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Abstract: Predicting thresholds of sediment motion is critical for a range of applications involving sediment transport. However, thresholds for sediment motion can vary over an order of magnitude for a single characteristic flow and bed configuration. Lacking simple ways to incorporate this variability, many assume thresholds are constant for rough, turbulent flow. Here, we quantify variability of incipient‐motion thresholds based on a commonly used grain‐scale force‐balance model, with model parameter distributions determined from published experiments. We show that variability in the threshold of motion within the 2D force‐balance model occurs predominantly due to variability in the lift coefficient and grain protrusion, and secondarily due to drag coefficient variability. For a known grain size, the mean threshold of motion, and variability about the mean, can be predicted from a family of power laws. These power laws can be altered with site‐specific parameter distributions, allowing for site‐specific application to well‐studied reaches and other planets. Using compiled flume and field data we show that constraining force‐balance parameter distributions with independent data results in narrower distributions of the predicted threshold of motion, consistent with constrained flume experiments. This analysis highlights that while the threshold of sediment motion is variable, the magnitude of variability is predictable within the force‐balance model based on site‐specific physical constraints of local flow and bed conditions.

River networks are composed of a mainstem and tributaries. These tributaries dissect landscapes, regulate water and habitat availability, and transport sediment and nutrients. Despite the importance of tributaries, we currently lack theory and data describing whether and how tributary length and spacing varies within watersheds, thereby limiting our ability to accurately describe river network geometry. We address this knowledge gap by analyzing 4,696 tributaries across six landscapes with varying climate, tectonic setting, and lithology. Our results show that both tributary length and spacing systematically increase with downstream distance along the mainstem river, following a power-law scaling. This power-law scaling can be modulated by basin shape, with tributaries becoming shorter and, in some cases, more closely spaced as basin elongate. Furthermore, the power-law scaling may break down in cases where river networks have been disturbed by pervasive faulting, raising the possibility that the scaling we observe is not unique to all branching networks, and instead may be universal across undisturbed fluvial networks. These findings can be used to improve predictions of river network geometry and potentially to distinguish fluvial river networks from other branching networks. [ABSTRACT FROM AUTHOR]

Waterfalls can form due to external perturbation of river base level, lithologic heterogeneity, and internal feedbacks (i.e., autogenic dynamics). While waterfalls formed by lithologic heterogeneity and external perturbation are well documented, there is a lack of criteria with which to identify autogenic waterfalls, thereby limiting the ability to assess the influence of autogenic waterfalls on landscape evolution. We propose that autogenic waterfalls evolve from bedrock bedforms known as cyclic steps and therefore form as a series of steps with spacing and height set primarily by channel slope. We identified 360 waterfalls split between a transient and steady-state portion of the San Gabriel Mountains in California, USA. Our results show that while waterfalls have different spatial distributions in the transient and steady-state landscapes, waterfalls in both landscapes tend to form at slopes >3%, coinciding with the onset of Froude supercritical flow, and the waterfall height to spacing ratio in both landscapes increases with slope, consistent with cyclic step theory and flume experiments. We suggest that in unglaciated mountain ranges with relatively uniform rock strength, individual waterfalls are predominately autogenic in origin, while the spatial distribution of waterfalls may be set by external perturbations. [ABSTRACT FROM AUTHOR]

Quantifying the time scales of sediment transport and storage through river systems is fundamental for understanding weathering processes, biogeochemical cycling, and improving watershed management, but measuring sediment transit time is challenging. Here we provide the first systematic test of measuring cosmogenic meteoric Beryllium‐10 (10Bem) in the sediment load of a large alluvial river to quantify sediment transit times. We take advantage of a natural experiment in the Rio Bermejo, a lowland alluvial river traversing the east Andean foreland basin in northern Argentina. This river has no tributaries along its trunk channel for nearly 1,300 km downstream from the mountain front. We sampled suspended sediment depth profiles along the channel and measured the concentrations of 10Bem in the chemically extracted grain coatings. We calculated depth‐integrated 10Bem concentrations using sediment flux data and found that 10Bem concentrations increase 230% from upstream to downstream, indicating a mean total sediment transit time of 8.4 ± 2.2 kyr. Bulk sediment budget‐based estimates of channel belt and fan storage times suggest that the 10Bem tracer records mixing of old and young sediment reservoirs. On a reach scale, 10Bem transit times are shorter where the channel is braided and superelevated above the floodplain, and longer where the channel is incised and meandering, suggesting that transit time is controlled by channel morphodynamics. This is the first systematic application of 10Bem as a sediment transit time tracer and highlights the method's potential for inferring sediment routing and storage dynamics in large river systems. Plain Language Summary: Understanding how long sediment takes to travel downstream in rivers, also known as sediment "transit time," is crucial for responsible watershed management and constraining global biogeochemical cycles. We aim to measure transit times for large rivers and determine the processes regulating this time scale. We present a new transit time proxy based on beryllium‐10 (10Be), a rare isotope produced in the atmosphere and delivered to Earth by rain. If river sediment collected downstream has more 10Be than sediment upstream, this indicates that sediment was trapped in the floodplain for many years before continuing to travel downstream. We collected river sediment at multiple locations from upstream to downstream along a large, undammed river, the Rio Bermejo in Argentina. We found that 10Be increased 230% from upstream to downstream, translating to a transit time of ~8,500 years. This long time scale implies that sediment and carbon delivered to rivers have enough time to be weathered or oxidized to CO2 before they are buried in the ocean. We also show that transit time is controlled by the river's shape and lateral mobility, suggesting that human alteration of channel shape or sediment supply will reduce a river's ability to use its floodplain for natural flood and erosion control. Key Points: Meteoric 10Be measured in depth‐integrated river suspended sediment geochemically records mean fluvial sediment transit timeIn the Rio Bermejo (Argentina), 10Be concentrations increase 230% over ~1,300 km transit distance, yielding a mean total transit time of 8.4 ± 2.2 × 103 yrReach‐scale analysis of 10Be‐derived transit times shows that tectonics, channel migration rates, and incision depth are primary drivers [ABSTRACT FROM AUTHOR]

