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Iron‐rich, ferruginous waters were the dominant geochemical regime for most of Earth's history. Modern ferruginous waters are found in stratified, sulfur‐poor lakes, and serve as crucial analogs for biogeochemical cycling throughout Earth's past. Here we present the first depth‐resolved data of physical structure, nutrients and trace elements from Lake Poso (Indonesia), a deep oligotrophic ancient lake. Lake Poso is ferruginous, with anoxia below ~ 90 m depth, placing it among the world's largest ferruginous lakes. Physical stratification is weaker than other tropical anoxic lakes, indicating sensitivity for paleoclimate reconstructions. Trace elements and nutrients are predominantly shaped by the oxic–anoxic transition. Manganese– and Fe oxyhydroxide–driven biogeochemical cycling occurs at distinct depth horizons, with Co and Ni controlled by Mn and showing shallow release in anoxic waters, while V, Cr, P, and As are controlled by Fe, with release in surface sediments and diffusive transport. Chromium is nonquantitatively removed in anoxic waters, in contrast to widespread assumptions in Cr‐based paleoreconstructions. Oxycline U and Se removal corresponds to a local N minimum, suggesting biological reduction and/or uptake. These first ferruginous water Se data also show removal in sediments, indicating sediment signals reflect multiple removal processes and informing Se‐based paleoreconstructions, while the absence of sediment U removal contrasts other anoxic basins. A comparison with other ferruginous lakes demonstrates how local influences drive deviations from expectations in other systems, and highlight common, generalizable ferruginous basin features. Therefore, these data will guide research in ferruginous settings across space and time, and improve paleoreconstructions from ferruginous sediment records. [ABSTRACT FROM AUTHOR]

Logjams create an upstream backwater of deepened, slower water, locally reducing bed shear stress. We compared hydraulic impact of logjam series across 37 geomorphically diverse reaches of mountain streams observed over 11 years in the US Southern Rockies. To enable reach‐scale comparison of logjam structure and spacing, we identified the modeled best‐fit effective resistance coefficient minimizing difference between outflow exiting a 1D channel with logjams present, and the same model channel with elevated channel resistance. Effective resistance increased with ratio of jam upstream depth to depth without a logjam, ratio of backwater length to average spacing, and decreased for randomly distributed jams due to close spacing, which reduced backwater impact. An analytic approximation and boundaries for region of relative spacing with steepest increase in effective resistance are provided. Our results can assist in targeting interventions to areas where hydraulic impact is greatest, providing value for money in nature‐based solution design. Plain Language Summary: In a river channel, logjams created by wood pieces create upstream backwater regions with slower, deepened water. By creating a backwater, logjams increase heterogeneity of habitat and sediment transport and increase connection between river channel and floodplain. We compared logjams in 37 reaches of mountain streams in the US Southern Rockies. The sites studied had high variation in logjam density, channel steepness, channel width, and floodplain width. To compare between reaches and identify the ability of logjam backwaters to slow water within a river channel, we found an effective channel resistance coefficient that produced similar model output as a reach containing a series of logjams. The effective resistance increased with ratio of jam upstream depth to flow depth without a logjam, and ratio of backwater length to average spacing. The highest rate of increase in effective resistance with more logjams in the river reach occurs for an intermediate range of inter‐jam spacing relative to backwater length. Engineered logjam and wood addition projects could target this range to provide the most benefit per intervention. Key Points: We compared the hydraulic impact of logjams, identified by effective resistance, across 37 reaches and 11 years in the Colorado RockiesEffective resistance increases with decreasing stream power, analytically linked to dependence on jam structure and spacingObserved random jam distribution reduces effective resistance compared to uniform distribution, due to backwater truncation [ABSTRACT FROM AUTHOR]

Tidal flats are valuable ecosystems that depend on complex biogeomorphic processes between organisms and sediment transport. Climate change has led to a rise in extreme weather events, such as storms. This, in turn, has increased sediment dynamics and created risks for the benthic communities inhabiting tidal flats. However, replicating sediment disturbances caused by extreme weather is difficult. To overcome this, we used the plow rake to enhance the natural tidal currents and wave conditions to simulate intensified sediment dynamics. The raking disturbance was repeated on two intertidal zones with different inundation frequencies and wind fetch levels to simulate the increasing frequency of storm impact on sediments due to climate change. We compared the measurements of sediment dynamics and macrofauna between plots that were raked and the control plots that were only influenced by natural hydrodynamics. Results showed that tidal flat sediments experienced erosion by 10–20 mm after six times biweekly raking treatments, depending on the site‐specific hydrodynamic conditions. Sediment dynamics served as a helpful tool for monitoring the species distribution regarding mobility traits: the high dynamic exposed sites were inhabited by mobile species, while the low dynamic sheltered sites were characterized by less‐mobile species. Moreover, the raking treatment decreased the abundance of species with immobile traits, yet the species composition did not experience significant change. Overall, the present findings indicate that tidal flats with low sediment dynamics and immobile macrofauna are at higher risk of declining abundance under intensified sediment disturbances than areas with high sediment dynamics and mobile macrofauna. [ABSTRACT FROM AUTHOR]

Understanding soil erosion and sediment transport from the hillslope scale to the regional scale is crucial for studies on water quality, soil‐water conservation, the lateral carbon cycle, environmental zoning and vulnerability. However, most existing erosion and sediment transport models are only applicable at the hillslope scale or for small watersheds with fine spatial resolutions (typically much less than 1 km). This study presents a process‐based soil erosion and sediment transport model for model applications designed for applications with coarse spatial (e.g., ≥10 km) and temporal (e.g., from hourly to daily) resolutions. This new model, referred to as VIC‐SED, effectively accounts for interactions between erosion and hydrological processes. This is achieved by tightly coupling the erosion processes with a hydrologically based Three‐layer Variable Infiltration Capacity (VIC‐3L) land surface model (LSM) and to a multi‐scale routing (MSR) model. VIC‐SED considers the impacts of (a) the spatio‐temporal variability of rainfall intensity on erosion processes and (b) soil moisture on the soil detachment process. VIC‐SED is evaluated in two watersheds. Results demonstrate that VIC‐SED is capable of reproducing water and suspended sediment discharges at coarse spatial resolutions and varying temporal scales varying from 15‐min to daily intervals. Our study indicates that the VIC‐SED model is a promising tool for studying and assessing the impacts of climate and land cover changes on suspended sediment yields over large regions using coarse spatial and temporal resolutions. Plain Language Summary: Soil erosion is a global issue impacting soil‐water conservation, making the qualification of sediment transport crucial for evaluating water quality and landscape evolution. Existing models predominantly focus on hillslopes or small watersheds, with limited applicability to larger spatial scales. This limitation is partly due to the complexity of erosion processes and the empirical formulations involved in the erosion and sediment transport. To address this gap, we developed a novel process‐based soil erosion and sediment transport model for large‐scale land surface modeling. This model is integrated with the sophisticated Three‐layer Variable Infiltration Capacity (VIC‐3L) hydrological model and a multi‐scale routing (MSR) scheme for water and sediment transport. It explicitly accounts for the spatial and temporal variability of rainfall intensity and formulates the impact of soil moisture on the soil detachment process. Results from two applications demonstrate the model's capability to predict soil erosion and sediment dynamics at large spatial and temporal scales. Key Points: A novel process‐based SED model is developed for applications with coarse spatial and temporal resolutions of precipitationThe SED model is coupled with VIC accounting for subgrid‐scale variability of rainfall intensity, soil, vegetation, and flow path lengthThe coupled VIC‐SED model is evaluated with two case studies and its model parameters are insensitive to spatial and temporal resolutions [ABSTRACT FROM AUTHOR]

Sedimentary landforms on Earth and other planetary bodies are built through scour, transport, and deposition of sediment. Sediment connectivity refers to the hypothesis that pathways of sediment transport do not occur in isolation, but rather are mechanistically linked. In dryland river systems, one such example of sediment connectivity is the transport of fluvially deposited sediment by wind. However, predictive tools that can forecast fluvial‐aeolian sediment connectivity at meaningful scales are rare. Here we develop a suite of models for quantifying the availability of river‐sourced sediment for aeolian transport as a function of river flow, wind regime, and land cover across 168 km of the Colorado River in Grand Canyon, USA. We compare and validate these models using topographic changes observed over 10 years in a coupled river sandbar‐aeolian dunefield setting. The models provide a path forward for directly linking fluvial hydrology with the management and understanding of aeolian landscapes. Plain Language Summary: Landscapes on Earth are built from sediment, which is deposited by a variety of processes, especially water and wind. In rivers located in dry regions, sediment is deposited by flowing water during river floods. Once this sediment dries, it is often then moved by wind across the landscape. Although we frequently observe this sediment connectivity between water and wind, we are not currently able to predict when and where windblown transport of river‐deposited sediment may occur. We used maps of sand in Grand Canyon along 168 km of the Colorado River, along with records of river flow, wind speed, and vegetation cover, to develop four models that predict how much sediment is available for wind to move. We compared the results of these models to actual windblown sediment transport at a study site in Grand Canyon, finding that when models predicted more sand was available at the site, we also observed more landscape changes there which were due to wind. Our research makes it possible to understand how changes in river flows affect the amount of sediment moved by wind. Key Points: We developed four methods for modeling the extent of exposed, bare sand available for windblown transport in river valleysOur models employ data on river flow, land cover, wind velocity, and vegetation to predict exposed, bare sand extent at corridor scalesWhere models predict increased areal extent of bare sand, we also observe a greater volume of topographic changes driven by wind [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) is suffering from a substantial decline in terrestrial water storage (TWS) in exorheic basins, threatening water resources that are critical for ∼2 billion people downstream. TWS changes are commonly estimated using gravity satellites through observations of the total terrestrial mass storage (TMS) change, with an implicit assumption of a negligible contribution from sediment transport. Through long‐term (2002–2017) sediment flux observations in seven headwater basins on the TP, we reveal that the gravity satellite‐derived TMS has decreased at a rate of 3.85 ± 0.23 Gt yr−1 in the seven basins, of which 0.35 ± 0.04 Gt yr−1 is contributed by sediment transport. Neglecting this contribution leads to an overestimation of the TWS loss by 10.1 ± 1.3%, equivalent to the annual water demand of an additional 0.62 million people in the surrounding nations. Regionally, the overestimation is surprisingly high in the Indus River and Yarkant River basins, reaching up to 50.8%–77.6%. Plain Language Summary: Accurate quantification of the change in the terrestrial water storage (TWS) on the Tibetan Plateau (TP) is imperative to improve the assessment of water availability that are critical for ∼2 billion people downstream. Through long‐term sediment flux observations, here we find that sediment transport makes a substantial contribution to gravity satellite‐derived TWS change in regions with a high erosion rate such as the TP, which has never been taken into consideration in previous studies. Neglecting this contribution leads to an overestimation of the TWS loss by about 10% on average for the seven headwater basins on the TP. The contribution is especially high in the Indus River and Yarkant River basins. Our findings improve the regional estimation of water availability and thus support climate adaptation and sustainable water resource management. Key Points: For the first time, we quantify the contribution of sediment transport to gravity satellite‐based estimation of TWS change on the TPNeglecting the contribution of sediment transport leads to an overestimation of the TWS loss by 10.1% on averageThe overestimated TWS loss is equivalent to the annual water demand of an additional 0.62 million people in surrounding nations [ABSTRACT FROM AUTHOR]

