities of sediment that they expel. Glacier sliding sculpts the landscape and creates sediment, while englacial debris advection and subglacial sediment transport can expel this material from the glacier terminus. In alpine regions, rockfall from mountain slopes adds debris to the glacier surface, which can be buried and advected down-valley as the glacier moves. When the debris melts out to the surface, it limits surface melt and changes the glacier’s surface slope. The reduced melt and change to the glacier surface slope impact the sediment transport capacity below the glacier.

We couple numerical models of debris-covered glacier dynamics and subglacial sediment transport to show that debris-covered glaciers are expected to erode at a slower rate than clean glaciers. The model outputs also demonstrate that sediment eroded below glaciers can accumulate below debris-covered portions due to limited sediment transport capacity. Reduced melt and surface slope limit the sediment transport capacity, resulting in sediment deposition. This sediment deposition creates till layers that may alter glacier dynamics. Additionally, the results demonstrate that debris cover can change the connectivity of sediment below glaciers and store glacially eroded sediment subglacially. These findings demonstrate the complicated interactions amongst glacier dynamics, hydrology, erosion, and sediment transport. The processes identified here have substantial implications for the transport of sediment in rapidly changing mountain landscapes experiencing glacier retreat and changing hydrology.

How to cite: Delaney, I. and Anderson, L.: Debris-covered glaciers encourage subglacial sediment accumulation and limit erosion, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15395, https://doi.org/10.5194/egusphere-egu23-15395, 2023.

Sediment transport plays an important role in the evolution of mountain landscapes and draws great attention to the quantification of its production, rate of transport, and interactions with anthropogenic activities. The initiation of sediment transport—or more specifically, the dislodgement of sediment particles from their resting places—depends on the magnitude and duration of local hydrodynamic forces disrupting the balance of stationary particles. In engineering practice, these driving forces are usually characterized by a surrogate measure: streamflow velocity, local realization or bulk-flow-averaged value, based on the physical scale under consideration. The implication of this approach is that the forces applied upon sediment are solely controlled by the oncoming streamflow and their magnitude can be determined uniquely by flow velocity. The direct link between velocity and force appears to be a general consensus, but the most recent research shows otherwise. In fact, noticeable discrepancies have been found between instantaneous force fluctuations and simultaneous velocity fluctuations regarding their magnitude and also the timing of their local peaks. Such discrepancies can be easily explained by the uncertain nature of a turbulent flow and underplayed by the use of a mean drag or a mean lift force coefficient to connect the main trends of the two properties. However, simplification of this kind may obscure underlying mechanisms that influence particle dislodgement in a more subtle way. Seepage effects, for instance, may change local hydrodynamic features around mobile particles and yet have received less attention, partly because seepage is difficult to identify and quantify at the instant of dislodgement. This aspect would become more critical at near-threshold conditions where local turbulent structures causing intermittent particle movements could interact with seepage at various degrees and deliver much different results. Our lack of understanding on such phenomena can limit the access to better interpretation and more accurate prediction of particle dislodgement.

This study was intended to fill that gap by experimentally exploring seepage effects at the moments of particle dislodgement over a coarse granular bed. Note that seepage identified in our experiments occurred spontaneously in the vicinity of a single target particle, not introduced into the granular bed by any additional facility. Dislodgement of this target particle was limited to a low range of transport rates, slightly varied with the degrees of particle exposure considered. Observations and measurements were conducted through a particle tracking velocimetry (PTV) system at a sampling rate of 500 fps, sufficiently high to fully resolve dislodgement and ambient fluid motion simultaneously. The obtained velocity data was used to analyze a set of hydrodynamic properties, including turbulence intensity, turbulence kinetic energy transport, Reynolds shear stress, velocity and pressure quadrant distribution, and most important, the relative importance of seepage on particle dislodgement. The results served as evidence supporting that, though inconspicuous in most cases, seepage can play a significant role in particle dislodgement at near-threshold conditions by changing local turbulence features.

How to cite: Shih, W. and Zhang, K.-J.: The influences of seepage flow on particle dislodgement over a coarse granular bed, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12104, https://doi.org/10.5194/egusphere-egu23-12104, 2023.

