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The Laurentide Ice Sheet's (LIS) interior had a dynamic polythermal base, but the spatiotemporal variations of subglacial processes related to ice divide migration and other transient changes remain largely unknown, limiting our understanding of regional glacial dynamics. Previous studies focused on the regional glacial landform record, while ice sheet models lacked detailed parameterization within these regions, leading to an overestimation of cold-based subglacial conditions' extent and duration. In this study, glacial sediment dispersal patterns as identified by heavy minerals, clasts, and multivariate statistics of till matrix geochemistry were used to assess ice sheet dynamics within the Quebec–Labrador sector of the LIS. The earliest ice-flow phase produced and transported till across the study area (>175 km). However, major oxide data from till matrix geochemistry show a correlation with underlying bedrock, and this relationship is relatively common in areas of thin till cover and resistant bedrock lithologies. These results suggest a switch from an early phase of widespread erosion and long, sustained sediment transport to one of more limited erosion, perhaps abrasion dominant and shorter transport. Till compositional data and related dispersal patterns add supporting evidence to earlier ice sheet reconstructions based on ice-flow indicators and 10Be data together suggesting a transition from widespread uniform warm-based conditions during the earliest ice flow, followed by the development of an ice divide, its migration, and more sporadic warm-based conditions. Consequently, a thorough understanding of ice-flow history is essential for ice sheet modelling and future mineral exploration programs in inner ice sheet regions of the LIS. [ABSTRACT FROM AUTHOR]

The measurement of cosmogenic nuclide (CN) concentrations in riverine sediment has provided breakthroughs in our understanding of landscape evolution. Yet, linking this detrital CN signal and relief evolution is based on hypotheses that are not easy to verify in the field. Models can be used to explore the statistics of CN concentrations in sediment grains. In this work, we present a coupling between the landscape evolution model Cidre and a model of the CN concentration in distinct grains. These grains are exhumed and detached from the bedrock and then transported in the sediment to the catchment outlet with temporary burials and travel according to the erosion–deposition rates calculated spatially in Cidre. The concentrations of various CNs can be tracked in these grains. Because the CN concentrations are calculated in a limited number of grains, they provide an approximation of the whole CN flux. Therefore, this approach is limited by the number of grains that can be handled in a reasonable computing time. Conversely, it becomes possible to record part of the variability in the erosion–deposition processes by tracking the CN concentrations in distinct grains using a Lagrangian approach. We illustrate the robustness and limitations of this approach by deriving the catchment-average erosion rates from the mean 10Be concentration of grains leaving a synthetic catchment and comparing them with the erosion rates calculated from sediment flux, for different uplift scenarios. We show that the catchment-average erosion rates are approximated to within 5 % uncertainty in most of the cases with a limited number of grains. [ABSTRACT FROM AUTHOR]

To investigate the influence of bedrock fragmentation by intensive tillage on the hydrological characteristics and soil erosion processes on slopes, two experimental treatments (soil–bedrock mixtures, WB, and pure soil, CK) in steel tanks were subjected to simulated rainfall under five rainfall patterns (constant, increasing, decreasing, decreasing–increasing, and increasing–decreasing) with the same total rainfall of 90 mm. For each rainfall event, runoff and sediment concentration were sampled at regular intervals. The flow velocity (v), effective/critical shear stress (τ/τc), Darcy–Weisbach resistance coefficient (f), unit stream power (p), and soil erodibility factor (Kr) were calculated to analyze the differences in hydrodynamic characteristics between the WB and CK. Our experimental findings show that significant differences in runoff volume and sediment yield were observed among different rainfall patterns and stages. Bedrock fragmenting significantly promoted runoff and sediment production under different rainfall patterns, with runoff volume and sediment yield increasing by averages of 59.29% and 71.62%, respectively. An increasing trend in average contribution rate of bedrock to runoff volume and sediment yield was observed across three distinct intensities: 6.37% and 4.61% for 30 mm h−1, 12.53% and 7.53% for 90 mm h−1, as well as 14.79% and 36.98% for 150 mm h−1, respectively. The v and p values under various rainfall patterns exhibited an increasing trend from the upper to the bottom slope positions, whereas the f and τ values showed an opposite trend, regardless of the WB and CK. Compared with the CK, the v, f, and p values for the WB increased by 23.34% to 48.94%, 1.59% to 53.16%, and 3.86% to 27.86%, respectively, whereas the τ value decreased by 1.52% to 22.19% for varying-intensity rainfall patterns. Among the variable rainfall patterns, the WB significantly increased sediment yield and also had a promoting effect on runoff generation. However, the WB displayed better erosion resistance compared to the CK under constant rainfall patterns. Therefore, varied-intensity patterns had a profound impact on bedrock-induced runoff and sediment transport processes. [ABSTRACT FROM AUTHOR]

