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The onset of glaciation in the late Cenozoic caused rapid bedrock erosion above the snowline; however, whether the influx of eroded sediment is recorded in continental weathering and basin accumulation rates is an ongoing debate. We propose that the transport of glacially eroded bedrock through the fluvial system damps the signal of rapid headwater erosion and results in steady basin‐integrated sediment flux. Using a numerical model with integrated glacial and fluvial erosion, we find that headwater bedrock erosion rates increase rapidly at the onset of glaciation and continue to fluctuate with climatic oscillation. However, bedrock erosion rates decrease in the downstream fluvial system because larger grain sizes from glaciers result in an increase in sediment cover effect. When erosion and sediment flux rates are averaged, long‐term sediment flux is similar to nonglacial flux values, while localized bedrock erosion rates in the glaciated landscape are elevated 2–4 times compared to nonglacial values. Our simulated values are consistent with field measurements of headwater bedrock erosion, and the pattern of sediment flux and fluvial erosion matches paraglacial theory and terrace aggradation records. Thus, we emphasize that the bedload produced from glacial erosion provides a missing link to reconcile late Cenozoic erosion records. Plain Language Summary: Glaciers are effective erosion agents and the start of mountain glaciation is thought to correspond to higher alpine erosion rates. However, at the landscape‐scale, there is little to no change in erosion rates across the start of glaciation. In order to connect across these scales, we use a computer model to calculate erosion rates and sediment transfer rates. We find that the increase in sediment size from glaciation will lower erosion rates in rivers. Larger sediment sits on the riverbed and protects the underlying rock from erosion. Despite rapid erosion by glaciers upstream of the river, the slower erosion rates in the river result in a constant rate of sediment transferred out. Therefore, sediment transfer from glacier to river systems is an important process for understanding erosion across landscape scales. Key Points: Bedrock erosion downstream of glaciers is damped through an increase in sediment cover effect caused by glacier‐produced sedimentBasin‐wide sediment flux is similar to unglaciated sediment flux at timescales of >100 kyGlacially derived bedload provides a missing link between rapid bedrock erosion rates and steady basin accumulation and weathering rates [ABSTRACT FROM AUTHOR]

Bedrock rivers are the pacesetters of landscape evolution in uplifting fluvial landscapes. Water discharge variability and sediment transport are important factors influencing bedrock river processes. However, little work has focused on the sensitivity of hillslope sediment supply to precipitation events and its implications on river evolution in tectonically active landscapes. We model the temporal variability of water discharge and the sensitivity of sediment supply to precipitation events as rivers evolve to equilibrium over 106 model years. We explore how coupling sediment supply sensitivity with discharge variability influences rates and timing of river incision across climate regimes. We find that sediment supply sensitivity strongly impacts which water discharge events are the most important in driving river incision and modulates channel morphology. High sediment supply sensitivity focuses sediment delivery into the largest river discharge events, decreasing rates of bedrock incision during floods by orders of magnitude as rivers are inundated with new sediment that buries bedrock. The results show that the use of river incision models in which incision rates increase monotonically with increasing river discharge may not accurately capture bedrock river dynamics in all landscapes, particularly in steep landslide prone landscapes. From our modeling results, we hypothesize the presence of an upper discharge threshold for river incision at which storms transition from being incisional to depositional. Our work illustrates that sediment supply sensitivity must be accounted for to predict river evolution in dynamic landscapes. Our results have important implications for interpreting and predicting climatic and tectonic controls on landscape morphology and evolution. Plain Language Summary: Rivers that carve rock are often used as markers of climate, tectonic, and rock‐type controls in evolving landscapes. To back out these controls from the present form of rivers which erode rock, thorough knowledge of how they evolve and how they respond to changes in climate and tectonics is required. Most models for the evolution of bedrock rivers assume that the rate of erosion always increases with river discharge, as the weight of water on the riverbed increases. Here, we challenge this assumption using a new model for river evolution which accounts for short‐timescale variations in river discharge and supply of sediment from hillslopes to channels. We find that in these systems, the largest floods may be less erosive than one would expect because they must transport large volumes of sediment, which makes it harder for them to erode fresh bedrock in channels. Our results are important because models of river evolution inform much of our understanding of interactions between Earth's surface and its atmosphere in rapidly changing parts of our planet. Key Points: We present a bedrock river evolution model that accounts for sediment supply sensitivity to precipitation events over long timescalesWhen sediment supply is highly sensitive to large precipitation events, bedrock incision is inhibited by sediment cover during floodsWe find evidence of an upper limit on water discharge for river incision driven by increased sediment cover in channels [ABSTRACT FROM AUTHOR]