Upstream knickpoint propagation is an important mechanism for channel incision, and it communicates changes in climate, sea level, and tectonics throughout a landscape. Few studies have directly measured the long-term rate of knickpoint retreat, however, and the mechanisms for knickpoint initiation are debated. Here, we use cosmogenic ³He exposure dating to document the retreat rate of a waterfall in Ka'ula'ula Valley, Kaua'i, Hawai'i, an often-used site for knickpoint-erosion modeling. Cosmogenic exposure ages of abandoned surfaces are oldest near the coast (120 ka) and systematically decrease with upstream distance toward the waterfall (<10 ka), suggesting that the waterfall migrated nearly 4 km over the past 120 k.y. at an average rate of 33 mm/yr. Upstream of the knickpoint, cosmogenic nuclide concentrations in the channel are approximately uniform and indicate steady-state vertical erosion at a rate of ~0.03 mm/yr. Field observations and topographic analysis suggest that waterfall retreat is dominated by block toppling, with sediment transport below the waterfall actively occurring by debris flows. Knickpoint initiation was previously attributed to a submarine landslide ca. 4 Ma; however, our dating results, bathymetric analysis, and landscape-evolution modeling support knickpoint generation by wave-induced sea-cliff erosion during the last interglacial sea-level highstand ca. 120-130 ka. We illustrate that knickpoint generation during sea-level highstands, as opposed to the typical case of sea-level fall, is an important relief-generating mechanism on stable or subsiding steep coasts, and likely drives transient pulses of significant sediment flux. [ABSTRACT FROM AUTHOR]

Fluvial bedrock incision sets the pace of landscape evolution and can be dominated by abrasion from impacting particles. Existing bedrock incision models diverge on the ability of sediment to erode within the suspension regime, leading to competing predictions of lowland river erosion rates, knickpoint formation and evolution, and the transient response of orogens to external forcing. We present controlled abrasion mill experiments designed to test fluvial incision models in the bedload and suspension regimes by varying sediment size while holding fixed hydraulics, sediment load, and substrate strength. Measurable erosion occurred within the suspension regime, and erosion rates agree with a mechanistic incision theory for erosion by mixed suspended and bedload sediment. Our experimental results indicate that suspension- regime erosion can dominate channel incision during large floods and in steep channels, with significant implications for the pace of landscape evolution. [ABSTRACT FROM AUTHOR]

Slow-moving landslides (earthflows) can dominate hillslope sediment flux and landscape erosion in hilly terrain with mechanically weak, fine-grained rock. Controls on the occurrence of slow-moving landslides are poorly constrained and need to be under stood for landscape evolution models, sediment budgets, and infrastructure and hazards planning. Here, we use airborne interferometric synthetic aperture radar (InSAR ) and aerial photographs to document 150 previously unidentified active earthflows along the central, creeping portion of the San Andreas fault, California. The earthflows move seasonally in response to winter rainfall, occur on hillslopes at ~20%-40% gradients (less than typically asso ciated with rapid, catastrophic landslides), and have similar morphological characteristics to earthflows in different climatic and tectonic settings. Although our data extend up to 10 km from the fault trace, ~75% of detected landslides occur within 2 km of the active fault. Topographic, pre cipi tation, and rock type metrics alone are not enough to explain the observed spatial distribution of earthflows. Instead, we hypothesize that earthflows cluster near the creeping San Andreas fault because of a fault-induced zone of reduced bulk-rock strength that increases hillslope susceptibility to failure. In addition, similar lithology, topography, and climate exist north of the creeping section of the fault, yet earthflows there are rare. This may be due to large-magnitude earthquakes episodically triggering coseismic rapid landslides, which preferen tially remove weak rock from the fault damage zone. Our analysis suggests that the necessary conditions for earthflow formation in central California include some combination of reduced rock strength, finegrained sedimentary rock, threshold precipitation and relief, and possibly the absence of t large-magnitude earthquakes. These conditions likely hold for earthflow development in other areas, and our work suggests that local variations in rock strength and seismicity, such as those associated with fault zones, t need to be taken into account in order to accurately predict earthflow occurrence. [ABSTRACT FROM AUTHOR]