Glaciers expel sediment as they melt, in addition to ice and water. As a result, changing glacier dynamics and melt produce changes to glacier erosion and sediment discharge, which can impact the landscape surrounding retreating glaciers, as well as communities and ecosystems downstream. Currently, numerical models that transport subglacial sediment on sub-hourly to decadal scales are one-dimensional, usually along a glacier's flow line. Such models have proven useful in describing the formation of glacial landforms, the impact of sediment transport on glacier dynamics, and the interactions among climate, glacier dynamics, and erosion. However, these models omit the two-dimensional spatial distribution of sediment and its impact on sediment connectivity – the movement of sediment between its detachment in source areas and its deposition in sinks. Here, we present a numerical model that fulfills a need for predictive frameworks that describe subglacial sediment discharge in two spatial dimensions (x and y) over time. SUGSET_2D evolves a two-dimensional subglacial till layer in response to bedrock erosion and changing sediment transport conditions. Numerical experiments performed using an idealized alpine glacier illustrate the heterogeneity in sediment transport and bedrock erosion below the glacier. An increase in sediment discharge follows increased glacier melt, as has been documented in field observations and other numerical experiments. We also apply the model to a real alpine glacier, Griesgletscher in the Swiss Alps, where we compare outputs with annual measurements of sediment discharge. SUGSET_2D accurately reproduces the general quantities of sediment discharge and the year-to-year sediment discharge pattern measured at the glacier terminus. The model's ability to match the measured data depends on the tunable sediment grain size parameter, which controls subglacial sediment transport capacity. Smaller grain sizes allow sediment transport to occur in regions of the bed with reduced water flow and channel size, effectively increasing sediment connectivity into the main channels. The model provides the essential components of modeling subglacial sediment discharge on seasonal to decadal timescales and reveals the importance of including spatial heterogeneities in water discharge and sediment transport in both the x and y dimensions in evaluating sediment discharge. [ABSTRACT FROM AUTHOR]

Glaciers are commonly conceptualized as bodies composed of snow and ice. Yet, many glaciers contain a substantial amount of rock, especially those abutting steep mountains. Mountain slopes erode, depositing rocks on glaciers below. This loose rock (or debris) is buried in glaciers and melts out lower down creating a debris cover. Debris cover reduces ice melt, which changes the shape and movement of glaciers. Glacier movement, specifically basal sliding, efficiently sculpts landscapes. To date, we know little about the impacts of surface debris on conditions below glaciers. To help remedy this, we run numerical model simulations which show that debris‐covered glaciers erode slower than glaciers unaffected by debris. Reduced melt under surface debris lowers sliding speeds and causes sediment to accumulate at the bed, potentially establishing conditions for surging. The influence of surface debris cover on the subglacial environment may hold substantial implications for alpine sediment storage and landscape evolution. Plain Language Summary: Glaciers are imagined as bodies composed entirely of snow and ice. But many glaciers, especially those in steep mountains contain a substantial amount of rock as well. Rocks are deposited on glaciers from steep mountain slopes above them. This loose rock (or debris), buried by snow, can completely cover glaciers' lower reaches as it melts out of the ice. Debris reduces melt, which changes the glacier shape and in turn how glaciers move down valley. To test how debris‐covered glaciers erode and sculpt landscapes we use numerical models. Our simulations show that debris‐covered glaciers erode landscapes at lower rates than glaciers unaffected by debris. The reduced amount of melt from debris cover causes glaciers to slide slower and causes sediment to accumulate under debris‐covered ice, shielding bedrock from erosion. The tendency for debris‐covered glaciers to build sediment layers at their bed may lead to the rapid speed up and advancement of glaciers, also known as surges. The influence of surface debris cover on the subglacial environment may hold substantial implications for alpine sediment storage and landscape evolution. Key Points: Erosion is reduced under debris‐covered ice because of compounding effects between reduced surface melt and the subglacial environmentTill accumulates under debris‐covered ice because surface melt is suppressed by surface debris coverDebris cover on glaciers promotes subglacial fluvial conditions that can store sediment in alpine environments, altering erosional patterns [ABSTRACT FROM AUTHOR]

