The idea that moving sediment drives bedrock erosion is not new. It was proposed qualitatively in the nineteenth century and confirmed by laboratory flume experiments in recent decades. What has been missing, the authors argue, is a complete coupling between the fate of individual grains and the erosion they cause. Bedrock erosion produces new sediment, depends on grains being in motion, and is simultaneously prevented when grains come to rest and blanket the bed. A river’s ability to cut rock is therefore inseparable from the full life cycle of its sediment load — erosion, transport, deposition and re-entrainment — and previous models handled that entanglement only partially, often with ad hoc corrections.

The core of the new theory is a deceptively simple question: when a grain is deposited on the bed, does it get lifted back into the flow, or does it strike the bedrock and chip it away? The researchers formalise this choice with a partitioning coefficient that describes the proportion of time the river spends eroding its sediment cover rather than abrading bedrock. When the flow is strong enough to sweep away everything that settles, every deposited grain is lifted back up, and the energy that would otherwise go into the cover instead goes into the rock. When deposition outpaces re-erosion, sediment accumulates, the bedrock is shielded, and abrasion stops.

From this partitioning, the team derives a key dimensionless quantity they call the bedrock coefficient: the volumetric fraction of bedrock eroded by each sediment impact, expressed as the ratio between the energy delivered by a grain and the rock’s resistance to abrasion. Combined with a transport length — the characteristic distance over which a river adjusts its sediment load toward equilibrium between erosion and deposition — this coefficient governs how quickly sediment-starved reaches approach capacity. Notably, the bedrock transport length that emerges from the equations is typically far longer than the ordinary bedload transfer length, meaning abrasion-driven rivers adjust over much greater distances than purely alluvial ones.

One of the study’s most satisfying results is theoretical rather than numerical. The classic cover-effect formulation of Sklar and Dietrich, published in 2004, describes bedrock erosion as proportional to the shortfall between actual sediment load and transport capacity. That expression was derived empirically, from flume observations. Davy and colleagues show that their coupled equations reproduce exactly the same cover term — but now as a consequence of sediment mass balance rather than curve fitting. The old empirical rule, in other words, turns out to be a natural outcome of treating grains as objects that must be lifted before they can impact again.

The framework also extends into two dimensions, incorporating lateral erosion of channel walls and lateral deposition on the floodplain edges of the channel. When the researchers analysed a river fed by uniform sediment supply from hillslopes, they found something counterintuitive: the sediment load can settle at a value that exceeds the classical transport capacity. Capacity, in this picture, is not an intrinsic property of the flow alone but depends on how much material the surrounding landscape delivers. For the South Fork Eel River in California, a case study borrowed from earlier abrasion work, the effect of hillslope supply becomes significant only when hillslope erosion rates approach metres per year — but the mathematics makes clear that supply and capacity cannot be treated independently.

To bring the theory to life, the team implemented the equations in their numerical platform River.lab/eros, which tracks small packets of water and sediment — dubbed precipitons — as they travel downstream solving the shallow-water equations and exchanging sediment with the bed and banks at every step. The approach lets channel width emerge from the simulation rather than being imposed, and it resolves both vertical incision and lateral erosion and deposition. Because realistic abrasion rates are roughly a million times slower than sediment transport, the authors exploit the quasi-steady behaviour of the sediment system to scale simulation times with rock resistance.

The showcase application is the Rheinfall at Schaffhausen, Switzerland — a twenty-metre-high knickpoint on the Hochrhein that formed about 15,000 years ago and has barely moved since. Using a lidar-derived digital elevation model at six-metre resolution and an inflow of 370 cubic metres per second, the team ran simulations with varying sediment concentrations. When the bedrock was treated as easily erodible sediment, the knickpoint simply diffused away, its slope flattening in place without upstream migration. But when abrasion governed the erosion, something dramatic happened: narrow canyons, thirty to fifty metres wide, propagated upstream from the base of the waterfall, carrying the knickpoint’s shape with them almost intact.