The presence of a strong interaction between tectonic deformation and surface processes is widely recognized. Still, the nature of this interaction is difficult to unravel and quantify. In the last decades, analogue landscape evolution models have been widely implemented and employed in different tectonic settings, to complement field campaign studies. Since the aim of these analogue models is to help the interpretation of data coming from natural prototypes, it is important to test how well empirical erosional laws built upon natural landscapes, explain analogue model behavior. We perform a series of experiments for a straight interpretation of the relationship between applied boundary conditions and analogue landscape evolution. The selected analogue material is composed of 40 wt.% of silica powder, 40 wt.% of glass microbeads, and 20 wt.% of PVC powder. The analogue material fills a rectangular box (30×35×5 cm3) placed over a reclined table. Over the box, a series of sprinklers generate a dense mist (i.e., rainfall) that triggers surface processes. The boundary conditions applied to the models are the imposed slope of the reclined table and the rainfall rate. We test three rainfall rates (9, 22, and 70 mm h-1) and three imposed slopes (10, 15, and 20°), analyzing how the combination of these boundary conditions results in different landscape metrics (e.g., basins length, basins width, drainage area, channel slope, erosional efficiency) and erosion rates. Results show that in models affected by high rainfall rates (70 mm h-1), the implemented analogue material is characterized almost no channelization, and erosion acts uniformly and diffusively over the models’ surface. Lower rainfall rates (9, 22 mm h-1) allow more discrete channelization instead. On the other hand, as expected, the imposed slope controls the amount of incision, so that the volumes of material removed by erosion increase moving from 10° to 20°. However, even if the maximum incision is generally controlled by the slope, the coupling with rainfall rate tunes the effectiveness of erosion. In this work we compare the imposed boundary conditions with the corresponding erosion rates, using geomorphic markers and landscape metrics to define if and how natural erosional laws apply to analogue landscape evolution models.

How to cite: Reitano, R., Clementucci, R., Conrad, E. M., Corbi, F., Lanari, R., Faccenna, C., and Bazzucchi, C.: Stream erosion in analogue models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8247, https://doi.org/10.5194/egusphere-egu23-8247, 2023.

Steep channel networks commonly show a transition from constant-gradient colluvial channels associated with debris flow activity and concave-fluvial channels downstream. The trade-off between debris flow and fluvial erosion in steep channels remains unclear, which obscures connections among topography, tectonics, and climate in steep landscapes. We analyzed steep debris-flow-prone channels across the western US and observe: 1) similar maximum channel gradients across a range of catchment erosion rates and geologic settings; and 2) lengthening colluvial channels with increases in sediment grain size. The correspondence between sediment grain size and colluvial channel length led us to test a hypothesis that steep channel gradients are controlled by thresholds of motion for mass-wasting failure of channel bed sediment and thresholds of sediment motion by fluvial entrainment. We calculated discharges needed to mobilize sediment by both mechanisms across channel networks in the San Gabriel Mountains (SGM) and northern San Jacinto Mountains in southern California (NSJM). Across steep colluvial channels in both landscapes, sediment is more likely to be mobilized by mass-wasting processes, which are less-sensitive to sediment grain size, in agreement with observations from imagery bracketing storms. Discharges with decadal recurrences are below fluvial entrainment thresholds but above mass-wasting entrainment thresholds for D50 (median) sediment sizes in colluvial channels. In lower-gradient concave channels downstream, discharges in decadal storms exceed fluvial entrainment thresholds but are lower than mass-wasting entrainment thresholds for D50-sized sediment. In both landscapes, fluvial channels progressively steepen downstream compared to gradients needed to mobilize sediment cover, which we interpret to reflect downstream increases in sediment flux. Coarser sediment supply in the NSJM than the SGM increases threshold discharges needed to mobilize sediment by fluvial entrainment, which increases total channel relief in the NSJM by (1) extending colluvial channels shaped by debris flows and (2) increasing fluvial channel gradients. Together, our analyses show how differing sensitivity of fluvial and debris flow processes to sediment grain size impacts the partitioning of colluvial and fluvial regimes in headwater channel networks.