In steep landscapes, river incision sets the pace of landscape evolution. Transport of coarse sediment controls incision by evacuating material delivered to river channels by landslides. However, large landslide-derived boulders that impede bedrock erosion are immobile even in major runoff-driven floods. Glacial lake outburst floods (GLOFs) mobilize these boulders and drive incision, yet their role in regional-scale erosion is poorly understood, largely because of their rarity. Here, we find a topographic signature consistent with widespread GLOF erosion in the Nepal Himalaya. Our interpretations emerge from the analysis of normalized channel steepness patterns, knickpoint distributions, and valley wideness. In rivers with glaciated headwaters that generate GLOFs, valleys stay narrow and relatively free of sediment, with bedrock often exposed to erosion. In turn, tributaries to these valleys are steep, allowing less efficient erosional regimes to keep pace with GLOF-driven incision. Where GLOFs are less frequent, valleys are more alluviated and incision stalls. Our results suggest that the extent of headwater glaciation may play an important role in the erosion of Himalayan river valleys and deserves more attention in future work. [ABSTRACT FROM AUTHOR]

Modeling the dynamics of alluvial rivers is theoretically and numerically more challenging than modeling erosion of bedrock channels in active mountain ranges. As a consequence, the majority of the approaches developed in the context of alluvial rivers are one-dimensional. However, recent advances in the numerics of fluvial landform evolution models allow for two-dimensional simulations of erosion and sediment transport over time spans of several million years. This study aims at finding out fundamental properties of rivers in a tectonically inactive foreland of a mountain range by investigating a simple reference scenario theoretically and numerically. This scenario consists of a mountain range and a foreland in a quasi-steady state wherein the material eroded in the mountain range is routed through the foreland. In order to understand the properties of foreland rivers, a subdivision into two classes – carriers and redistributors – is introduced. Carriers originate in the mountain range and are thus responsible for the large-scale sediment transport to the ocean. In turn, redistributors are rivers whose entire catchment is located in the foreland. Using the concept of carriers and redistributors, it is shown that the drainage network in the foreland permanently reorganizes so that a steady state in the strict sense is impossible. However, the longitudinal profiles of carriers are described well by a steady-state approximation. Their concavity index is considerably greater than that of rivers in the mountain range. Carriers predominantly deposit sediment at high rates, while redistributors erode at much lower rates. Despite the low erosion rates, the sediment flux from redistributors into carriers is a major component of the overall sediment budget and finally the main driver of the highly dynamic behavior of the carriers. [ABSTRACT FROM AUTHOR]