The present paper develops a new framework to predict the mean flow through an array of cylinders in which the flow around the array (array‐scale) and the flow around individual cylinders (element‐scale) are modeled separately using actuator disc theory and empirical drag models respectively, and then coupled through the net drag force. Applying this framework only requires knowledge of the array geometry and incident flow. The framework is validated using high‐fidelity direct numerical simulations for arrays of between 7 and 109 cylinders having different arrangements (staggered, concentric, random) and bounding shapes (circular, square) in both two‐ and three‐dimensional flows. In general, the framework outperforms existing models which require calibration and are only valid for part of the practical parameter space. The demonstrated scale separation suggests different combinations of element‐scale and array‐scale models/theories may be used for other arrangements of bluff bodies. Plain Language Summary: The temporally and spatially averaged velocity, or mean flow, within an array of cylinders is important for many practical applications including, for example, estimating the rates of sediment transport and nutrient uptake within a patch of aquatic vegetation, the hydrodynamic forces on offshore structures, and the efficiency of power generation with a turbine farm. However, a general predictive tool for mean flow is currently lacking. To develop this predictive capacity, this study uses the idea of scale separation to integrate models from different research fields, including the actuator disc theory originally developed for design of turbines and propellers, and element‐scale drag models initially proposed for aquatic vegetated flows, to develop a unifying, predictive, analytical framework to estimate the mean flow. The framework is validated using high‐fidelity direct numerical simulations of arrays of cylinders with different arrangements (staggered, concentric, random) and array shapes (square, circular). With only knowledge of the array geometry and incident flow, the framework predicts the mean flow exceptionally well across a wide range of practical flow conditions. This leads to a step‐change in our ability to predict the fraction of incoming flow passing through a porous array, hydrodynamic load on offshore structures, and sediment flux within aquatic vegetation systems when combined with bed load transport models. The framework could be an important guide for coastal and riverine restoration projects using aquatic vegetation and the design of offshore structures and turbine farms. Key Points: A predictive framework based on an idea of scale separation is proposed for the mean velocity of the flow passing through an array of cylindersThe framework is versatile, being applicable for a wide range of practically meaningful cylinder arrangements, array shapes and array flow blockagesHigh‐fidelity, three‐dimensional direct numerical simulations are employed to investigate flow through the array of cylinders and validate the new predictive framework [ABSTRACT FROM AUTHOR]

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]

Previous studies have demonstrated that vegetation‐generated turbulence can enhance erosion rate and reduce the velocity threshold for erosion of non‐cohesive sediment. This study considered whether vegetation‐generated turbulence had a similar influence on natural cohesive sediment. Cores were collected from a black mangrove forest with aboveground biomass and exposed to stepwise increases in velocity. Erosion was recorded through suspended sediment concentration. For the same velocity, cores with pneumatophores had elevated turbulent kinetic energy compared to bare cores without pneumatophores. However, the vegetation‐generated turbulence did not increase bed stress or the rate of resuspension, relative to bare cores. It was hypothesized that the short time‐scale fluctuations associated with vegetation‐generated turbulence were not of sufficient duration to break cohesion between grains, explaining why elevated levels of turbulence associated with the pneumatophores had no impact on the erosion threshold or rate. Plain Language Summary: Mangrove habitat grows by retaining sediment. To restore these systems, it is necessary to understand how vegetation influences the transport and retention of sediment. This study used sediment cores collected from the interior of a mangrove forest to study how the aboveground roots, called pneumatophores, influence hydrodynamic conditions and sediment transport, and in particular the onset and rate of sediment erosion. Individual pneumatophores generate eddies that enhance turbulence, compared to conditions without pneumatophores. In sandy soil, vegetation‐generated turbulence can enhance erosion. However, in this study, vegetation‐generated turbulence did not increase the rate of erosion for natural cohesive (muddy) sediment, suggesting that the mangrove forest interior has naturally greater resistance to erosion and sediment loss. Key Points: For the same velocity, cores with pneumatophores had higher turbulent kinetic energy (TKE) compared to cores without pneumatophoresUnlike sands, the inception of erosion and erosion rates for cohesive sediment were better predicted by bed shear stress than by TKEModelers should parameterize erosion within vegetation differently for cohesive and non‐cohesive sediment [ABSTRACT FROM AUTHOR]

Bedload sediment transport plays an important role in the evolution of rivers, marshes and deltas. In these aquatic environments, vegetation is widespread, and plant species have unique morphology. However, the impact of real plant morphology on flow and sediment transport has not been quantified. This study used model plants with real plant morphology, based on the aquatic species Phragmites australis, Acorus calamus and Typha latifolia. The frontal area of these species increases away from the bed, which leads to higher near‐bed velocity than would be predicted from depth‐average frontal area. A plant morphology coefficient was defined to quantify the impact of vertically‐varied plant frontal area. Laboratory experiments confirmed that the plant morphology coefficient improved the prediction of near‐bed velocity, near‐bed turbulent kinetic energy and bedload transport rate in canopies with realistic morphology. Plant morphology can alter transport rates by up to an order of magnitude, relative to the assumption of uniform morphology. Plain Language Summary: Aquatic vegetation is a crucial component of river, marsh and delta ecosystems. It has a significant impact on landscape evolution by altering sediment transport. Each vegetation species has a unique shape, but the impact of plant shape on sediment transport has not been previously investigated. This study explained how plant shape impacts sediment transport. Results show that differences in plant shape can change velocity and turbulence close to the channel bed, and thus alter sediment transport rate by an order of magnitude. Plants with greater vertical variation in shape, and specifically less plant volume near the channel bed, produced greater sediment transport. A parameter was defined to quantify the impact of plant shape on flow and sediment transport. By incorporating this parameter, our new model improves the prediction of sediment transport in vegetated regions. This study provides a method for describing the impact of plant shape on flow and sediment transport, which is important to the management and restoration of rivers, marshes and deltas. Key Points: Plant morphology influences the near‐bed velocity, turbulent kinetic energy (TKE) and bedload transport rate inside emergent canopiesA new model accounts for plant morphology in the prediction of near‐bed velocity and TKE and bedload transportFor each canopy solid volume fraction, plants with greater vertical variation in frontal area produce greater bedload transport [ABSTRACT FROM AUTHOR]

Hydrogeomorphology studies river dynamics, focusing on the interactions between flow structure, sediment transport, and the morphologies that characterize rivers and their watersheds. It provides an analytical framework and tools for better integrating knowledge of river dynamics into river management in the broadest sense, and more specifically, into river restoration as well as into the assessment and prevention of risks associated with fluvial hazards. In Quebec, hydrogeomorphology is emerging as a significant contribution to risk assessment and management approaches, and is at the heart of a paradigm shift in river management whereby process restoration aims to increase the resilience of fluvial systems and societies, and improve the quality of fluvial environments. This contribution outlines the trajectory of hydrogeomorphology in Quebec, based on scientific publications by Quebec geographers, and discusses the discipline's aims in research and knowledge integration for river management. Messages clés: Les géographes du Québec ont contribué fortement au développement des connaissances et outils de l'hydrogéomorphologie.L'hydrogéomorphologie a évolué d'une science fondamentale à une science où les connaissances fondamentales sont au service de la gestion des cours d'eau.L'hydrogéomorphologie et le cortège de connaissances et d'outils qu'elle promeut font de cette discipline une partenaire clé pour une gestion holistique des cours d'eau. [ABSTRACT FROM AUTHOR]