How to cite: Neely, A. and DiBiase, R.: Sediment controls on the transition from debris flow to fluvial channels in steep mountain ranges, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3633, https://doi.org/10.5194/egusphere-egu23-3633, 2023.

Channel networks exert a key control on drainage basins shape and dynamics in mountainous landscapes, including the transfer of water and sediment throughout basins, and thus hydrosedimentary hazards. Landscape dissection by channels results from the competition between hillslope processes and channelized erosion processes such as overland flow or debris flows. Although the competition between fluvial processes and hillslope processes has been modeled in simple cases, more data is still needed to constrain the localization and characteristic of this transition, especially in channels where debris-flows occur. High resolution LiDAR DEMs open new perspectives for the extensive extraction of channel heads. Several channel extraction methods exist but none is yet robust on fast eroding landscapes where channels are not always fluvial and sometimes initiate in rough bedrock areas.
Therefore we developed the CO²CHAIN method which identifies the hillslope to channel transition in drainage basins based on relative changes of local and upstream measures of flow convergence. We calibrate CO²CHAIN by fitting its results to channel head mapping made by geomorphologists on four contrasted basins in the United States and France. Compared to state-of-the-art channel extraction methods, and without any recalibration, it achieves similar and higher accuracy in moderate and high erosion-rate basins, respectively.
This allows us to identify the first order  channels (the most upstream part of mountain channels) to better understand how their morphology (slope, minimum drainage area, length) are linked to catchment mean erosion rates. We applied our method to a few LiDAR DEMs of mountain catchments with varied mean erosion rates and where debris flows have been identified. It appears that,  where channels begin, the drainage area and  slope  are correlated and this correlation depends on the catchment mean erosion rate and the local hilltop curvature. In order to understand what controls the transitions between hillslopes and debris flow channels, we also studied preliminarly the distribution of hillslope length and slope gradient and of channel gradient  throughout the catchments. This could give us insight into the processes shaping the bedrock channels and allow us to test long-term geomorphic models for debris flow erosion.

How to cite: Lurin, A., Marc, O., Meunier, P., and Carretier, S.: Detecting and characterizing the hillslope to channel transition in high erosion rate, bedrock landscapes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5533, https://doi.org/10.5194/egusphere-egu23-5533, 2023.

The west Coast of central Sumatra is situated above the Sumatra subduction zone and is characterized by short (< 100 km) and very steep (> 20°) bed rock channels draining the volcanic arc towards the Indian Ocean. Incision and fluvial adaptation of these catchments are commonly attributed to tectonic uplift along the subduction zone and emplacement of magma bodies within the volcanic arc. Recent recognition of a regional sea-level high stand between 4,500 to 5,000 years ago and the low shelf depth of ~70 m below sea level, may indicate that base-level fluctuations had a stronger control on river network dynamics than previously thought. Here, we explore the impact of these processes on the fluvial morphology and the incision history of 31 catchments draining the volcanic arc of west-central Sumatra. Landscape evolution simulations using PyBadlands, geomorphic metrics of the normalized-steepness index, knickpoint detection and c-analysis derived from 30 m SRTM-satellite data demonstrate that the morphometric response of smaller catchments (drainage area: < 500 km²) is different from that of larger catchments (drainage area: > 500 km²). The reduction in overall drainage area due to the last postglacial sea-level rise forced smaller catchments to oversteepen to adjust to the new conditions. Instead, larger catchments responded by upstream drainage area expansion and capture of previously eastward flowing rivers, thus maintaining an overall lower gradient. Furthermore, the mid-Holocene high-stand and subsequent sea-level drop resulted in a major regional base-level fall and creation of a knickzone along the entire West Coast of Sumatra that is currently migrating up the fluvial network and is located at an elevation of around 200 m. Our results imply that (1) the tempo of fluvial incision between catchments along the West Coast of Sumatra may be out of phase with the uplift of the volcanic arc; (2) the drainage area reduction due to postglacial sea-level rise controlled fluvial catchment evolution; and (3) we observe a catchment size depended threshold at which catchments either oversteepen or incise headward to adjust for drainage area loss. This response should be applicable to all natural occurring fluvial bedrock channels that experience drainage area loss and should be modulated by runoff and erodibility. The process is exceptionally well visible in Sumatra as there exists no orographic rainfall gradient along the mountain front.