Open conduit modeling of cave stream floods can yield useful information about water velocities and shear stresses, which can in turn be used to estimate sediment transport capabilities. All such calculations require roughness coefficients for estimating energy losses and a priori knowledge of either discharge or flow depths to set model boundary conditions. However, the difficulties associated with observing in-cave floods generally preclude measuring discharge; roughness coefficients must be assumed based on channel properties. To overcome these challenges, we monitored stream flow depths in Fullers Cave, Greenbrier County, West Virginia using pressure transducers, and simultaneously measured stage and discharge in a karst window immediately upstream of the cave. Five pressure transducers were deployed opportunistically along a 93-meter-long reach in a 10+ meter high canyon averaging 1.5 to 3 meters wide. Stage-discharge relationships were determined for the karst window using an electromagnetic flow meter for floods with peak discharges of 1.66 m3 s-1 or less. The collected data was used to obtain the empirical Manning's n roughness values, head losses, and energy gradients. Calculated floodwater velocities are comparable to values obtained from scallops on passage walls. Major energy losses were observed where breakdown partially occludes the passage. At peak flow, Manning n values average 0.053 for reaches typified as cobble-floored canyons, but n was 0.069 in the breakdown reach. Roughness values declined exponentially with increasing discharge, but friction slopes calculated using head losses show more complex relationships with discharge. Notably, n values back calculated using bed gradients differ from those calculated using measured head losses by as little as 12%, so the use of bed gradients in roughness estimations will generally yield reasonable approximations of flow conditions. Fullers Cave experiences significantly larger open conduit floods than we observed, so additional work is needed to estimate roughness coefficients for higher discharges. Our empirical roughness coefficients can be applied to similar cave passages in other caves and contexts, including modeling slot canyonlike channels, and our methods demonstrate a technique for measuring hard to obtain data. The addition of data for open conduit conduits significantly expands the range of environments that can be modeled using empirical data beyond pipe-full caves. Applications include studying flooding, sediment transport, and bedrock erosion process. All of these topics will be addressed in Fullers in the future. [ABSTRACT FROM AUTHOR]

Bedrock river-gorge incision represents a fundamental landscape-shaping process, but a dearth of observational data at >10 yr timescales impedes understanding of gorge formation. I quantify 102 yr rates and processes of gorge incision using historical records, field observations, and topographic and image analysis of a human-caused bedrock meander cutoff along the North Fork Fortymile River in Alaska (USA). Miners cut off the meander in 1900 CE, abruptly lowering local base level by 6 m and forcing narrowing and steepening of the channel across a knickpoint that rapidly incised upstream. Tectonic quiescence, consistent rock erosivity, and low millennial erosion rates provide ideal boundary conditions for this 102 yr gorge-formation experiment. Initial fast knickpoint propagation (23 m/yr; 1900-1903 CE) slowed (4 m/yr; 1903-1981 CE) to diffusion (1981-2019 CE) as knickpoint slope decreased, yielding an ~350-m-long, 6-m-deep gorge within the pre-1900 CE channel. Today, diffusion dominates incision of a 500-m-long knickzone upstream of the gorge, where sediment transport likely limits ongoing adjustments to the anthropogenic cutoff. Results elucidate channel width, slope, discharge, and sediment dynamics consistent with a gradual transition from detachment- to transport-limited incision in fluvial adjustment to local base-level lowering. [ABSTRACT FROM AUTHOR]

The Labyrinth in the McMurdo Dry Valleys of Antarctica is characterized by large bedrock channels emerging from beneath the margin of Wright Upper Glacier. To study the morphodynamics of large subglacial channels cut into bedrock, we develop herein a numerical model based on the classical theory of subglacial channels and recent results on bedrock abrasion by saltating bed load. Model results show that bedrock abrasion in subglacial channels with pressurized flow reaches a maximum at an intermediate distance up-ice from the glacier snout for a wide range of sediment grain sizes and sediment loads. Close to the snout, the velocity is too low and the sediment particles cannot be mobilized. Far from the snout, the flow accelerates and sediment is transported in suspension, thus limiting particle impacts at the channel bottom and reducing abrasion. This non-monotonic relationship between subglacial flow and bedrock abrasion produces concave up bottom profiles in subglacial channels and potential cross-section constrictions after channel confluences. Both landforms are present in the bedrock channels of the Labyrinth. We therefore conclude that these geomorphic features are a possible signature of bedrock abrasion, rather than glacial scour, and reflect the complex interplay between transport rate, sediment load, and transport capacity in subglacial channels. [ABSTRACT FROM AUTHOR]