The sediment supply proved to be the master variable. With no incoming sediment, the knickpoint eroded only briefly, consuming the pre-existing patches of sediment on the bed before grinding to a halt — a result that mirrors the real Rheinfall, whose outflow from Lake Constance is virtually sediment-free because the lake traps Alpine debris. At low to moderate concentrations, the knickpoint retreated upstream while keeping its height and steep slope of roughly ten percent. At higher concentrations, sediment deposition raised the base of the fall and flattened it. In every case, erosion first carved a narrow canyon — where reduced width amplifies shear stress and impact rates — and the river widened only after the knickpoint had passed, a two-stage evolution the authors attribute to any contrast between bedrock and sediment erodibility.

Quantitatively, knickpoint retreat rates scaled with sediment flux to the power of about 0.8, and decreased with rock resistance faster than inversely — a preliminary result the authors treat cautiously, since canyon geometry and grid resolution may confound the scaling. The broader message is harder to dispute: whether a waterfall retreats as a migrating cliff or dissolves as a diffusing bump depends on the delicate balance between grains being lifted and grains striking rock. By grounding that balance in explicit sediment dynamics, the new model gives landscape evolution studies a physically consistent way to read the erosion histories written in bedrock rivers — and explains why some of the world’s great waterfalls hold their ground while others march relentlessly upstream.

Subject of Research: Coupled modelling of sediment-driven bedrock incision and knickpoint retreat in rivers

Article Title: Lift or impact: modelling bedrock incision coupled with sediment dynamics

Article References: Davy, P., Schwanghart, W., Mey, J., Darcel, C., & Landgraf, A. (2026). Lift or impact: modelling bedrock incision coupled with sediment dynamics. Earth Surface Dynamics, 14(4), 635-651. https://doi.org/10.5194/esurf-14-635-2026

Image Credits: AI Generated

DOI: 10.5194/esurf-14-635-2026

Keywords: bedrock incision, sediment transport, abrasion, knickpoint, cover effect, landscape evolution, river erosion, transport capacity, Rheinfall, canyon formation, geomorphology, numerical modelling

The research, led by Gunnel Göransson and colleagues at the Swedish Geotechnical Institute, examines the Göta River, the Nors River, the Säve River and the Ångerman River, all of which flow through fine-grained or mixed sediments where landslide susceptibility is high. The five case studies, conducted between 2009 and 2022, were carried out within a national programme for assessing and mapping future landslide hazards along watercourses. Together they form one of the most sustained efforts anywhere in the world to connect climate-driven fluvial erosion with probabilistic geotechnical slope-stability analysis.

The methodological framework that emerged was iterative, refined case by case, but consistently followed seven steps: compiling previous studies and measurements, conducting hydroacoustic surveys and sediment investigations, running hydrodynamic models to derive erosion parameters, selecting future flow scenarios, modelling erosion, validating results and assessing uncertainty, and finally integrating erosion forecasts into geotechnical stability analyses. Multibeam echosounder surveys mapped bathymetry, side-scan sonar characterized bedforms, backscatter analysis classified sediments, and physical sampling determined how erodible the riverbed materials actually were.

Hydrodynamic modelling sat at the heart of each assessment. The teams used two-dimensional models, including Delft3D, TELEMAC-2D, MIKE 21 C and MIKE 21 FM, to simulate water levels, flow velocities and bed shear stresses under a range of hydrological conditions. Two-dimensional modelling was deliberately chosen as the best balance between computational demand, data requirements and accuracy; three-dimensional simulation, while more detailed, was not considered justified given the uncertainties inherent in forecasting erosion over nearly a century. Erosion estimates rested on the relationship between calculated bed shear stresses, critical shear-stress thresholds and sediment erodibility coefficients, with cohesive sediments handled through the Partheniades formulation within a GIS framework.

The choice of erosion model depended on the geology. In clay-dominated systems such as the Göta, Nors and Säve rivers, the GIS-based analyses were supplemented with the Bank Stability and Toe Erosion Model, known as BSTEM, which explicitly couples hydraulic toe erosion with geotechnical bank-failure mechanisms. The Ångerman River, by contrast, is dominated by frictional sediments such as silt, sand and gravel, so the team adopted a full morphodynamic approach using the MIKE 21 C multi-fraction model, representing sediment transport and channel evolution explicitly. Future flows were derived from downscaled RCP climate projections supplied by the Swedish Meteorological and Hydrological Institute, alongside alternative hydropower regulation strategies.