Keywords: Postglacial sea-level rise, fluvial geomorphology, landscape evolution, erosion, subduction zone, SE Asia

How to cite: Rohrmann, A., Fischer, A., Hülscher, J., and Bernhardt, A.: Last postglacial sea-level fluctuations control fluvial morphodynamics along the West Coast of Sumatra, Indonesia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8352, https://doi.org/10.5194/egusphere-egu23-8352, 2023.

Bedrock rivers are the keystone to understanding landscape evolution as they control the rate of geomorphic responses to climatic and tectonic signals. Bedrock erosion is driven by channel hydraulics, which are not well understood for complex bedrock river morphologies. Some bedrock rivers exhibit a constriction-pool-widening morphology and associated plunging flow where the core of maximum velocity follows the bed into deep pools. Previous observations suggest that moderate discharges enhance the erosive potential of plunging flows, and that plunging flows are the dominant driver of morphology and downcutting in reaches where they are present. However, there are very few observations of plunging flows in natural environments, so their frequency and cumulative impact on landscape evolution is still unclear. Here we examine Acoustic Doppler Current Profiler and Multibeam Echosounder data collected in the Fraser Canyon of British Columbia, Canada to better understand the general properties and frequency of plunging flows. The entire 375 kms of the Fraser Canyon were analyzed for indicators of plunging flows to estimate their frequency. Results suggest that plunging flows appear to be abundant and are correlated with high shear stresses which are concentrated into deep bedrock pools. When examined using common erosion modelling techniques, observations suggest that reaches with plunging flows are incising at a much faster rate than non-plunging reaches. This work shows that considering reach scale hydraulics is critical for understanding the morphogenesis of large bedrock rivers and the mountainous landscapes that they drain. The abundance of plunging flows in large bedrock rivers suggests the importance of integrating complex flow patterns into bedrock erosion models, informs patterns of bedrock erosion at the reach scale, and begs for further investigation into the distribution of fluid shear stresses in complex bedrock channel morphologies.

How to cite: Hurson, M., Venditti, J., Rennie, C., and Church, M.: The abundance of plunging flows in bedrock rivers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8203, https://doi.org/10.5194/egusphere-egu23-8203, 2023.

On November 1^st, 2018 the Big Bar Landslide temporarily blocked Fraser River, the most productive salmon-bearing watershed in Canada, presenting a barrier to upstream salmon migration in 2019 and 2020.  The landslide is an example of an ecohazard, similar to a natural hazard, but with immediate and direct impacts on the biosphere, rather than on people and infrastructure.  Like natural hazards, ecohazards have cascading effects, where a geophysical process triggers additional events, often with dramatic consequences.  The Big Bar Landslide originated from collapse of a steep bedrock wall and deposited 89,000 m³ of rock into one of the narrowest sections of the Fraser River, damming the channel for over 7 hours, and impounding 650,000 m³ of water.  The rockfall debris formed a bank to bank step with an ‘overfall’ that was ~4 m at low flow and ~7 m at high flow.  The overfall resembled a waterfall, but without a freefall into a plunge pool.  A backwater formed upstream that extends ~750 m upstream at low flow, but several kilometers at high flow trapping incoming sediment.  The overfall generated a hydraulic barrier to upstream salmon passage, and significantly impeded salmon migration to the Upper Fraser Basin in 2019 and 2020, but rock work has partially ameliorated the impact.  Fish passage monitoring indicates success in passing the landslide in 2019 was species and discharge dependent with population-specific estimates ranging from <1% to over 80% success.  Passage success was particularly low for early timed populations exposed to the highest flow in 2019; so few fish successfully migrated to the spawning grounds that there was a risk of functional extinction of those runs.  There is an ongoing risk to all salmon populations above the landslide until hydraulic conditions at the slide stabilize.  The event will have cascading effects on upstream ecosystems, Indigenous peoples who rely on the salmon fishery throughout the Fraser River Basin, and commercial ocean fisheries, but the extent of the cascading effects is not yet known.  Collapse of bedrock canyon walls, like the one that started the ecohazard cascade in the Fraser River, are geologically commonplace, but the risk they pose to migratory fish populations in mountainous river systems is largely unknown.