Waterfall plunge pools experience cycles of sediment aggradation and scour that modulate bedrock erosion, habitat availability, and hazard potential. We calculate sediment flux divergence to evaluate the conditions under which pools deposit and scour sediment by comparing the sediment transport capacities of waterfall plunge pools (Qsc_pool) and their adjacent river reaches (Qsc_river). Results show that pools fill with sediment at low river discharge because the waterfall jet is not strong enough to transport the supplied sediment load out of the pool. As discharge increases, the waterfall jet strengthens, allowing pools to transport sediment at greater rates than in adjacent river reaches. This causes sediment scour from pools and bar building at the downstream pool boundary. While pools may be partially emptied of sediment at modest discharge, floods with recurrence intervals >10 yr are typically required for pools to scour to bedrock. These results allow new constraints on paleodischarge estimates made from sediment deposited in plunge pool bars and suggest that bedrock erosion at waterfalls with plunge pools occurs during larger floods than in river reaches lacking waterfalls. [ABSTRACT FROM AUTHOR]

• Overlapping crater populations constrain the total era of martian valley formation. • Martian valleys may have formed intermittently over hundreds of millions of years. • Long-term erosion rates would have been similar to the modern Atacama Desert. A number of studies have constrained the cessation of martian valley network activity to around the Noachian-Hesperian boundary, but the total time period over which these features formed remains poorly constrained. Most attempts to ascertain valley network formative timescales rely on an intermittency factor that varies widely for terrestrial rivers and is unknown for Mars. In this contribution, superposition relationships are used to identify crater populations that pre- and postdate valley network incision, resulting in maximum cumulative valley formation times of ∼108 years. These timescales correspond with long-term erosion rates of ∼0.25 m/Myr, similar to those calculated for the Middle to Late Noachian as well as the central Atacama Desert since the Late Miocene. Assuming sediment transport rates previously calculated for other martian valley networks, this implies a minimum fluvial intermittency of ∼10−5, several orders of magnitude lower than modern day Earth rivers but similar to values independently calculated for some martian fluvial deposits. This low intermittency suggests that martian valley incision may have been significantly hindered by erosion into existing bedrock or by boulder armoring of the valley floor. Alternatively, unlike river valleys on Earth, runoff events driving martian valley network activity may have been non-seasonal, Milankovitch-like cycles, with long eras of quiescence between periods of intensified fluvial activity. [ABSTRACT FROM AUTHOR]

This study examines the role of topography as a dynamic boundary condition that limits the efficiency of fluvial erosion in the post-glacial Teton Range landscape. The volume of sediment currently stored in two major catchments was estimated using high-resolution LiDAR and geometric reconstructions of depth to bedrock. Seismic reflection data in Moran Bay reveals post-glacial sediment preserved behind a submerged moraine, which isolates the bay from the larger Jackson Lake depocenter. The volume of post-glacial sediment stored in the canyons and bay totals 173.82 ± 19.5336.0 (×10−3 km3), which translates to a catchment-wide sediment production rate of 0.17 ± 0.02 mm/yr. The rock-equivalent sediment volume in Moran Bay is 4.4 ± 0.9 (×10−3 km3), only ∼2.6% of the total post-glacial volume. While the estimated sediment production rate in the canyons is similar to the uplift rate, the denudation rate derived from Moran Bay sediment is 0.004 ± 0.001 mm/yr, implying highly inefficient post-glacial sediment transport. The fluvial system has been disequilibrated by glacial erosion such that interglacial valley profiles lack the steepness needed to transport sediment, delaying sediment evacuation until the next glacial advance, or until uplift sufficiently steepens the fluvial system so that it regains efficiency. Furthermore, colluvial production rates in the deglaciated valleys are close to long-term denudation and uplift rates, suggesting that once topography has been equilibrated to glacial erosion processes, subsequent glaciers do not need to produce much bedrock erosion, but mainly sweep out accumulated sediment to maintain equilibrium. • Sediment budget quantified for two catchments in the Teton Range, Wyoming. • Flattened post-glacial topography limits efficient fluvial erosion. • Sediment transport delayed until next glacial advance or uplift steepens system. • Colluvial production rates are close to long-term denudation and uplift rates. • Subsequent glaciers may not need to produce much bedrock erosion. [ABSTRACT FROM AUTHOR]

