The Art of Landslides: How Stochastic Mass Wasting Shapes Topography and Influences Landscape Dynamics

Bedrock landslides shape topography and mobilize large volumes of sediment. Yet, interactions between landslide-produced sediment and fluvial systems that together govern large-scale landscape evolution are not well understood. To explain morphological patterns observed in steep, landslide-prone terrain, we explicitly model stochastic landsliding and associated sediment dynamics. The model accounts for several common landscape features such as slope frequency distributions, which include values in excess of regional stability limits, quasi-planar hillslopes decorated with straight, closely spaced channel-like features, and accumulation of sediment in valley networks rather than on hillslopes. Stochastic landsliding strongly affects the magnitude and timing of sediment supply to the fluvial system. We show that intermittent sediment supply is ultimately reflected in topography. At dynamic equilibrium, landslide-derived sediment pulses generate persistent landscape dynamism through the formation and breaching of landslide dams and epigenetic gorges as landslides force shifts in channel positions. Our work highlights the importance of interactions between landslides and sediment dynamics that ultimately control landscape-scale response to environmental change

River inundation suggests ice-sheet runoff retention

AbstractThe Greenland ice sheet is experiencing dramatic melt that is likely to continue with rapid Arctic warming. However, the proportion of meltwater stored before reaching the global ocean remains difficult to quantify. We use NASA MODIS surface reflectance data to estimate river discharge from two West Greenland rivers – the Watson River near Kangerlussuaq and the Naujat Kuat River near Nuuk – over the summers of 2000–12. By comparison with in situ river discharge observations, ‘inundation–discharge’ relations were constructed for both rivers. MODIS-based total annual discharges agree well with total discharge estimated from in situ observations (86% of summer discharge in 2009 to 96% in 2011 at the Watson River, and 106% of total discharge in 2011 to 104% in 2012 at the Naujat Kuat River). We find, however, that a time-lapse camera, deployed at the Watson River in summer 2012, better captures the variations in observed discharge, benefiting from fewer data gaps due to clouds. The MODIS-derived estimates indicate that summer discharge has not significantly increased over the last decade, despite a strong warming trend. Also, meltwater runoff estimates derived from the regional climate model RACMO2/GR for the drainage basins are higher than our reconstructions of river discharge. These results provide indirect evidence for a considerable component of water storage within the glacio-hydrological system.</jats:p

Warming-driven erosion and sediment transport in cold regions

We synthesized a global inventory of cryosphere degradation-driven increases in erosion and sediment yield, e.g., suspended load, bedload, particulate organic carbon, and riverbank/slope erosion. This inventory includes 76 locations from the high Arctic, European mountains, High Mountain Asia and Andes, and 18 Arctic permafrost-coastal sites, and they were collected from ~80 studies

Mendenhall Glacier basal ice-sediment outcrop samples, 2021

We gathered field observations of debris-rich ice layers from an alpine glacier margin to help inform models of glacial sediment transport across landscapes. Basal ice and entrained sediment samples were taken from the margin of Mendenhall Glacier, Juneau, AK, in August 2021. Along the land-terminating margin of the glacier, we identified five representative outcrops of debris-rich ice with obvious stratigraphic layers. Samples were taken in duplicate at intervals from the base of the debris-rich layers to the clean ice above. After removing the outermost layer of sediment, which is typically enriched by surface melt, we weighed the samples in the field, then transported them back to the Sedimentology and Plant Lab at the Institute of Arctic and Alpine Research in Boulder, Colorado. In the lab, samples were freeze-dried to remove the ice fraction, then weighed again to obtain sediment mass, and thus sediment concentration

Doomed to drown? Sediment dynamics in the human-controlled floodplains of the active Bengal Delta

