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Tectonic and geomorphic processes drive landscape evolution over different spatial and temporal scales. In mountainous environments, river incision sets the pace of landscape evolution, and hillslopes respond to channel incision by, e.g., gully retreat, bank erosion, and landslides. Sediment produced during stochastic landslide events leads to mobilization of soil and regolith on the slopes that can later be transported by gravity and water to the river network during phases of hillslope–channel geomorphic coupling. The mechanisms and scales of sediment connectivity mitigate the propagation of sediment pulses throughout the landscape and eventually drive the contribution of landslides to the overall sediment budget of mountainous catchments. However, to constrain the timing of the sediment cascade, the inherent stochastic nature of sediment and transport through landsliding requires an integrated approach accounting for different space scales and timescales. In this paper, we examine the sediment production on hillslopes and evacuation to the river network of one landslide, i.e. the Schimbrig earthflow, affecting the Entle River catchment located in the foothills of the Central Swiss Alps. We quantified sediment fluxes over annual, decadal, and millennial timescales using respectively unmanned aerial vehicle (UAV)–structure-from-motion (SfM) techniques, classic photogrammetry, and in situ produced cosmogenic radionuclides. At the decadal scale, sediment fluxes quantified for the period 1962–1998 are highly variable and are not directly linked to the intensity of sediment redistribution on the hillslope. At the millennial scale, landslide occurrence perturbs the regional positive linear relationship between sediment fluxes and downstream distance as the landslide-affected Schimbrig catchment is characterized by a decrease in sediment fluxes and a strong variability. Importantly, the average decadal sediment flux of the Schimbrig catchment is 2 orders of magnitude higher than millennial sediment fluxes computed over the same spatial extent. The discrepancy between decadal and millennial sediment fluxes, combined to the highly variable annual sediment evacuation from the hillslopes to the channel network suggest that phases of hillslope–channel geomorphic coupling are short and intermittent. During most of the time, the first-order catchments are transport-limited and sediment dynamics in the headwaters are uncoupled from the fluvial systems. In addition, our unique spatio-temporal database of sediment fluxes highlights the transient character of the intense geomorphic activity of the Schimbrig catchment in a regional context. Its decadal sediment flux is of the same order of magnitude as the background sediment flux going out of the entire Entle River catchment. Over the last 50 years, the Schimbrig catchment, which represents ca. 1 % of the entire study area, provides 65 % of the sediments that the entire Entle catchment will supply over the millennial scale. These results suggest that episodic supply of sediment from landslides during intermittent phases of hillslope–channel geomorphic coupling are averaged out when considering sediment fluxes at longer timescales and larger spatial scales. [ABSTRACT FROM AUTHOR]

The western Peruvian region is prone to erosion and geomorphic change. Extreme precipitation events lead to rapid change in river channel and floodplain morphology due to bank erosion and debris flows delivering detrital material to the fluvial system. Monitoring geomorphic events and their associated topographic changes at high spatial and temporal resolutions remains a challenge. Here, we used an Uncrewed Aerial Vehicle - Post-Processing Kinematic - Structure from Motion (UAV-PPK-SfM) approach that includes co-registration of point clouds by using relative Ground Control Points (GCPs). This workflow adjusts each elevation model to a reference model using invariant features that did not change their position or form over time. We applied this technique to monitor landscape change (2019–2021) in an area of 0.3 km2 located in the Cañete River basin. Our results showed that a minimum observable elevation change of 0.56 m (95 % confidence interval) can be achieved using this workflow, beyond which an actual elevation change can be separated from systematic error. Using object-based classification techniques on the aerial images, we separated geomorphic dynamics from land cover changes. This allowed us to isolate the effect of geomorphic processes, and quantify rates related to gully erosion, river scouring, bank erosion, and sediment deposition. Within the study area, a hotspot of geomorphic change corresponded to an ephemeral tributary channel. The gully channel incising an alluvial fan is highly dynamic, showing bank erosion of 0.75 to 3.2 m and net export of 37 m3 of sediment in the 25-month study period. Given that the monitoring period did not include high intensity rainfall events, the study illustrates how geomorphic activity in ungauged Andean river basins, such as the Cañete valley, may be considerably underestimated in literature. [Display omitted] • Western Peruvian Andes is prone to rapid geomorphic change in ephemeral channels. • Hotspots of surface elevation change are detectable using UAV-PPK-SfM workflow. • Object-based classification techniques separate geomorphic dynamics from land cover changes. • Co-registration outperforms image co-alignment techniques in dynamic environments. • Specific sediment yields of about 9500 t km−2 yr−1 due to river undercutting and bank erosion in ephemeral channels. [ABSTRACT FROM AUTHOR]

In-situ cosmogenic radionuclides (CRN) have become a widely used tool in the geomorphic community to constrain geomorphic process rates. In many applications, catchment-wide denudation rates have been derived from in-situ produced 10Be concentrations in samples of river sand. These applications assume that sediment production and delivery rates within the catchment are reasonably steady and that river sediment is well mixed and all parts of the catchment are represented in proportion to their denudation rates. These assumptions are not necessarily met in active mountain ranges where deep-seated landslides or gully systems are contributing with deeply shielded soil or regolith to the river network. In this study, we analyse the variation in CRN-derived denudation rates as a function of the magnitude and frequency of geomorphic events. We first modified the CRN dynamics model of Yanites et al. (2009) to represent landslides as a proper spatial Poisson process and then adapted the code for the open-source programming language R. Then, we designed scenarios with different landscape and landslide configurations, taking into account the following four variables: the drainage area, the background erosion rate, the landslide return period and the landslide area distribution coefficient. The latter defines the shape of the magnitude-frequency landslide distribution curve, i.e. the probability of occurrence of landslides of a given magnitude. Finally, we run the scenarios for a time span of 200 kyr, and iterated 100 times each scenario to simulate the intrinsic variability of geomorphic events in landslide-prone landscapes. All scenarios show that the CRN-derived denudation rates are equal or higher than the volumetrically-derived denudation rates. The overestimation of CRN-derived denudation rates is larger when large landslides are more likely to occur and the landslide return period is shorter. An increase in background erosion reduces the landslide effect on CRN surface and river sediment concentrations. In addition, the stochastic approach shows that the variability of CRN-derived denudation rates is strongly controlled by the landslide area distribution coefficient and landslide return period. As such, the magnitude-frequency distribution of landslides influences both the variability and the accuracy of CRN-derived denudation rates. Our results show that this parameter, along with the landslide return period, should be taken into account when assessing the validity of CRN-derived denudation rates. [ABSTRACT FROM AUTHOR]