How to cite: Venditti, J. G., Menounos, B., Heathfield, D., Patterson, D. A., Robinson, K. A., Moore, J. W., and Seagren, E. and the Big Bar Landslide Research Team: Ecohazard posed by a rockslide that blocked salmon migration in the Fraser River, British Columbia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5393, https://doi.org/10.5194/egusphere-egu23-5393, 2023.

Landslides are major drivers of landscape evolution. Mass-wasting events connect hillslope and channel processes through the downslope transferal of sediment, which can impact fluvial systems in a multitude of ways. Coarse sediment delivered into the channel can impact flow dynamics, alter river incision rates through the tools and cover effect, deflect reach-scale river alignment, and disrupt riverine ecosystems. The feedbacks between landslides, fluvial processes, and landscape evolution remain largely unexplored despite increasingly detailed landslide inventories, enabled by the availability of airborne lidar mapping and high-resolution topographic data. To better understand the nature of these feedbacks across varying lithologies, tectonic conditions and valley morphologies we explore post-glacial landslides (~14 ka to present) in the Fraser River valley in southwest British Columbia, Canada. We created a landslide inventory using existing literature and 2,560 km² of new airborne lidar along the Fraser Canyon corridor, a 375-km stretch of the Fraser River, which flows through multiple regions with distinct climate, morphology, and geology. We documented ~300 landslides with areas between 2 x 10³ and 2 x 10⁶ m². Failure types include translational bedrock slides, earthflows, and rock avalanches, which vary systematically with bedrock geology and valley morphometrics. We find more translational bedrock and earthflow failures in the broader, U-shaped valley of the northern Fraser River canyons, where sedimentary, metasedimentary, and volcanic bedrock are more common. Rock avalanches are more common in the southern Fraser River canyon, where valley walls are composed of plutonic and metamorphic rocks, however the southern region has fewer landslide features overall. These findings suggest individual failures and their combined impact on the post-glacial evolution of the Fraser River are sensitive to both the geologic and glacial history of a particular stretch of river.

How to cite: Steelquist, A., Seagren, E., Carr, J., Baird, K., Heathfield, D., Menounos, B., Larsen, I., Dingle, E., and Venditti, J.: Distribution and characteristics of landslides in the Fraser River Corridor, Southwestern British Columbia, Canada, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5346, https://doi.org/10.5194/egusphere-egu23-5346, 2023.

Each year a variable portion of adult Pacific salmon in the Fraser River, British Columbia, Canada die trying to retrace and ascend the river network to their natal spawning grounds. A major factor in migration failure is the severe hydraulic conditions experienced in the Fraser Canyon where encounter velocities can exceed upstream swim speeds of adult salmon, creating a migration barrier. Hydraulic barriers are defined as reaches of river where upstream fish migration is delayed due to high water velocity. A few barriers have been identified along the river and have structures in place designed to help facilitate fish passage. We explore other locations in the Fraser River that are apt to be hydraulic barriers to fish migration based on measured centerline velocity. We classify the barriers as either 1) plunging flows in canyons where the channel is deep and the fastest velocities are observed deep in the water column, 2) rapids where flow is fast and shallow over one or more bedrock steps, or 3) overfalls where fast flow occurs over a step with a substantial drop in elevation. We used drone footage at various discharges and Large-Scale Particle Image Velocimetry (LSPIV) to examine flow structure at typical plunging flows, rapids and overfalls. Surface velocities for the discharges when salmon species are known to be migrating upstream were then compared with published salmon swimming capabilities to determine which locations are likely to create the greatest barriers to salmon migration. We find that there are twenty-two sites, sixteen measured and six suspected high velocity locations, that are potential hydraulic barriers. Overfalls present the greatest barrier to salmon migration, creating vertical barriers in addition to high velocity across the entire width of the channel in narrow laterally constricted reaches. Rapids have high velocity in the segments of the water column where salmon typically swim, but often have back eddies along the banks for fish to rest. Plunging flows in canyons have high depth-averaged velocities, and higher bank velocities as a result of turbulent upwelling along the walls, but typically lower surface velocities than the overfalls and rapids. Pacific salmon populations are already threatened by external factors – such as climate change, habitat degradation, fishing, and disease – and cannot afford to have these impacts amplified by additional barriers to migration. Our observations provide important information for salmon conservation and can be used to better understand salmon migration which in turn helps to inform future mitigation efforts to improve salmon survival rates.