The erosion projections were then translated into future channel cross-sections that served as direct input to slope-stability calculations performed with Slope/W, following Swedish geotechnical practice. Both present-day and year-2100 conditions were analysed, accounting for projected changes in groundwater, pore-water pressures and erosion-modified slope geometry. To capture uncertainty, the team applied the Point Estimate Method, a computationally efficient alternative to Monte Carlo simulation, deriving failure probabilities from statistical distributions of shear strength, unit weight, pore pressure and geometry. These probabilities were classified into five classes and combined with consequence classes in a GIS-based risk matrix to produce landslide risk maps sensitive to climate change.

Among the most striking findings is the role of hydropower regulation. In heavily regulated rivers, erosion driven by operational flow management can exceed that caused by climate-related changes in discharge, potentially obscuring the climate signal altogether. On the Ångerman River, short-term regulation generated rapid fluctuations in water level and flow velocity that dwarfed projected climate-driven hydrological changes. The study also revealed that the temporal resolution of discharge data matters enormously: simulations based on 14-day averaged flows suggested reduced future erosion, while high-resolution data capturing short-duration flow peaks indicated the opposite, because those peaks contribute disproportionately to bed shear stress but vanish when flows are averaged.

The synthesis also clarified how erosion interacts with inherent susceptibility. In areas already highly prone to landslides, even minor erosion can significantly raise the probability of a catastrophic failure, whereas in stable terrain substantial erosion is needed before risk begins to climb. Sediment composition determines which climate-related driver dominates: in cohesive clay valleys such as those of the Göta, Nors and Säve rivers, fluvial erosion is the dominant climate-related trigger of slope instability, while in the coarser deposits of the Ångerman valley, changes in groundwater and pore-water pressure are likely to matter more.

The authors are candid about uncertainty. Scarcity of sediment-transport measurements, limited repeated bathymetric surveys, positional inaccuracies on steep underwater slopes and the sheer difficulty of simulating long-term fluvial geomorphology all constrain quantitative precision. Their response is a call for recurrent, systematic monitoring, particularly comprehensive bathymetric surveys, and for assessments that are treated as living documents, updated regularly and immediately after any landslide occurs, since such an event would fundamentally reshape river morphology. The projections are best read as semi-quantitative tools for identifying erosion-prone areas and prioritizing detailed investigations rather than as precise predictions.

Perhaps the most valuable export of the study is its transferable workflow. Because it is grounded in fundamental principles of hydrology, hydraulics, sediment transport and erosion, the framework can be adapted beyond Sweden by adjusting the hydrological forcing, whether the driver is monsoon rainfall, tropical cyclones, drought or rapid glacier retreat. The authors emphasize that success depends on interdisciplinary collaboration across geomorphology, hydrology, sediment transport, hydraulics and geotechnics, and on adaptive rather than static management. As extreme precipitation intensifies worldwide, the lesson from the Swedish rivers is clear: the ground beneath riverside communities is being reshaped now, and the only responsible way to plan for it is to model, monitor and adapt continuously.

Subject of Research: Projecting climate-induced river erosion to assess future landslide susceptibility in Swedish river valleys

Article Title: Projecting climate-induced river erosion for assessing future landslide susceptibility: methodological insights and lessons learned from five Swedish cases

Article References: Göransson, G., Odén, K., Bergdahl, K., & Bolin, P. (2026). Projecting climate-induced river erosion for assessing future landslide susceptibility: methodological insights and lessons learned from five Swedish cases. Environmental Earth Sciences, 85(15), Article 396. https://doi.org/10.1007/s12665-026-13128-4

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13128-4

Keywords: river erosion, landslide susceptibility, climate change, slope stability, hydrodynamic modelling, sediment transport, bathymetry, hydropower regulation, geotechnical risk, Sweden, Göta River, morphodynamics