The Ganges-Brahmaputra-Meghna (Bengal) Delta in Bangladesh has been described as a delta in peril of catastrophic coastal flooding because sediment deposition on delta plain surfaces is insufficient to offset rates of subsidence and sea level rise. Widespread armoring of the delta by coastal embankments meant to protect crops from flooding has limited natural floodplain deposition, and in the tidally dominated delta, dikes lead to rapid compaction and lowered land surface levels. This renders the deltaic floodplains susceptible to flooding by sea level rise and storm surges capable of breaching poorly maintained embankments. However, natural physical processes are spatially variable across the delta front and therefore the impact of dikes on sediment dispersal and morphology should reflect these variations. We present the first ever reported sedimentation rates from the densely populated and human-controlled floodplains of the central lower Bengal Delta. We combine direct sedimentation measurements and short-lived radionuclides to show that transport processes and lateral sedimentation are highly variable across the delta. Overall aggradation rates average 2.3 ± 9 cm y–1, which is more than double the estimated average rate of local sea level rise; 83% of sampled sites contained sediment tagged with detectable 7 Be, indicating flood-pulse sourced sediments are widely delivered to the delta plain, including embanked areas. A numerical model is then used to demonstrate lateral accretion patterns arising from 50 years of sedimentation delivered through smaller order channels. Dominant modes of transport are reflected in the sediment routing and aggradation across the lower delta plain, though embankments are major controls on sediment dynamics throughout the delta. This challenges the assumption that the Bengal Delta is doomed to drown; rather it signifies that effective preparation for climate change requires consideration of how infrastructure and spatially variable physical dynamics influence sediment dispersal on seasonal and decadal time scales

Selected field photos from the Mendenhall Glacier in 2021

We gathered field observations of debris-rich ice layers from an alpine glacier margin to help inform models of glacial sediment transport across landscapes. Basal ice and entrained sediment samples were taken from the margin of Mendenhall Glacier, Juneau, AK, in August 2021. Along the land-terminating margin of the glacier, we identified five representative outcrops of debris-rich ice with obvious stratigraphic layers. Samples were taken in duplicate at intervals from the base of the debris-rich layers to the clean ice above. After removing the outermost layer of sediment, which is typically enriched by surface melt, we weighed the samples in the field, then transported them back to the Sedimentology and Plant Lab at the Institute of Arctic and Alpine Research in Boulder, Colorado. In the lab, samples were freeze-dried to remove the ice fraction, then weighed again to obtain sediment mass, and thus sediment concentration

Doomed to drown? Sediment dynamics in the human-controlled floodplains of the active Bengal Delta

The Ganges-Brahmaputra-Meghna (Bengal) Delta in Bangladesh has been described as a delta in peril of catastrophic coastal flooding because sediment deposition on delta plain surfaces is insufficient to offset rates of subsidence and sea level rise. Widespread armoring of the delta by coastal embankments meant to protect crops from flooding has limited natural floodplain deposition, and in the tidally dominated delta, dikes lead to rapid compaction and lowered land surface levels. This renders the deltaic floodplains susceptible to flooding by sea level rise and storm surges capable of breaching poorly maintained embankments. However, natural physical processes are spatially variable across the delta front and therefore the impact of dikes on sediment dispersal and morphology should reflect these variations. We present the first ever reported sedimentation rates from the densely populated and human-controlled floodplains of the central lower Bengal Delta. We combine direct sedimentation measurements and short-lived radionuclides to show that transport processes and lateral sedimentation are highly variable across the delta. Overall aggradation rates average 2.3 ± 9 cm y–1, which is more than double the estimated average rate of local sea level rise; 83% of sampled sites contained sediment tagged with detectable 7 Be, indicating flood-pulse sourced sediments are widely delivered to the delta plain, including embanked areas. A numerical model is then used to demonstrate lateral accretion patterns arising from 50 years of sedimentation delivered through smaller order channels. Dominant modes of transport are reflected in the sediment routing and aggradation across the lower delta plain, though embankments are major controls on sediment dynamics throughout the delta. This challenges the assumption that the Bengal Delta is doomed to drown; rather it signifies that effective preparation for climate change requires consideration of how infrastructure and spatially variable physical dynamics influence sediment dispersal on seasonal and decadal time scales

Water depth data for Elephant Butte Reservoir near the mouth of the Rio Grande (March and May 2022)

Water depth within the Rio Grande and Elephant Butte Reservoir was determined with a Lowrance Hook Reveal X Series single beam sonar attached to a packraft in March and May 2022. The purpose of this data collection effort was to determine the location of the subaqueous Rio Grande channel, as well as the depth from which to collect sediment-water samples

Pebble count data for two gravel bars on the Rio Grande above Elephant Butte Reservoir, New Mexico, USA (September 2022)

Wolman pebble counts were conducted on point bars along the Rio Grande channel upstream of Elephant Butte Reservoir in September 2022 to characterize the typical grain sizes of gravels that are commonly found on point bars, mid-channel bars, and on the Rio Grande channel bed (Wolman, 1954)