The dating of moraine deposits can present challenges to standard geochronological methods; terrestrial cosmogenic nuclide (TCN), sediment luminescence and radiocarbon dating may suffer from problems of incomplete resetting (by inheritance, intermittent cover/exposure, transport under unfavourable conditions) and/or a lack of suitable (organic) material. Rock surface luminescence burial dating (RSLBD) offers an alternative approach with considerable potential in dating moraines. In RSLBD, large cobbles/boulders are targeted, rather than smaller grains usually used in luminescence dating. The age limit of RSLBD is much higher than that of radiocarbon dating, and rocks are much more readily available than organic material. In contrast to TCN dating, the effect of exposure prior to deposition can be measured. In this study, we sampled a broad selection of primarily granitic boulders of various sizes and shapes (e.g. different degrees of roundness and sphericity) from the Vimmerby Moraine, a prominent and accessible feature in southern Sweden dated using TCN to 14.4±0.9 ka. Our study was designed to test whether morphological characteristics can be used to discriminate in favour of the most light‐exposed boulders and minimize measurements of non‐exposed boulders. As expected, not all RSLBD ages can be attributed to the same depositional event, but the majority of the resulting ages provide a mean age of 13.0±1.5 ka, consistent with the cosmogenic nuclide dating of the Vimmerby Moraine. Despite the apparently successful TCN study, the luminescence–depth profiles measured in the buried surfaces of the sampled clasts indicate that >50% of these moraine boulders were exposed to light (and cosmic radiation) before final deposition, implying some (presumably small) TCN inheritance; seven of the 16 boulders identified as light exposed were sufficiently bleached to be useful for RSLBD. These results and their implications in regard to transport and deposition of the sampled cobbles are critically discussed and evaluated. [ABSTRACT FROM AUTHOR]

Understanding the material properties and physical conditions of basal ice is crucial for a comprehensive understanding of Antarctic ice‐sheet dynamics. Yet, direct data are sparse and difficult to acquire. Here, we employ ultra‐wideband radar to map high‐backscatter zones near the glacier bed within East Antarctica's Jutulstraumen drainage basin. Our backscatter analysis reveals that the basal ice in an area of ∼10,000 km2 is composed of along‐flow oriented sediment‐laden basal ice units connected to the basal substrate, extending up to several hundred meters thick. Three‐dimensional thermomechanical modeling supports that these units form via basal freeze‐on of subglacial water that originated from further upstream. Our findings suggest that basal freeze‐on, and the entrainment and transport of subglacial material play a significant role in an accurate representation of material, physical, and rheological properties of the Antarctic ice sheet's basal ice, ultimately enhancing the accuracy and reliability of ice‐sheet modeling. Plain Language Summary: We investigate the lowermost portion of the ice column of the Antarctic ice sheet, known as basal ice. This part holds crucial information about how the ice sheet behaves when it flows. Using the principle of echo‐location, we use radar technology to scan the ice in East Antarctica's Jutulstraumen drainage basin. We discover that a significant portion of the basal ice in this region is filled with sediment and can be several hundred meters thick. To better understand how this distinctive type of ice forms, we used a mathematical model that suggested that these ice units likely form when water underneath the ice is transported and refreezes at particular locations. Our findings highlight the importance of these freezing and mixing of materials from beneath the ice. Understanding where these ice units are located provides important information for ice‐flow models to ultimately better understand how the Antarctic ice sheet behaves in the future. Key Points: We have identified high‐scattering basal ice in Jutulstraumen Glacier's onset region using radar, reaching several hundred meters from the bedBackscatter analysis suggests that the basal ice units contain unstratified point scatters and cause little radio‐glaciological loss3D thermo‐mechanical modeling implies that freeze‐on of basal meltwater generated upstream likely initially formed the basal ice units [ABSTRACT FROM AUTHOR]

A high‐resolution wave model is crucial for accurate modeling of sediment and organic material transports, but its computational costs hinder direct coupling to an ecosystem model. We developed a machine learning model using long short‐term memory to simulate large‐scale, high‐resolution waves. Trained with numerical wave model (NWM) outputs and wind data from nine locations, our model successfully replicates NWM results for daily mean significant wave height and period in Chesapeake Bay with identical spatial resolution. Compared to the NWM, the data‐driven model has root‐mean‐square errors below 6 cm for daily mean significant wave height and 1 s for the wave period in the bay. It demonstrates excellent model skills and can accurately forecast daily mean significant wave height and period at NOAA wave stations comparable to NWMs. Using minimal wind data and having a short runtime, our data‐driven model shows promise as an alternative for wave forecasting and coupling with sediment and ecological models. Plain Language Summary: Accurately modeling sediment and organic transport necessitates the use of a high‐resolution numerical wave model (NWM). However, the computational expenses of running an NWM within an ecosystem model pose significant challenges. We conducted a study using a machine learning approach based on long short‐term memory to replicate the results of an NWM. The data‐driven wave model (DWM) was trained with the NWM outputs and wind data from nine locations. The trained DWM can replicate the NWM results of daily mean significant wave height and period in Chesapeake Bay with identical spatial resolution and the same predictive skill with a root‐mean‐square error of less than 6 cm and 1 s, respectively for the model domain. The model successfully forecasted daily mean significant wave height and period at NOAA wave stations, showing great forward prediction capability. Notably, the DWM only relied on a minimal amount of wind data from selected locations and had a short runtime. These findings indicate that machine learning is promising as an alternative approach to NWM for wave forecasting. Moreover, it can serve as an alternative wave model and be coupled to ecological models for simulating sediment transport, light attenuation, and resuspension of organic materials. Key Points: A machine learning‐based wave model reproduces the numerical wave model (NWM) results at the same spatial resolution with good accuracyThe data‐driven model, trained on daily significant wave height and period using NWM outputs, has comparable wave prediction skillThe fast‐running data wave model, driven by wind data from nine locations, can forecast waves and couple with sediment and ecological models [ABSTRACT FROM AUTHOR]

Feedbacks between sediment and plants in the submersed aquatic vegetation (SAV) beds of the Susquehanna Flats help modulate sediment delivery into the upper Chesapeake Bay from its major tributary (Susquehanna River). Recent modeling work has shown that SAV can steer the river plume, directly controlling sediment transport and fate. However, transport mechanisms likely differ between flood and non‐flood conditions and depend on event timing. An opportunity to evaluate these insights in the field occurred with several flood events in summer 2018. Sediment and biomass samples were collected after the flood in the large, continuous main SAV bed of the Flats and in smaller patches to the west of the main bed to evaluate the effect of SAV bed size (width). Sediment characteristics (grain size, organic content) and deposition rates were compared to previous observations during non‐flood conditions in the main bed in 2014–2015 and in the smaller patches in 2019. Overall, sediment deposition rates could be predicted by an empirical model including sediment load, vegetation biomass, and SAV bed width, reflecting the role of vegetation in steering the Susquehanna River plume and modulating sediment input to the upper Chesapeake Bay. Understanding spatiotemporal patterns of sedimentation is essential for realistic estimates of sediment and nutrient (especially nitrogen and carbon) storage in freshwater SAV systems, which have not received as much attention as their saltwater counterparts. [ABSTRACT FROM AUTHOR]

Shorelines face growing threats due to climate change and diminishing sand supply. Coastal headlands, common rocky features along coastlines, are crucial in shaping hydrodynamics and sediment transport. Yet, the influence of future climate conditions, including sea‐level rise (SLR) and intensified storm energy on complex shorelines with headlands has remained relatively unexplored. In this study, we model changes in hydrodynamics and headland bypassing under different SLR and higher storm wave scenarios. Our findings reveal the formation of circulation cells on both sides of a headland, where wave energy converges around the headland zone. Future climate conditions result in larger storm waves on the beach. However, SLR enhances nearshore currents through a landward shifting of the circulation cells, while higher storm waves intensify offshore flow currents due to the seaward movement of the cells. This effect, in turn, increases the potential for headland sediment bypassing. Plain Language Summary: Coastal headlands, prominent rocky features along open coastlines, play a crucial role in protecting nearby beaches from strong waves and erosion. They also affect how sand is exchanged between different beaches. We use a model to explore how future climate conditions including sea‐level rise (SLR) and larger storm waves influence coastal waves, littoral currents, and the transport of sand around headlands. Our findings reveal that headlands converge wave energy forming circulation flow cells. SLR results in stronger nearshore currents, driven by the landward movement of the rotating flow cells. In contrast, larger storm waves can move the rotating flow cells seaward, thereby increasing offshore current strength and the potential for sand transport around the headland. Understanding how coastal waves, flow and sediment transport change under future climate conditions will help determine coastal resilience. Key Points: The formation of circulation cells and wave energy convergence around headlands during storms reduce bottom shear stress at the beachSea‐level rise increases wave heights, currents, and bed shear stress in the nearshore because of landward shifting of the circulation cellsHigher storm waves expand the surf zone, shift circulation cells seaward, and enhance potential headland sediment bypassing [ABSTRACT FROM AUTHOR]

The presence of a late Quaternary ice sheet/ice shelf over the East Siberian Sea has been proposed in several papers. Here, we further document its duration/resilience based on the sedimentary, bulk mineralogical, and geochemical (organic matter content and its stable isotopic composition, U‐Th series) properties of a core raised from the southernmost Mendeleev Ridge. The chronostratigraphy of the studied core was mainly built from the 230Th excess (230Thxs) distribution and decay downcore. At the core‐top, peaking 230Thxs values during the early MIS 3 and mid‐MIS 1 encompassing an MIS 2 hiatus were observed. As documented in several papers, these peaks suggest seasonally open ice conditions over proximal continental shelves. Below, the interval spanning MIS 4 and possibly MIS 5d records major ice‐rafting events illustrated by overall high coarse‐fraction contents. Underlying MIS 5e, down to MIS 11, the sediment depicts relatively low sand (1.7±2.5 dw%), high clay (33.5±4.7 dw%), and very low organic carbon (0.10±0.06 dw%) contents, and low δ13Corg values (−24.3±0.9‰). This section is interpreted as recording fine sediment transport by deep currents and/or meltwater plumes below a resilient ice cover, only interrupted by a few short‐duration events. These events include (i) detrital carbonate pulses assigned to deglacial events along the NW Laurentide Ice Sheet margin (Termination (T) III), and (ii) intervals with some planktonic foraminifer occurrences, likely relating to their advection from open areas of the Arctic Ocean (MIS 5e, 9 and 11). All Terminations, but TII and the early MIS 3, show peaking Mn/Al values linked to the submergence of Arctic shelves under a rising sea level. We conclude that the resilient ice cover, likely an ice shelf, has been present over the southern Mendeleev Ridge during most of the interval after the Mid‐Pleistocene Transition and was favoured by the low summer insolation of the MIS 14 to 10 interval. [ABSTRACT FROM AUTHOR]