How to cite: Wright, M., Hurson, M., Patterson, D., Robinson, K., Baerg, J., and Venditti, J.: A typology of hydraulic barriers to salmon migration in a bedrock river, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2123, https://doi.org/10.5194/egusphere-egu23-2123, 2023.

Heavy rainfall events in mountainous areas can trigger thousands of destructive landslides, which pose a risk to people and infrastructure and significantly affect the landscape. In Nepal, monsoon rainfall triggers hundreds of landslides across the country every year and represent a large share of total sediment production. However, a persistent problem is that monsoon-induced landslides are mapped using multi-spectral satellite images, where cloud cover makes it impossible to constrain precisely landslide timing. This has hampered our understanding of how rainfall is driving landsliding during the monsoon and how the hillslope susceptibility to rainfall could be modulated after earthquakes.

Sentinel-1 SAR images offer a solution to this problem since SAR can be acquired through cloud cover, is sensitive to landslides, and the regular acquisition strategy of the satellite means that images are acquired every 12 days on two tracks globally. A newly developed method based on Sentinel-1 amplitude time series can be used to assign 12-day time windows for 30% of the landslides in an inventory (Burrows et al., NHESS, 2022).

We apply this method to optically-derived inventories of monsoon triggered landslides across Nepal from 2015-2019, obtaining timing information for hundreds of landslides during this period. We use this new landslide timing information alongside satellite rainfall data (GPM IMERG calibrated using rain gauges to better account for orographically-induced precipitation) to further our understanding of landslide triggering during the monsoon. We are able to identify spatio-temporal clusters of landslides that are concurrent with intense peaks in rainfall during the 2017 and 2019 monsoon seasons. This suggests that cloudburst events during the monsoon can drive a large share of the mass-wasting volume associated with a given year.

We also observe that, during the 2015 monsoon season, a large number of landslides failed earlier and after much less rainfall than in 2017-2019. This may reflect weakening of the hillslope following the M_w 7.8 Gorkha earthquake, which occurred around six weeks before the onset of the monsoon season in 2015, triggering co-seismic landslides across central and eastern Nepal and resulting in elevated numbers of landslides during the 2015 monsoon. Using the satellite rainfall data, we model the evolution of soil water content through time for every landslide. By using the modelled soil moisture at the time of failure in the Factor of Safety equation, we can obtain an estimate of cohesion for every landslide. By comparing cohesions for the 2015 dataset against those from 2017-2019, we suggest that the early landsliding in 2015 could be explained by a cohesion loss of 1-5 kPa.

How to cite: Burrows, K. and Marc, O.: Monsoon-triggered landslide timings derived from Sentinel-1 reveal cloudburst triggering and suggest earthquake-induce hillslope weakening, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9329, https://doi.org/10.5194/egusphere-egu23-9329, 2023.

Landslides are of wide interest because they shape topography and influence fluvial sediment transport. To investigate the duration of landsliding's influence on fluvial sediment discharge, we analyzed suspended sediment concentration measurements at 77 gauging stations across Taiwan over an 11-year period after Typhoon Morakot in 2009, the wettest typhoon on record in Taiwan which generated nearly 20,000 landslides. At each gauging station, we computed annual rating curves for suspended sediment concentration as a power-law function of the centered water discharge, which isolates the efficiency of suspended sediment discharge from temporal variations in water discharge. Among the 40 stations in basins that were strongly impacted by landsliding, the discharge-normalized rating curve coefficient ã increased by a mean factor of 4.89 within 1-2 years of Morakot, while the rating curve exponent b exhibited no systematic response to Morakot (mean factor of 1.21). Elevated values of ã declined exponentially at 26 of the 40 stations with a median characteristic timescale of 13.2 years (interquartile range: 7.6-20.7) and tended to respond faster in basins with more intense landsliding. In contrast, at the 37 stations that were not impacted by landsliding, neither ã nor b exhibited a systematic response to Morakot. To quantify the effect of landsliding on sediment discharge, we compared the measured sediment discharges after Morakot to the hypothetical sediment discharges that would have occurred if no change in sediment transport efficiency had occurred at the time of Morakot, calculated by applying the post-Morakot water discharge history to pre-Morakot rating curves. This analysis suggests that Morakot-induced landsliding increased sediment discharge by as much as >10-fold in some basins in southeast Taiwan in the 1-2 years after Morakot. Post-Morakot changes in ã were positively correlated with landslide intensity for approximately seven years after Morakot, and changes in b were negatively correlated from 2011 to 2014, indicating that the influence of landsliding on rating curves diminished within a few years. Together, these results suggest that Morakot-induced landsliding amplified fluvial sediment fluxes for a relatively short time (< 10 years). To the extent that these results are applicable to rivers in other landscapes, this suggests that rivers may more efficiently transport landslide-derived sediment for relatively short times.

