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The battering of bedrock by bedload collisions is the primary mechanism by which bedrock rivers erode and landscapes evolve. The energy imparted via impacts acts to detach bedrock via the growth and intersection of surface fractures. We present impact experiments designed to test the influence of particle impact energy on bedrock erosion rates. We found that erosional efficiency increased with increasing impact energy. Notably, these increases in efficiency are not captured by a widely‐used mechanistic bedrock erosion model. Observed increases in erosional efficiency were linked with enhanced elastic energy dissipation captured by differences in the coefficient of restitution. We suggest that this increase in energy dissipation is indicative of enhanced crack extension for high velocity impacts. Our experiments indicate a clear energy‐dependence for bedrock detachment processes that is not yet captured by bedrock incision models but may be integrated into long‐term erosion rates and landscape evolution. Plain Language Summary: Bouncing sediment grains batter the bedrock beds of rivers during floods. As these impacts collide with the bed, they slowly chip away at the riverbed bedrock. It's perhaps intuitive that a grain with more energy when it collides with the riverbed is able to chip away more bedrock. But, how much more destructive are these high energy impacts? We designed an experiment where we hit a bedrock sample with a steel ball at a range of impact energies, over and over again, up to 30,000 times, and measured how much material was chipped away. High energy impacts eroded more material, but this increased efficiency of erosion is not fully captured by impact‐based erosion models. We hypothesize that for higher energy impacts, a larger fraction of the incoming energy is used to crack the bedrock, more easily eroding material. We liken the relative influence of these high energy impacts to a battering ram, where only a few strikes can easily break down a door, whereas lower energy impacts, similar to a simple knock, are unlikely to achieve the same result. We suggest that this effect may be important in determining the influence of extreme events on bedrock erosion over long timescales. Key Points: Impact experiments demonstrate that bedrock erosional efficiency is enhanced with increasing impact energyExperimentally‐derived coefficients of restitution and rock resistance coefficients indicate enhanced elastic energy dissipation with increasing impact energyResults suggest that more energetic impacts enhance crack extension, increasing erosional efficiency [ABSTRACT FROM AUTHOR]

Abstract The battering of bedrock by bedload collisions is the primary mechanism by which bedrock rivers erode and landscapes evolve. The energy imparted via impacts acts to detach bedrock via the growth and intersection of surface fractures. We present impact experiments designed to test the influence of particle impact energy on bedrock erosion rates. We found that erosional efficiency increased with increasing impact energy. Notably, these increases in efficiency are not captured by a widely‐used mechanistic bedrock erosion model. Observed increases in erosional efficiency were linked with enhanced elastic energy dissipation captured by differences in the coefficient of restitution. We suggest that this increase in energy dissipation is indicative of enhanced crack extension for high velocity impacts. Our experiments indicate a clear energy‐dependence for bedrock detachment processes that is not yet captured by bedrock incision models but may be integrated into long‐term erosion rates and landscape evolution.

Large, rarely mobile boulders are observed globally in mountainous bedrock channels. Recent studies suggest that high concentrations of boulders could be associated with channel morphological adjustment. However, a process‐based understanding of large boulder effects on channel morphology is limited, and data are scarce and ambiguous. Here, we develop a theory of steady‐state channel width and slope as a function of boulder concentration. Our theory assumes that channel morphology adjusts to maintain two fundamental mass balances: (a) grade, in which the channel transports the same sediment flux downstream despite boulders acting as roughness elements and (b) bedrock erosion, by which the channel erodes at the background tectonic uplift rate. Model predictions are normalized by a reference, boulder‐free channel width and slope, accounting for variations due to sediment supply, discharge, and lithology. Models are tested against a new data set from the Liwu River, Taiwan, showing steepening and widening with increasing boulder concentration. Whereas one of the explored mechanisms successfully explains the observed steepening trend, none of the models accuratly account for the observed width variability. We propose that this contrast arises from different adjustment timescales: while sediment bed slope adjusts within a few floods, width adjustment takes a much longer time. Overall, we find that boulders represent a significant perturbation to fluvial landscapes. Channels tend to respond by forming a new morphology that differs from boulder‐free channels. The general approach presented here can be further expanded to explore the role of other hydrodynamic effects associated with large, rarely mobile boulders. Plain Language Summary: Large boulders are a significant feature in mountainous landscapes. Recent studies suggested that boulders residing in rivers interfere with the flow and sediment transport, forcing their geometry, specifically width and slope, to change. Our ability to understand and predict such changes is challenged by scarce field data and a general lack of models capable of explaining the processes underlying channel geometry adjustment in the presence of boulders. Here, we develop a theory and several models for the variation of channel width and slope as with channel boulder coverage. Our theory builds on the assumption that the geometry of boulder‐bed channels evolves to a new configuration to maintain steadiness of erosion rate and sediment transport. Predictions from the various models are tested against data from the steep Liwu River in Taiwan. These data show that width and slope increase with more boulders. We find that channel slope increases to overcome the greater resistance to sediment transport due to the boulders. In contrast, the scattered nature of the width data and the overall models inability to explain width variability likely reflect a longer adjustment period for width than for slope. This study demonstrates the important role of boulders in shaping landscapes. Key Points: We develop a theory for steady‐state reach‐scale channel morphology responding to large, rarely mobile boulders in bedrock riversPredictions of boulder‐bed channel width and slope are derived based on grade equilibrium and bedrock erosional balanceTheory is tested against new data from the Liwu River, Taiwan, showing steepening and widening with increasing boulder concentration [ABSTRACT FROM AUTHOR]