Alpine glaciers are rapidly retreating due to global warming and this has been associated with enhanced supply of subglacially derived sediment to downstream environments. We present the first high frequency quantitative record on how the signal of sediment exported from an Alpine glacier is filtered by its proglacial forefield. The data, covering two climatically distinct glacier melt seasons, show that the signal of subglacial bedload export is strongly filtered over short distances, unlike suspended load whose signal is less impacted. The reason is related to the interplay of short particle advection lengths with strong morphodynamic forcing. The subglacial sediment export signal is thus rapidly replaced by one combining (a) the local forcing by stream hydraulics and (b) the reworking of the proglacial braid plain itself. These findings have implications for estimating subglacial erosion rates, natural hazard mitigation, sediment management for hydropower plants and ecological succession related to rapid glacier retreat. Plain Language Summary: Alpine glaciers have been retreating at increasing rates since the early 20th century due to the current climate warming. As a consequence, new, powerful and rapidly changing rivers have developed in the newly emerged terrain in front of shrinking glaciers (i.e., proglacial forefields). These environments are known to be among the most rapidly changing landscapes on Earth. In this study we present the first combined high frequency quantitative data on sediment export from an Alpine glacier for two melt seasons with very different climatic conditions. Results show that the low potential transport distances of particles traveling on the river bed together with the continuously changing shape of these streams, rapidly modify the glacier‐dictated export signal (i.e., subglacial sediment evacuation rates in time). For sediment transported in suspension in the river, that loss of information is less evident. These findings are important for the understanding of how much, when, and which kind of sediment are delivered to the populated places downstream in the valleys, in order to mitigate natural hazards, but also for sediment management in hydropower plants and ecological succession in the context of rapid glacier retreat. The results are also key to improving estimation of glacial erosion rates. Key Points: Seismically inferred sediment transport anatomy reveals strong filtering of subglacial bedload export signal due to proglacial processesFiltering operates at a sub‐daily scale, and is due to the short advection lengths of bedload particles in the proglacial forefieldProglacial filtering is a key factor affecting (pro‐)glacial sediment transport dynamics, sediment connectivity and ecological succession [ABSTRACT FROM AUTHOR]

The increase in bedrock channel width and the decline in bedrock channel slope with increasing drainage area are fundamental characteristics of mountainous landscapes. Compared with slope, little is known about controls on steady‐state bedrock channel width. We model steady‐state bedrock channel width by iteratively solving models for bed and bank erosion by impacting bedload, using measurable physical parameters, including uplift rate, water discharge, sediment supply, grain size, rock strength, and bank angle. The results indicate that width is largely controlled by sediment flux, rather than water discharge. The commonly used width‐discharge scaling relation is an artifact of the covariance of sediment flux and water discharge. Scaling up from cross‐section to drainage basin scales, our model reproduces the width‐drainage area scaling relation and suggests that the scaling exponent is largely controlled by the downstream change in the fraction of total sediment supply transported as bedload. Plain Language Summary: Bedrock rivers become wider and less steep as water flows from upstream to downstream in mountainous landscapes. The reasons why bedrock channels widen are poorly understood. Here, we develop a method to predict typical channel width based on the mechanism of bedrock erosion by impacting sediment particles. Our method can be tested in natural bedrock rivers from measurable parameters. Our results show that channel width is largely controlled by the amount of sediment supplied from upstream, rather than the amount of water flow. We show the widely observed increase in bedrock river width with drainage area is due to the increase in sediment supply not water discharge. Key Points: A new model is developed for steady‐state bedrock channel width based on bed and bank erosion by bedload particle impactsModel results indicate that the width‐drainage area scaling relation exponent is largely set by the coarse sediment fractionThe width‐discharge scaling relation is an artifact of the covariance of sediment flux and water discharge [ABSTRACT FROM AUTHOR]

Infragravity waves are key components of the hydro‐sedimentary processes in coastal areas, especially during extreme storms. Accurate modeling of coastal erosion and breaching requires consideration of the effects of infragravity waves. Here, we present InWave, a new infragravity wave driver of the Coupled Ocean‐Atmopshere‐Waves‐Sediment Transport (COAWST) modeling system. InWave computes the spatial and temporal variation of wave energy at the wave group scale and the associated incoming bound infragravity wave. Wave group‐varying forces drive free infragravity wave growth and propagation within the hydrodynamic model of the coupled modeling system, which is the Regional Ocean Modeling System (ROMS) in this work. Since ROMS is a three‐dimensional model, this coupling allows for the combined formation of undertow currents and infragravity waves. We verified the coupled InWave‐ROMS with one idealized test case, one laboratory experiment, and one field experiment. The coupled modeling system correctly reproduced the propagation of gravity wave energy with acceptable numerical dissipation. It also captured the transfer of energy from the gravity band to the infragravity band, and within the different infragravity bands in the surf zone, the measured three‐dimensional flow structure, and dune morphological evolution satisfactorily. The idealized case demonstrated that the infragravity wave variance depends on the directional resolution and horizontal grid resolution, which are known challenges with the approach taken here. The addition of InWave to COAWST enables novel investigation of nearshore hydro‐sedimentary dynamics driven by infragravity waves using the strengths of the other modeling components, namely the three‐dimensional nature of ROMS and the sediment transport routines. Plain Language Summary: Infragravity waves have periods between 25 and 250 s and are the result of wind‐wave groups, or "sets." When wind‐waves of similar periods travel together, they group, resulting in varying wave heights within the groups. This wave height variation at the group scale forces ocean surface infragravity waves. Coastal circulation, flooding, sand transport, and erosion are strongly influenced by these infragravity waves, especially during extreme storms. Therefore, it is important that we are able to model infragravity waves. We present a novel infragravity wave component of the Coupled‐Ocean‐Atmosphere‐Wave‐Sediment Transport (COAWST) modeling system: InWave. By coupling InWave and a circulation model, COAWST is now able to account for the main processes needed to predict coastal hazards due to extreme storms. This new coupled system is verified by reproducing observations from idealized numerical cases, a laboratory experiment on dune erosion, and a field experiment. Results show a good agreement with observations. Key Points: InWave is a new infragravity wave driver of the Coupled Ocean‐Atmosphere‐Wave Sediment Transport (COAWST) modeling systemCoupling InWave with the Regional Ocean Modeling System within COAWST enables the modeling of infragravity waves and three‐dimensional flowsThe coupled system is verified by reproducing laboratory and field observations, with good hydrodynamic and morphodynamic performance [ABSTRACT FROM AUTHOR]

We describe a framework for the simultaneous estimation of model parameters in a partial differential equation using sparse observations. Markov Chain Monte Carlo sampling is used in a Bayesian framework to estimate posterior probability distributions for each parameter. We describe the necessary components of this approach and its broad potential for application in models of unsteady processes. The framework is applied to three case studies, of increasing complexity, from the field of cohesive sediment transport. We demonstrate that the framework can be used to recover posterior distributions for all parameters of interest and the results agree well with independent estimates (where available). We also demonstrate how the framework can be used to compare different model parameterizations and provide information on the covariance between model parameters. Plain Language Summary: We describe a framework for the simultaneous estimation of multiple unobserved parameters by combining observations of a tracer with a numerical model. This framework uses Bayesian inference techniques established in statistical literature to estimate the unobserved parameters of interest used in the model with uncertainty quantification. We explain the key components of this framework in simple terms to encourage its use for analyzing other unsteady processes and performing quantitative inference on parameters that are difficult or impossible to measure directly. We then demonstrate the framework's efficacy by applying it to three case studies from the field of cohesive sediment transport that all use the transport equation (advection‐diffusion). Inferred parameter values show good agreement with independent estimates, where available. Key Points: Probabilistic framework to estimate unobserved erosion model parameters using sparse measurements collected above the seabedGeneral approach can be updated with any model parameterization and quantitatively comparedThe framework is applicable to many similar data sets with both unsteady or quasi‐steady forcing and response [ABSTRACT FROM AUTHOR]