How to cite: Ruetenik, G., Ferrier, K., and Marc, O.: Decadal scale influence of rainfall induced landslides on fluvial sediment transport following typhoon Morakot, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9763, https://doi.org/10.5194/egusphere-egu23-9763, 2023.

Slope failures and deep seated landslides are usually considered as the most efficient processes for hillslope erosion in active orogens. Erosion in the central Himalaya (Nepal) confirms such assertion (Morin et al., 2018), with in addition the probable predominance of the very large landslides in the erosive budget of the range (Marc et al., 2019).

In this contribution, we report geological evidence for a giant rockslide that occurred around 1190 AD (¹⁴C burial age, ³⁶Cl exposure age, and IRSL dating) in the Annapurna massif (central Nepal), and filled a wide glacial cirque (Sabche cirque) and the Seti valley farther downstream with a rock-avalanche deposit up to 1km thick, made of finely-crushed breccia. This rockslide, which involves a total rock volume of ~23 km³, decapitated the paleo-Annapurna IV, a paleo-summit culminating likely above 8000 m of altitude.

Such giant rockslides are rare but not uncommon in central Himalaya with former description of pluri-kilometric rockslides involving up to ten cubic kilometres mass wasting (e.g. Weidinger et al., 2002). They have major implications for landscape evolution since they could represent the main mode of erosion of the high glaciated peaks, leading to the sudden reduction of ridge crest elevation by several hundred meters.

They also have major implications on the fluvial network and how it responds to such massive and sudden supply of sediments. Downstream of the Seti river, the Pokhara Basin is filled by ~5 km³ of mostly conglomeratic sediment emplaced by fluvial, hyperconcentrated, or turbulent, sediment-laden flows. Extensive ¹⁴C dating of organic fragments found in the fine-grained units of the Pokhara sediments (Schwanghart et al., 2016; this study) provide robust constraints on the timing of aggradation. Onset of aggradation around 1200 AD (as well as the calcareous clasts whose only possible origin is the Sabche cirque) indicates that the conglomerates are the result of the active erosion of the Sabche rock-avalanche breccia. The aggradation lasted for about a century at an average rate of 1m/yr. During that period, the erosion of the fine-grained breccia has been extremely efficient and rapid: of the initial 27 km³ of rockslide debris, less than 10% can still be observed today in the Sabche cirque. During that period, the rate of sediment yield delivered by the Seti river was comparable to the highest measured values of post-volcanic eruption sediment transport. It led to an overwhelming content of carbonate-rich material in Narayani river sediments up to 300 km downstream, in the Narayani megafan in the middle of the Gangetic plain. Compared to the Narayani basin (30,000 km²), which exports annually ~0.05km³/yr of sediment, post-collapse erosion of the Sabche breccias (only 0.2% of the Narayani total basin area) would have increased this annual flux by a factor of 3 over 100-150 years.

This particular example illustrates how the erosional/sediment routing system in active mountain range, as well as the landscapes, can be durably affected by one single extreme event.

How to cite: Lavé, J., Guérin, C., Valla, P., France Lanord, C., Benedetti, L., Morin, G., and Galy, V.: Giant collapse of a high Himalayan peak and its major consequences downstream, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13230, https://doi.org/10.5194/egusphere-egu23-13230, 2023.