Lithology is an important control on the efficiency of bedrock incision and thus the pace of landscape evolution. Rock strength is commonly considered the limiting lithologic factor that resists erosion. Yet, rock strength is a dynamic property that oscillates during advection of rock to the channel surface as damaging processes weaken rock and erosion exposes fresh rock. We approach the problem by investigating damage on bedrock surfaces that vary by the frequency they are eroded in channels of different lithologies. Our data set includes measurements of channel slope and width to characterize channel morphology, and Schmidt hammer rebound, P wave velocity, slake durability, and porosity to characterize the mechanical properties of channel surfaces. The average damage accumulation rate of lithologies ranges over 41% of the mean. We find a range of damage patterns among the different lithologies. Local surface damage increases with erosion frequency in channels comprised of coarse‐grained bedrock but decreases with erosion frequency in channels comprised of fine‐grained bedrock. We interpret these patterns to develop from lithological influences on weathering, abrasion, and the threshold of damage to erosion. The cross‐channel damage patterns between channel floors and margins are well correlated with stream power demonstrating links between microstructural rock properties, reach‐scale morphology, and landscape‐scale processes. We conclude that the morphodynamics of bedrock channels are sensitive to the lithological influences on the direction and magnitude of feedback in the coevolution of bedform morphology and the mechanical properties of the surface. Plain Language Summary: Rivers shape the landscape by carving out valleys into rock. Some types of rock are harder to carve than other types of rock, but it is not clearly understood why because there are many physical and chemical weathering processes that attack rock other than the river. Using several dry river channels, we take many measurements of rock strength along the bedrock surface that forms the river channels. We take advantage of the different channel surfaces that are either shielded or exposed to the downstream flow of river sediment to get a sense of how the bedrock is damaged by sediment impacts and by weathering. We find that the size and type of minerals that make up the bedrock influence the pattern of rock damage that develops through complex feedbacks that occur as bedrock material is either weakened or eroded thereby exposing fresher material that lies underneath. These damage patterns in turn control the shape of the channel, and therefore, the erosive potential of the flowing water. Our findings demonstrate how microscale rock properties can influence the shaping of landscapes. Key Points: Erosion frequency and surface damage modulate the spatial variability in mechanical properties of bedrock channelsGrain size and mineralogy control damage by weathering and abrasion, which influence the slope, width, and roughness of bedrock channelsThe relation between rock damage and rock‐erodibility is unique for different lithologies [ABSTRACT FROM AUTHOR]

In most landscape evolution models, extreme rainfall enhances river incision. In steep landscapes, however, these events trigger landslides that can buffer incision via increased sediment delivery and aggradation. We quantify landslide sediment aggradation and erosional buffering with a natural experiment in southern Taiwan where a northward gradient in tectonic activity drives increasing landscape steepness. We find that landscape response to extreme rainfall during the 2009 typhoon Morakot varied along this gradient, where steep areas experienced widespread channel sediment aggradation of >10 m and less steep areas did not noticeably aggrade. We model sediment export to estimate a sediment removal timeline and find that steep, tectonically active areas with the most aggradation may take centuries to resume bedrock incision. Expected sediment cover duration reflects tectonic uplift. We find that despite high stream power, sediment cover may keep steep channels from eroding bedrock for up to half of a given time period. This work highlights the importance of dynamic sediment cover in landscape evolution and suggests a mechanism by which erosional efficiency in tectonically active landscapes may decrease as landscape steepness increases. [ABSTRACT FROM AUTHOR]

Landscape morphology reflects drivers such as tectonics and climate but is also modulated by underlying rock properties. While geomorphologists may attempt to quantify the influence of rock strength through direct comparisons of landscape morphology and rock strength metrics, recent work has shown that the contact migration resulting from the presence of mixed lithologies may hinder such an approach. Indeed, this work counterintuitively suggests that channel slopes within weaker units can sometimes be higher than channel slopes within stronger units. Here, we expand upon previous work with 1-D stream power numerical models in which we have created a system for quantifying contact migration over time. Although previous studies have developed theories for bedrock rivers incising through layered stratigraphy, we can now scrutinize these theories with contact migration rates measured in our models. Our results show that previously developed theory is generally robust and that contact migration rates reflect the pattern of kinematic wave speed across the profile. Furthermore, we have developed and tested a new approach for estimating kinematic wave speeds. This approach utilizes channel steepness, a known base-level fall rate, and contact dips. Importantly, we demonstrate how this new approach can be combined with previous work to estimate erodibility values. We demonstrate this approach by accurately estimating the erodibility values used in our numerical models. After this demonstration, we use our approach to estimate erodibility values for a stream near Hanksville, UT. Because we show in our numerical models that one can estimate the erodibility of the unit with lower steepness, the erodibilities we estimate for this stream in Utah are likely representative of mudstone and/or siltstone. The methods we have developed can be applied to streams with temporally constant base-level fall, opening new avenues of research within the field of geomorphology. [ABSTRACT FROM AUTHOR]

Abstract: We use the two-dimensional (2D) electrical resistivity imaging (ERI) method to detect the contact between sediment and bedrock in a modern fluvial system. We performed eight 2D electrical resistivity surveys along the Peikang River in Kuohsing area, central Taiwan, with a pole–pole array of 1-m electrode spacing to produce a spatial resolution of about 1 m. The resistivity data were inverted into subsurface electrical structures using the least-squares inversion techniques. Then, the contact between the bedrock and sediment was found by using an image processing technique called Laplacian edge detection (LED), which represents an objective approach in interpreting various geophysical images. Numerical modeling demonstrates that isoresistivity lines are not in good agreement with the hypothetical interfaces between the bedrock and sediment, but the zero-lines of the Laplacian operation are strikingly accurate. Our results indicate a well-defined boundary in the resistivity structure that can be used to estimate the quantity of sediments covering bedrock, thus highlighting the utility of this technique in studies of landscape evolution, sediment transport, and sediment budgets. [Copyright &y& Elsevier]