Alluvial rivers aggrade, incise, and adjust their sediment‐transport rates in response to changing sediment and water supply. Fluvial landforms, such as river terraces, and downstream stratigraphic archives may therefore record information about past environmental change. Using a physically based model describing sediment transport and long‐profile evolution of alluvial rivers, we explore how their responses to environmental change depend on distance downstream, forcing timescales, and whether sediment or water supply is varied. We show that amplitudes of aggradation and incision, and therefore the likelihood of terrace formation, are greater upstream and in shorter and/or wetter catchments. Aggradation and incision, and therefore terrace ages, may also lag behind environmental change. How sediment‐transport rates evolve depends strongly on whether water or sediment supply is varied. Diverse responses to environmental change could arise in natural alluvial valleys, controlled by their geometry and hydrology, with important implications for paleo‐environmental interpretations of fluvial archives. Plain Language Summary: Rivers carry sediment from upland regions, where it is produced by weathering and erosion, to low‐lying areas where it is deposited and stored. This process is sensitive to climate: for example, wetter conditions enhance rivers' capacity to transport sediment, causing rivers to cut into their beds, which in turn remobilizes sediment that is transported downstream. Changing climate may also lead to enhanced sediment production and supply to rivers, causing them to accumulate sediment in their beds. Climate cycles can therefore produce alternating periods of sediment accumulation and removal, resulting in ancient floodplains being abandoned and preserved alongside modern rivers ("terraces"), and in variations in the amounts of sediment delivered downstream. Terraces and patterns of sediment accumulation downstream could therefore provide valuable records of past climatic change. We develop a mathematical model that describes the effects of climatic change on river‐bed height and sediment‐transport rates. We show how the amount and timing of river in‐filling and down‐cutting, and the timescales over which they occur, depend on the rate of climatic change, on river length and associated drainage‐basin area, and on the amount of water it carries. These predictions will facilitate more detailed interpretations of terrace records along rivers and of the sedimentary deposits that they leave behind. Key Points: Alluvial rivers respond in diverse ways to cyclical climatic change depending on their geometry and hydrology and on the forcing frequencyTerrace formation is favored upstream and in shorter and wetter catchments and may lag appreciably behind environmental changeVariation in sediment output lags variation in sediment supply but can lead and be amplified with respect to variation in water supply [ABSTRACT FROM AUTHOR]

Near the threshold of grain motion, sediment transport is "on‐off" intermittent, characterized by large but rare bursts separated by long periods of low transport. Without models that can account for the effects of intermittency, measurements of average sediment flux can be in error by up to an order of magnitude. Despite its known presence and impact, it is not clear whether on‐off intermittency arises from the grain activity (the number of moving grains) or grain velocities, which together determine the sediment flux. We use laboratory flume experiments to show that the on‐off intermittency has its origins in the velocity distributions of grains that move by rolling along the bed, whereas grain activity is not on‐off intermittent. Incorporating the types of intermittency we identify into stochastic models of sediment transport could yield improved predictions of sediment flux, including physically based estimates of the uncertainty in time‐averaged sediment flux. Plain Language Summary: Sediment, such as sand and gravel, is transported across the surface of the Earth and other planets by wind and water. Predicting the amount of sediment that can be transported given a certain flow rate is crucial for predicting how a landscape will change over time. For low flow rates, little to no grain motion occurs. Just beyond the flow rate required for motion, sediment transport occurs mostly in rare bursts. This so‐called "on‐off" intermittency creates difficulties when trying to measure the average transport rate, which must be done over longer time periods as the bursts become larger but less frequent. While on‐off intermittency has been identified in previous studies of sediment transport, there is currently no understanding of its physical origin. We use a series of experiments in a small laboratory river to show that the on‐off intermittency is due to large fluctuations in the speed of the grains rolling on the bed, and that the sediment transport becomes less bursty as more grains are lifted off the river bed and into the fluid. Our results will pave the way for better measurements and predictions of sediment transport in rivers. Key Points: We use grain tracking data from laboratory experiments to separately study the statistics of grain velocities and grain activityWe show that on‐off intermittency has its origins in the velocity distributions of grains, not in the grain activityOn‐off intermittency comes from grains rolling on the bed, and disappears as more and more grains are lifted into the bulk of the flow [ABSTRACT FROM AUTHOR]

Human intervention makes river channels adjust their slope and bed surface grain size as they transition to a new equilibrium state in response to engineering measures. Climate change alters the river controls through hydrograph changes and sea level rise. We assess how channel response to climate change compares to channel response to human intervention over this century (2000–2100), focusing on a 300‐km reach of the Rhine River. We set up a schematized numerical model representative of the current (1990–2020), non‐graded state of the river, and subject it to scenarios for the hydrograph, sediment flux, and sea level rise. We conclude that the lower Rhine River will continue to adjust to past channelization measures in 2100 through channel bed incision. This response slows down as the river approaches its new equilibrium state. Channel response to climate change is dominated by hydrograph changes, which increasingly enhance incision, rather than sea level rise. Plain Language Summary: Humans have modified rivers to enable boat traffic, to protect people against flooding, and to provide them with freshwater and energy. When the shape of a river changes, the amount of sand and gravel (sediment) that can move along its bed also changes. In response, rivers change their slope and bed characteristics, so that they can transport as much sediment as they receive from higher up in the basin. This results in changes in bed level, which becomes higher or lower, causing problems for navigation and flood protection. Climate change makes this worse, because it changes the amount of water flowing down the river, and sea level. This further affects the amount of sediment that can move down the river, therefore causing additional bed level change. Here we study how climate change affects the lower Rhine River (Germany‐Netherlands), over the 21st century. This river has been heavily modified by humans, and its bed has been lowering over hundreds of kilometers. With a computer model, we simulate how different scenarios of climate change affect this behavior. We foresee that the ongoing bed level lowering will continue in the upcoming decades, and that it will be enhanced by climate change. Key Points: Human intervention will continue to govern channel response in the lower Rhine River by 2100, mainly through channel bed incisionClimate change leads to sea level rise and hydrograph adjustment, the latter being dominant and causing enhanced incisionChannel response to human intervention slows down as the river approaches its equilibrium state, but response to climate change accelerates [ABSTRACT FROM AUTHOR]

The evolution of the East Asian winter monsoon (EAWM) during the Holocene remains controversial because of the lack of reliable proxies. In this study, a link between cross‐front transport and the EAWM was found in the North Yellow Sea, and the EAWM intensity could be imprinted in the leaked sediment, which permits reliable reconstruction of the EAWM on well‐preserved offshore deposition. By comparison with instrumental data, the wind speed of the EAWM since 4.4 ka was quantitatively reconstructed for the first time at high resolution (∼5 yr). The results show that the EAWM gradually weakened in the mid‐to‐late Holocene, which was likely controlled by solar insolation, and that there has been a negative relationship between the EAWM and temperature on the centennial scale since ∼2.0 ka; this relationship was reversed by increasing CO2 emissions during the Current Warm Period. Plain Language Summary: The East Asian monsoon is characterized by a distinct seasonal reversal of monsoon flow driven by temperature differences between the Pacific Ocean and the East Asian continent. It influences more than 2 billion people around East Asia by regulating the risk of drought or flooding. However, the evolution of the East Asian winter monsoon (EAWM) in the mid‐to‐late Holocene remains elusive, limiting our understanding of its driving force and projections under climate change. This study analyzed sediment transport dynamics under varying winter monsoons in the North Yellow Sea. It was found that coastal sediments could be transported offshore, and their pathways differed under different strengths of the winter monsoon. Based on the sedimentary records of offshore muddy deposition, the EAWM proxy was extracted, and the wind speed of the EAWM from 4.4 ka was quantitatively reconstructed. A gradual weakening trend during 4.4–0.7 ka and a dramatic strengthening after 0.2 ka were found in the EAWM; the EAWM was negatively related to the temperature on the centennial scale since ∼2.0 ka, but this relationship was reversed by an increase in CO2 emissions since ∼1800 CE, indicating a shift in the controlling mechanism. Key Points: A link between cross‐front transport and the East Asian winter monsoon (EAWM) was found in the shelf sea, and the leaked sediment was used to reconstruct the EAWMThe wind speed of the EAWM since 4.4 ka was quantitatively reconstructed at high resolution (∼5 yr) for the first timeCorrelation between the EAWM and the temperature on the centennial scale since ∼2.0 ka was reversed by the increasing CO2 at the Current Warm Period [ABSTRACT FROM AUTHOR]

In meandering rivers, interactions between flow, sediment transport, and bed topography affect diverse processes, including bedform development and channel migration. Predicting how these interactions affect the spatial patterns and magnitudes of bed deformation in meandering rivers is essential for various river engineering and geoscience problems. Computational fluid dynamics simulations can predict river morphodynamics at fine temporal and spatial scales but have traditionally been challenged by the large scale of natural rivers. We conducted coupled large‐eddy simulation and bed morphodynamics simulations to create a unique database of hydro‐morphodynamic data sets for 42 meandering rivers with a variety of planform shapes and large‐scale geometrical features that mimic natural meanders. For each simulated river, the database includes (a) bed morphology, (b) three‐dimensional mean velocity field, and (c) bed shear stress distribution under bankfull flow conditions. The calculated morphodynamics results at dynamic equilibrium revealed the formation of scour and deposition patterns near the outer and inner banks, respectively, while the location of point bars and scour regions around the apexes of the meander bends is found to vary as a function of the radius of curvature of the bends to the width ratio. A new mechanism is proposed that explains this seemingly paradoxical finding. The high‐fidelity simulation results generated in this work provide researchers and scientists with a rich numerical database for morphodynamics and bed shear stress distributions in large‐scale meandering rivers to enable systematic investigation of the underlying phenomena and support a range of river engineering applications. Plain Language Summary: River flows in nature follow spectacular meandering paths from upstream to downstream. Such flows are turbulent and interact with mobile sediment and the shape of the channel through an intricate set of complex feedback mechanisms that impact flow dynamics, sediment motion, and streambank stability leading to migration of river dunes and erosion of the channel banks. Computer models are key tools for understanding these mechanisms but have traditionally faced challenges in representing turbulent flow at the scales of natural rivers. We generated 42 synthetic, meandering rivers with various geometrical and flow characteristics representative of those encountered in nature. We then simulate numerically the flow over the mobile bed for each of these rivers to predict changes in the bed geometry. We analyze the predicted results to determine general trends in the predicted shape of the riverbed and the distribution of shear stresses near the riverbed. The resulting library of simulation results provides a data set of unprecedented resolution and detail for a wide class of river meanders encountered in nature enabling engineers and scientists to develop a predictive understanding of river morphodynamics to develop mathematical approaches for river restoration, and infrastructure design and protection against floods. Key Points: A series of large‐scale river meanders with various geometries and flow characteristics were simulated to represent natural meandersThe bed deformation of the meanders was obtained using a coupled hydro‐ and morpho‐dynamic model under bankfull flow conditionsBy analyzing the morphodynamic simulation results of the meanders, general trends in bed deformations and bed shear stress were found [ABSTRACT FROM AUTHOR]