Natural bedrock rivers exhibit diverse erosional morphologies. Although the formation of alternate bars on bedrock has been noted in previous studies, the influence of these alternate bars on bedrock erosion has not been clarified. In this study, the authors propose a model for bedrock-alluvial channels that reproduces both bar formation and erosional morphology. In addition, the authors report on numerical simulations to evaluate the influence of sediment supply on the state of the bed at and over the bedrock surface. The numerical results illustrate the formation of different morphologies for different supply rates. When the sediment supply rate is close to transport capacity, mixed alluvial-bedrock alternate bars form. These bars are analogous to purely alluvial alternate bars. A meandering thread of alluvial material migrates downstream over a uniformly eroding bedrock surface. When the sediment supply rate is well below capacity, however, multiple incisional troughs (grooves) form on the bedrock surface, and alluvial cover tends to concentrate in the troughs. [ABSTRACT FROM AUTHOR]

Bedrock erosion in rivers sets the pace of landscape evolution, influences the evolution of orogens and determines the size, shape and relief of mountains. A variety of models link fluid flow and sediment transport processes to bedrock incision in canyons. The model components that represent sediment transport processes are increasingly well developed. In contrast, the model components being used to represent fluid flow are largely untested because there are no observations of the flow structure in bedrock canyons. Here we present a 524-kilometre, continuous centreline, acoustic Doppler current profiler survey of the Fraser Canyon in western Canada, which includes 42 individual bedrock canyons. Our observations of three-dimensional flow structure reveal that, as water enters the canyons, a high-velocity core follows the bed surface, causing a velocity inversion (high velocities near the bed and low velocities at the surface). The plunging water then upwells along the canyon walls, resulting in counter-rotating, along-stream coherent flow structures that diverge near the bed. The resulting flow structure promotes deep scour in the bedrock channel floor and undercutting of the canyon walls. This provides a mechanism for channel widening and ensures that the base of the walls is swept clear of the debris that is often deposited there, keeping the walls nearly vertical. These observations reveal that the flow structure in bedrock canyons is more complex than assumed in the models presently used. Fluid flow models that capture the essence of the three-dimensional flow field, using simple phenomenological rules that are computationally tractable, are required to capture the dynamic coupling between flow, bedrock erosion and solid-Earth dynamics. [ABSTRACT FROM AUTHOR]

Many rivers in Taiwan have steep slopes, are subject to typhoon-induced flood flows, and contain soft bedrock that is exposed at many locations and easily erodible. The occurrence of extensive bedrock erosion has been a major threat to river infrastructure at many locations. Soft bedrock erosion, therefore, is an important process to consider for river projects in Taiwan. In this study, bedrock erosion models are reviewed. A specific model is proposed by combining two existing models incorporating both the hydraulic and abrasive scour mechanisms. The proposed bedrock erosion model is incorporated into a two-dimensional mobile-bed model, and the integrated model is tested by simulating bedrock erosion downstream of the Chi-Chi weir on the Choshui River in Taiwan. A calibration study is performed to determine appropriate values of the model parameters based on two and a half years of measured data. The model is then assessed based on a verification study that compares model predictions of bedrock erosion of the same reach to two additional years of measured data. The bedrock erosion model is found to be suitable for the river reach studied. Further improvement, however, is still necessary, which points to potential future research. [ABSTRACT FROM AUTHOR]