Bedrock river erosion is driven by channel hydraulics, which are not well understood for complex morphologies. Many bedrock rivers exhibit a constriction‐pool‐widening (CPW) morphology associated with submerged plunging flows. These flows cause velocity profile inversions resulting in high velocities near the bed and low velocities on the water surface. The first observations documenting plunging flows were from relatively low discharge, and it is unclear whether they persist during floods. Here we show that plunging flows persist and get stronger at flood discharge, increasing bedrock erosion potential by particle impacts. Flood‐discharge plunging flows contact the bed and maintain high velocities farther through the CPW structure, and evacuate large volumes of sediment from the pools. These reach‐scale processes are not represented in large‐scale landscape evolution models, yet these erosion mechanisms set the pace of landscape evolution, begging for a re‐evaluation of process representation in landscape evolution models. Plain Language Summary: Incision in bedrock rivers sets the pace of landscape evolution by controlling the rate of geomorphic responses to climatic and tectonic signals, yet the processes driving incision occur at much finer scale than those captured by landscape evolution models. Local bedrock river incision is driven by flow structures that are not well understood. Rivers typically flow fastest near the surface and slowest near the bed, but many bedrock rivers have channel morphologies that cause this velocity/depth relation to invert. The fastest‐flows submerge toward the bed enhancing near‐bed velocities, sediment transport, and consequently the potential for bedrock incision by particle impacts. However, the first observations of these "plunging flows" were from relatively low discharges and it is not clear if they persist during floods. Here we show that plunging flows get stronger during floods, which clears sediment cover that protects the underlying bedrock and increases bedrock incision potential. The length of the plunging flows matches their coincident pools which are common features of bedrock rivers, explaining why these pools exist. Formation of deep scour pools by complex flow structures in bedrock‐confined rivers is the mechanism that drives incision, begging for a re‐examination of the models used to explore landscape evolution. Key Points: Plunging flows occurring in bedrock rivers increase near‐bed velocity, sediment transport, and rock erosion potential in scour poolsPlunging flows persist and get stronger as the discharge and flow velocity increasePlunging flows at moderate discharges remove sediment cover, exposing bedrock surfaces to bedload impacts [ABSTRACT FROM AUTHOR]

This is at a low resolution, like all other figures embedded in this manuscript for reviewer convenience. High resolution illustrations are uploaded to Catena server. [Display omitted] • First investigation of dirt cracking in a temperate Mediterranean (Csb) climate. • Carbonate wedging and fracture fill wetting and drying widens fractures. • 87Sr/86Sr ratios indicate calcium in carbonate coatings comes from host rock. • Nanoscale HRTEM imagery reveals dissolution co-occurs with carbonate precipitation. • Lead deposits from automobile emissions used as a time diagnostic. Dirt cracking physically wedges open fractures in desert bedrock via the synergistic processes of carbonate precipitation and the wetting and drying of clays in fracture fill. We use back-scattered and high-resolution electron microscopy along with 87Sr/86Sr analyses to find that dirt cracking also occurs in the hypermaritime Mediterranean climate of Victoria, British Columbia, Canada. 87Sr/86Sr analyses of calcium carbonate from four different sampling locations do not reflect an ocean source, but 87Sr/86Sr ratios correspond with the known composition of the sampled rocks. Carbonates derived from decay of host-rock minerals precipitate in narrow fractures, widening them. In addition to calcium carbonate, iron carbonate, barium carbonate, and calcium phosphate also precipitate in fractures. Dissolution of silicates, in association with carbonate precipitation, aids in the further penetration of carbonates. Fines falling into fissures includes smectites that undergo wetting (expansion) and drying (contraction) that further promotes fracture widening. Lead contamination, probably from mid-20th century automobile emissions, occurs from heavy metal scavenging by iron oxides. As studies of dirt cracking are few in number, and since the carbonate precipitation process has been observed in Antarctica, monsoonal Asia, and now in a Mediterranean climate, it is possible that dirt cracking is a common rock-decay process in biogeochemical settings dry enough for carbonate precipitation in rock fissures. [ABSTRACT FROM AUTHOR]