Deltas are fragile ecosystems threatened by sea‐level rise, sediment starvation, and subsidence. Erosional/depositional processes in these systems mainly depend on the sediment supply and the spatial divergence in bed shear stress induced by hydrodynamic forces. Thus, quantifying the spatiotemporal variability of the flow velocity field is essential for forecasting their fate. To calibrate/validate models, field measurements alone are not sufficient because such data only characterize the hydrodynamic conditions in localized areas. Remote sensing has potential to fill this data gap. We developed a methodology to estimate flow velocities from a map of suspended sediment concentration (SSC) measured by the NASA airborne spectrometer AVIRIS‐NG within the Wax Lake Delta, Louisiana. We extracted streaklines from remotely sensed SSC estimates, and quantified water fluxes and velocities based on the distance between them. Our study demonstrates that the velocity field in deltas can be estimated by leveraging the synoptic information offered by remote sensing. Plain Language Summary: River deltas are coastal environments particularly susceptible to sea‐level rise. Sediment deposition can counteract land loss from sea‐level rise, but we have a poor understanding of where and when land is built through sediment accretion. Improving the accuracy of hydrodynamic models is pivotal to predicting the fate of sediment that rebuilds sinking land, but we lack methods to test these models against observations over large areas. Here, we introduce a new method to measure water discharge, flow direction and speed from remotely sensed data. The method takes advantage of water flow patterns revealed in data collected from a NASA instrument flown on an airplane. Our study shows the utility of remotely sensed data to improve the performance and reliability of models that are needed to predict the fate of river deltas. Key Points: We propose a novel methodology to compute water fluxes and flow velocities in deltas from maps of suspended sediment concentrationOur method quantifies flow velocities based on the distance between streaklines, under the assumption that streaklines act as streamlinesFlow velocities obtained via our methodology match well with those derived from a numerical model in a sub‐region of the Wax Lake Delta [ABSTRACT FROM AUTHOR]

Effective coastal management requires a fundamental understanding of the impacts of anthropogenic activities on sediment dynamics, yet it is challenging to isolate individual impacts in heavily altered regions. The Ayeyarwady Delta, Myanmar has been extensively deforested for agriculture but has few levees/polders. In this study, the relative resilience to subsidence was compared between a 45‐year‐old agricultural field and a nearby mangrove‐forest preserve. At both sites, water velocity and turbidity were measured in tidal channels, topography was mapped, and sediment cores were collected during 2018–2019. There was net sediment import at both sites due to sediment trapping by vegetation. Relative elevations were equivalent, suggesting that the field has aggraded at rates similar to the forest (0.7 cm/year). Unleveed fields may be less vulnerable to subsidence than leveed fields. However, uncertainties remain and the decision to replace mangroves with agricultural fields should weigh all the benefits provided by each environment. Plain Language Summary: Many tropical river deltas are densely populated agricultural centers, and these coastal landscapes are threatened by sea‐level rise. In these managed regions, farmers remove mangrove forests to expand fields and construct levees to control the annual inundation of fields. Unfortunately, levees prevent sediment delivery and can contribute to rapid sinking of the land surface. In the Ayeyarwady Delta, Myanmar, there are relatively few leveed agricultural fields. Here, we assess the resilience to sinking of a deforested but unleveed rice field with respect to a nearby, natural mangrove‐forest preserve in the center of the Ayeyarwady Delta. Land‐surface elevation, sediment transport, and sediment accumulation were measured at both sites in 2018–2019. The land‐surface elevation and average flooding per tide were equivalent in the field and forest. There were similar sediment retention rates, suggesting that rice crops can help trap sediment in a manner similar to mangrove trees. Consequently, unleveed fields are less likely to sink than leveed fields, despite mangrove removal. However, mangrove forests can provide additional benefits and should be preserved where possible. Key Points: Elevation and sediment import are equivalent in an unleveed agricultural field and a mangrove preserve in the Ayeyarwady Delta, MyanmarUnleveed agricultural fields in deltas are less vulnerable to subsidence than leveed or poldered fields [ABSTRACT FROM AUTHOR]

Through a field experiment on a sandy beach at Pingtan, China, we propose the first model to characterize the variability of apparent von Kármán parameter κa ${\kappa }_{a}$ in the thermally and sedimentologically stratified air flows. The model shows that κa ${\kappa }_{a}$ is negatively correlated to flux Richardson number as well as sediment transport rate. The atmospheric stability can be either from thermal stratification or sedimentological stratification, neglecting either of them can lead to substantial errors in predicting shear velocity and sediment transport rate. Plain Language Summary: In natural conditions, atmospheric boundary layer flows are often non‐neutral due to thermal stratification. However, recent studies indicate sedimentological stratification may also have a similar effect while almost no research was conducted to examine their combined effect on the flows. We present the first model to characterize the apparent von Kármán parameter κa ${\kappa }_{a}$, a parameter representing the scale of flow eddy size, as a function of thermal and sedimentological stabilities. This work provides a feasible means to correct the potentially substantial errors when estimating shear velocity and predicting sediment transport rate for sedimentologically and thermally stratified air flows. Key Points: Apparent von Kármán parameter for thermally and sedimentologically stratified air flows was obtainedApparent von Kármán parameter has a negative, linear relationship with flux Richardson number and sediment transport rateNeglecting thermal and sedimentological stratifications can lead to substantial errors in shear velocity and transport rate estimations [ABSTRACT FROM AUTHOR]

The Coastal Dune Model, used to examine controls on dune development and evolution, includes negative feedback between the wind and the evolving topography that results in a scale‐invariant beach‐dune profile. While a limited number of reference profiles appear to support the Coastal Dune Model assumption of scale‐invariance, previously published studies and conceptual models suggest that the beach‐dune profile is not scale‐invariant. Using a combination of journal articles, textbooks, and remotely sensed data from Prince Edward Island, Washington state, and Texas, this short communication finds that there is no evidence for a scale‐invariant beach‐dune profile within and between field sites. Variability in the beach‐dune profile is due to site‐specific and scale‐dependent controls on the beach and the dune that are often highlighted in the conceptual models that authors use to depict the controls on sediment supply and transport from the beach to the dune. Results highlight a need to critically evaluate the assumptions and applications of models such as the Coastal Dune Model, and to explore how and when sites can be considered representative and in support of model results. Key Messages: There is no scale‐invariant beach‐dune profile as assumed in recent models.Beach‐dune profiles vary in response to site‐specific and scale‐dependent controls on sediment supply and transport.Further study is required to determine how and when sites can be considered representative. [ABSTRACT FROM AUTHOR]

Overwash is the cross‐shore transport of water and sediment from a waterbody over the crest of a sand or gravel barrier beach, and washover is the resulting sedimentary deposit. Washover volume, and alongshore patterns of washover distribution, are fundamental components of sediment budgets for low‐lying coastal barrier systems. Accurate sediment budgets are essential to forecasting barrier system sustainability under future climate‐driven forcing. However, comprehensive surveys of three‐dimensional washover morphology are challenging to deliver. Here, we use the results of a physical experiment, analysis of lidar data, and examples of washover characteristics reported in the literature to develop scaling relationships for washover morphometry that demonstrate volume can be reasonably inferred from planform measurements, for washover in natural (non‐built) and built barrier settings. Gaining three‐dimensional insight into washover deposits from two‐dimensional information unlocks the ability to analyze past aerial imagery and estimate contributions from washover flux to sediment budgets for past storms. Plain Language Summary: When a coastal storm drives water up and over the crest of a beach, sediment gets picked up by the flow and deposited behind the beach. That deposit, which might settle among dunes, on a marsh, or on the streets of a neighborhood, is called washover. Knowing the total volume of washover delivered by a storm event helps coastal managers account for the amount of sediment that moves through a beach system over time, known as a sediment budget. An accurate sediment budget provides an indication of environmental functioning and sustainability. However, washover volume is expensive to measure over large spatial scales—so despite its importance to sediment budgets, washover volume often goes unquantified. What if three‐dimensional washover volume could be estimated from the two‐dimensional shape and size of washover deposits visible in aerial photos? Here, we created washover deposits in a laboratory and compared them to washover in field settings to show that the area of a washover deposit is a good predictor of its volume. This relationship between area and volume unlocks information about past storm deposition captured in aerial photo archives, and enables estimation of washover volume where direct measurement of volume is unavailable or impossible. Key Points: Washover volume can be estimated from washover areaScaling relationships for washover size and shape link measurements from a physical experiment and field examplesEstimation of washover volume from aerial imagery is a step toward quantifying storm‐driven contributions to sediment budgets [ABSTRACT FROM AUTHOR]

The sediment composition of the seabed governs its mobility, hence determining sediment transport and morphological evolution of estuaries and tidal basins. Bed sediments often consist of mixtures of sand and mud, with spatial gradients in the sand/mud content. This study aims at increasing the understanding of processes driving the sediment composition in tidal basins, focusing on depositional processes. We show that bed sediments in the Wadden Sea tend to be either mud‐dominated or sand‐dominated, resulting in a bimodal distribution of the mud content where the two modes represent equilibrium conditions. The equilibria depend primarily on the sediment deposition fluxes, with bimodality originating from the dependence of suspended sand/mud concentrations on the local bed composition. Our analysis shows that bimodality is a phenomenon that is not only specific for the Wadden Sea; it can be expected for a wide range of suspended sediment concentrations and thus also in other systems worldwide. Plain Language Summary: The bottom of coastal seas is often composed of two sediment types, namely sand and mud. The evolution of the seabed, and thus of our coasts, depends on how these sediment types move. A correct representation of the spatial sand‐mud patterns in the bed is therefore essential if we want to understand and predict coastal evolution. Studying a large data set we found that sediment beds tend to be either sandy or muddy, with lower chances of finding something in between (i.e., bimodality). Although segregation of sand and mud has often been attributed to erosion processes, we found that this is mainly the result of depositional processes, and especially of the suspended sediment concentrations. Sand concentrations largely depend on local conditions, while local mud concentrations are much more affected by conditions elsewhere and/or earlier. The difference in the suspended sand and mud concentrations leads to multiple equilibria in the bed composition—resulting in the previously mentioned bimodality—for a very wide range of conditions. Therefore, we also expect bimodality and sharp sand‐mud transitions in other systems worldwide. Key Points: Mud contents in the sediment bed can be bimodally distributed, resembling preferential sand‐mud segregationBimodality represents the existence of two stable equilibrium conditions, which are a result of the deposition fluxes of sand and mudBimodality is expected for a large range of suspended sediment concentrations in sand‐mud systems [ABSTRACT FROM AUTHOR]

A physically rational model for river width is critical to predict macroscopic landscape evolution driven by fluvial sediment transport. Growing evidence suggests that rivers widen until the stress exerted by the fluid on the bed surface is close to the critical entrainment stress of the bank material. In this study, we test the limits of this model as a closure assumption in dynamically evolving river systems. We consider a simple laboratory channel with a fixed water discharge, monodisperse bed material, no sediment supply, and an initial relief that was sufficiently large to guarantee a finite transport capacity. Over time, the transport capacity approaches zero through changes in channel morphology. Concurrent measurements of width and sediment load illustrate how theory developed for equilibrium rivers fails to predict transient channel processes. Similarity to first order trends in natural rivers suggest that many rivers could be far from equilibrium. Plain Language Summary: River channel width influences the rates and locations of large‐scale geomorphic change. Recent studies argue that channel width is set by a balance between fluid forces and gravity acting on sediment particles that make up the banks and bed. Here, we test whether this principle constrains the width of a disequilibrium laboratory river. Width remains roughly constant throughout the experiment, diverging from physical theory but mirroring observations of natural rivers. Based on this result, we suggest that many rivers on Earth's surface could be far from equilibrium. Key Points: Sediment discharge approaches zero in a laboratory river with no sediment supplyWidths depart from near‐threshold model predictionComparison with global database indicates many rivers could be far from equilibrium [ABSTRACT FROM AUTHOR]

Internal waves (IW) are crucial contributors to the transport of sediment, heat, and nutrients in coastal areas. While IW have been extensively studied using point measurements, their spatial variability is less well understood. In this paper we present a unique set of high‐resolution infrared imagery collected from a helicopter, hovering over very energetic shoaling and breaking IW. We compute surface velocities by tracking the evolution of thermal structures at the ocean surface and find horizontal velocity gradients with magnitudes that are more than 100 times the Coriolis frequency. Under the assumption of no vertical shear we determine vertical velocities from the obtained horizontal divergence estimates and identify areas of the wave undergoing breaking. The spatial variability of the internal wave occurs on scales from a few to a few hundred meters. These results highlight the need to collect spatio‐temporal observations of the evolution of IW in coastal areas. Plain Language Summary: Oceanic internal waves (IW) form between heavier and lighter ocean water in the ocean interior, covering a wide range of temporal and spatial scales. They are amongst some of the key processes in the ocean—they facilitate transport of heat, sediment, and nutrients which have important implications for plankton and other ocean lifeforms, and serve as a pathway of energy from large to small scale flows. Yet, they are often measured using a point based approach (e.g., a buoy), which is unable to provide a full picture of the spatial variability of these waves and hence might not capture crucial behavior. Here, a unique set of high resolution airborne measurements is presented, covering several kilometers of a shoaling (and breaking) internal wave packet. Velocities at the sea surface are determined from infrared video imagery, using the temperature of the water as a tracer. By applying simple models to the velocity data (no change of horizontal velocity with depth), we show that variations in the vertical transport and in breaking characteristics are significant on many different spatial scales. This work is a step forward in understanding and quantifying the spatial variability of IW and processes associated with their breaking induced flow. Key Points: We present novel high‐resolution airborne observations of the kinematics of shoaling and breaking internal wavesWe find that the active breaking regions of the internal wave drive strong convergence/divergenceThe spatial variability of the internal wave is shown to occur over a broad range of scales, from a few to a few hundred meters [ABSTRACT FROM AUTHOR]

Understanding the influence of vegetation on flow and sediment transport is essential to wetland protection and restoration projects. Laboratory experiments were conducted to investigate the sediment pickup (or entrainment) over bare and vegetated beds with model vegetation (i.e., cylinder arrays) of different solid volume fractions, ϕ $\phi $, and diameters, d $d$. For the vegetated beds, measured pickup rate was closely correlated with turbulent kinetic energy, kt ${k}_{t}$, and τ $\tau $‐based model developed for bare channels significantly underestimated the pickup rate measured in vegetated channels. Building on previous theoretical analyses and observations, this study proposed two kt ${k}_{t}$‐based predictions for pickup rate that include the impact of both bed‐ and vegetation‐generated turbulence. The new models improve the prediction of pickup rate within vegetated regions. Plain Language Summary: Aquatic vegetation abounds in rivers, lakes, and coastal regions. It plays an important role in ecosystems, for example, it enhances water quality, stabilizes riverbanks, and promotes soil carbon storage. During the past decades, however, the area of vegetated ecosystems has significantly decreased, leading to a growing interest in wetland restoration. To improve restoration outcomes, it is essential to understand how vegetation impacts flow and sediment motion. This study revealed that traditional models describing sediment erosion in bare channels underestimated by orders of magnitude the erosion rate in vegetated regions. This was attributed to the influence of vegetation‐generated turbulence on sediment erosion. New predictions for sediment erosion rate that incorporate the influence of turbulence were proposed and validated. The new description of erosion can improve the modeling of landscape evolution and marsh restoration. Key Points: Time‐averaged bed‐shear stress models significantly underestimated sediment pickup rate in vegetated channelsNew turbulence‐based models predicted sediment pickup rate in both bare and vegetated channelsPickup rate resulting in either suspended load or bedload can be described by the same law [ABSTRACT FROM AUTHOR]

Shallow coastal muddy sediments are important for global nutrient cycling and carbon storage and provide a habitat for diverse communities of organisms. Bottom shear stress from physical disturbances such as storms and hurricanes resuspend coastal muds, disrupting their cohesive structure. How biological vs. physical processes affect reconsolidation of physically disturbed natural muds, however, is poorly understood. We compare the recovery of sediment properties related to compaction and cohesion of muds following physical disturbance. We hypothesized that recovery of cohesion would take longer than compaction and that changes in surface and subsurface cohesion would occur on different timescales. We collected muddy sediment cores and resuspended the top 5 cm in the lab to simulate storm disturbance. At several timepoints following resuspension (1–30 d), we measured sediment properties providing metrics of compaction (sediment height, porosity, grain size distribution, sound speed) and of surface and subsurface cohesion (erodibility, fracture behavior, exopolymeric substances [EPS]). Compaction and cohesion increased rapidly over the first 1–2 d after resuspension and continued to slowly increase for 2 weeks. Between 2 and 4 weeks, however, subsurface cohesion increased and turbidity decreased, consistent with sediment restabilization. In contrast, subsurface compaction decreased and eroded mass increased, suggesting destabilization. We attribute this apparent destabilization to delayed activity of small‐bodied infauna. Surprisingly, EPS concentration did not explain cohesion changes. These results highlight the importance of including biological parameters when predicting the recovery of sediment structure following a physical disturbance. This work has implications for understanding coastal sediment transport dynamics in frequently disturbed sediments. [ABSTRACT FROM AUTHOR]

Methane (CH4) produced in anoxic sediments plays a significant role in the carbon economy of many lakes and reservoirs. CH4 released from sediments first crosses the bottom boundary layer (BBL), the layer of water overlying the lakebed where currents are slowed by friction with the sediments below. Physical and biogeochemical conditions in the BBL, which can fluctuate hourly to daily with basin‐wide internal waves (seiches), likely influence CH4 transport from sediments into the hypolimnion. In this study, we estimated CH4 fluxes across the BBL of a eutrophic lake using a novel in situ flux gradient approach adapted from marine applications. For 2–6 h periods throughout the spring and summer, we estimated CH4 fluxes across the BBL using simultaneous measurements of CH4 concentrations, turbulent mixing, and thermal stratification. Sub‐daily variation in CH4 fluxes was high, and CH4 fluxes sometimes changed several‐fold within hours. These rapid shifts in BBL fluxes were likely influenced by fluctuations in seiche‐driven variations in the intensity of BBL turbulent mixing. Fluxes increased from spring to summer, concurrent with the development of lake stratification, and fueled an accumulation of CH4 below the thermocline. Throughout the summer, CH4 flux across the BBL exceeded CH4 accumulation below the thermocline, suggesting significant methanotrophy in the hypolimnion, consistent with incubation‐based oxidation rates. Our results are the first to demonstrate sub‐daily and seasonal variability in the timing and magnitude of CH4 fluxes within a lake BBL, and highlight a need to quantify such variability in other lentic systems. [ABSTRACT FROM AUTHOR]

Fluvial cross strata are depositional products of bedform migration that record formative flow and sediment transport conditions on planetary bodies. Bedform evolution varies with transport stage even under constant flow depths, but our understanding of how prevailing sediment transport conditions affect preserved cross strata is limited. Here, we analyzed experimental bedform evolution and preserved set thickness spanning threshold‐of‐motion to suspension‐dominated transport conditions at multiple equilibrium flow depths. Results show that bedform trough depth and mean preserved set thickness have a parabolic dependence on transport stage, with maximum values observed at intermediate transport stages. Our results indicate that transport stage is a key control on the flow‐depth‐normalized set thickness but set thickness is a poor indicator of flow depth. Thus, the dependence of bedform dimensions on transport stage should be considered in paleohydraulic reconstruction, and the analysis of set thickness may aid in the estimation of ancient fluvial sediment flux. Plain Language Summary: Alluvial rivers have ripples and dunes (i.e., bedforms) that migrate across the riverbed, resulting in characteristic signatures called cross strata found in river deposits on Earth and Mars. Cross strata encode information about the flow and sediment transport conditions under which bedforms evolved. For over a century, the relation between cross strata thickness and formative flow conditions has been studied. However, we do not understand how prevailing sediment transport conditions control set thickness distributions. We used high‐resolution topography of bedform evolution from controlled physical experiments where transport stage—a parameter that quantifies sediment transport intensity—was systematically changed across multiple flow depths. We discovered that preserved cross set thickness is primarily controlled by transport stage, and that the mean set thickness of preserved deposits is a poor indicator of flow depth. We develop a new mathematical function that describes the dependence of mean set thickness on the formative transport stage. Our work indicates that the transport‐stage dependence of set thickness should be a primary consideration in paleohydraulic reconstruction, and that analysis of cross strata may aid the estimation of paleo‐sediment‐transport conditions. Key Points: Mean bedform trough depths and set thickness show a parabolic dependence on transport stage peaking under mixed‐load transport conditionsMean set thickness is a poor indicator of flow depth, but depth‐normalized mean set thickness is strongly influenced by transport stageTransport stage exerts a key control on preserved fluvial strata and should be considered in paleo‐hydraulic reconstructions [ABSTRACT FROM AUTHOR]

Anthropogenic stressors on large rivers have significantly modified riverine sediment transport. However, the impacts of dredging and sand and gravel (S&G) mining are poorly monitored. We quantify these impacts on the lower Arkansas River, USA, where anthropogenic processes are well documented. The construction of dams caused a 98% reduction in suspended sediment discharge (Qss). Since dam construction, fluvially transported suspended sediment (4.4 ± 0.5 Mtyr−1) and suspended sand discharge (Qsand) (1.1 ± 0.1 Mtyr−1) are of the same order as the dredging rate (1.2 ± 0.1 Mtyr−1) and S&G mining rate (1.7 ± 0.1 Mtyr−1). While the modern sediment deficit on the Arkansas River is smaller than in rivers documented in developing countries, dredging and mining significantly alter the sediment balance in this large river, indicating that extraction management is needed worldwide, especially in developing economies, to improve ecosystem sustainability in large rivers. Plain Language Summary: Large river systems provide many services, including water resources, barge transport, and sand and gravel for construction. Dredging for navigation and sand and gravel (S&G) mining are pervasive in many rivers, but good estimates of these activities are difficult to find. Based on the long‐period records of sediment extraction in the lower Arkansas River (LAR), U.S.A, we show that the river's sediment discharge is of the same order as the sediment extraction rates by dredging and S&G mining. We show that the LAR has a sediment deficit between the cities of Van Buren and Little Rock because the amount of outgoing sediment, including dredging and S&G mining, is greater than the amount of incoming sediment. Similar situations exist and are even worse in large rivers in developing countries, indicating actions are needed to manage sediment extraction to achieve sustainable development of large rivers worldwide. Key Points: Navigation dredging and sand and gravel mining contribute significantly to the sediment discharge decline on the lower Arkansas River (LAR)Engineering sediment extraction is the same order as the magnitude of modern fluvial sediment transport on the LARSediment deficit on the LAR is smaller than in rivers documented in developing countries [ABSTRACT FROM AUTHOR]

Freshwater salinization is a common yet underappreciated environmental problem in rivers, yet how this process impacts the transport of dissolved oxygen (DO) in the hyporheic zone remains unknown. Using flume experiments and numerical simulations, we have demonstrated that the morphological dynamics of the DO transition zone are controlled by the dimensionless number Da·Ra−2, a measure of the rate of aerobic respiration to free convection. At low Da·Ra−2, the oxic front breaks up into fingers which subsequently grow, and leads to accelerating of DO transport and shrinking of the anaerobic zone in the underlying sediment. As Da·Ra−2 increases, the growth rate of the instability decreases, and the DO plume fingers are suppressed and delayed compared with the saltwater‐freshwater interface, the mixing area between saltwater and freshwater. These results indicate that the freshwater salinization syndrome can have significant impacts on the functioning of aquifer ecosystems unless regulated and managed effectively. Plain Language Summary: Increases in salinity, alkalinity, and major ions occur in a wide range of freshwater ecosystems around the world, and the freshwater salinization syndrome is becoming a serious water quality problem and a major chemical signature of the Anthropocene. The goal of this study is to understand how dissolved oxygen (DO), which is beneficial to the survival of benthic invertebrates, is transported in the sediment in response to freshwater salinity fluctuation. Flume experiments and numerical simulations are performed to investigate the development of DO plume in the riverbed. The experimental outcomes, obtained through high‐resolution imagining techniques, show that the convection induced by saltwater density gradients in the hyporheic zone could influence DO plume development. Our numerical model is capable of describing the transport behaviors of DO before and after salinization in the surface water. A dimensionless number Da·Ra−2 is proposed to predict DO transport dynamics in the river with freshwater salinization syndrome. The implications and potential threats of freshwater salinization for the river ecosystem, including the biogeochemical processes and contaminant attenuation in the hyporheic zone, are also discussed. Key Points: Experiments and simulations are performed to investigate the morphology of dissolved oxygen (DO) transition zone in a salinity‐impacted bedformAsynchronous transport of salt and DO is controlled by a dimensionless number Da·Ra−2Whether density‐driven flow can be a DO pump depends on the intensity of aerobic respiration [ABSTRACT FROM AUTHOR]

Seagrass meadows are important carbon sinks in the global coastal carbon cycle yet are also among the most rapidly declining marine habitats. Their ability to sequester carbon depends on flow–sediment–vegetation interactions that facilitate net deposition, as well as high rates of primary production. However, the effects of seasonal and episodic variations in seagrass density on net sediment and carbon accumulation have not been well quantified. Understanding these dynamics provides insight into how carbon accumulation in seagrass meadows responds to disturbance events and climate change. Here, we apply a spatially resolved sediment transport model that includes coupling of seagrass effects on flow, waves, and sediment resuspension in a seagrass meadow to quantify seasonal rates of sediment and carbon accumulation in the meadow. Our results show that organic carbon accumulation rates were largely determined by sediment accumulation and that they both changed non‐linearly as a function of seagrass shoot density. While seagrass meadows effectively trapped sediment at meadow edges during spring–summer growth seasons, during winter senescence low‐density meadows (< 160 shoots m−2) were erosional with rates sensitive to density. Small variations in winter densities resulted in large changes in annual sediment and carbon accumulation in the meadow; meadow‐scale (hundreds of square meters) summer seagrass dieback due to marine heatwaves can result in annual erosion and carbon loss. Our findings highlight the strong temporal and spatial variability in sediment accumulation within seagrass meadows and the implications for annual sediment carbon burial rates and the resilience of seagrass carbon stocks under future climate change. [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]

Field measurements of aeolian sediment transport during major storm events are challenging in coastal settings due to the common coincidence of high winds with elevated water levels that can cover beaches. This work demonstrates a new application of lidar to remotely measure properties of wind‐blown sand transport at high spatial resolution. A data set of spatially continuous saltation heights was remotely developed from rapid (7.1 Hz) lidar scanning along a two‐dimensional transect across a beach during wind speeds of between 3.1 and 15.8 m/s associated with the passing of Tropical Storm Isaias. Outputs from the lidar demonstrate the ability to remotely observe the threshold velocity for the initiation of motion for sand transport and show that the upper limit of the saltation layer rises with increasing wind speed. Plain Language Summary: Wind can be an important contributor to landscape changes in sandy desert and coastal environments. There are complex mechanisms by which wind moves sand that are not yet entirely understood. To answer questions about how, when, and why sand is transported by wind, experiments commonly use point‐based sensors that are unable to capture the full spatial and temporal complexity of sand movement very close to the ground. This paper shows that new laser sensors can be utilized to remotely measure properties of sand transport simultaneously at multiple points and with increased detail compared to prior approaches. High resolution, remote measurements from a tropical storm event are used to show how high wind‐blown sand is lifted off of the ground surface and how different wind speeds influence these sand transport patterns. Key Points: Wind‐driven sand transport can be measured using repeat terrestrial laser scanningRemote measurements are used to characterize time and space variable saltation heights during energetic wind forcingMaximum saltation heights respond to changes in wind speed on a coastal beach [ABSTRACT FROM AUTHOR]

River deltas are home to large populations and can be composed of complex channel networks which convey flows of matter to the shoreline. Knowledge of flow within individual channels is needed to quantify the distribution of discharge across the delta, and thus its sustainability over time. Due to a lack of field measurements at the local channel scale, researchers leverage remote sensing data to estimate the partitioning of flow. We compare data from 15 river deltas to discharge partitioning estimates based on channel network graphs derived from remote sensing imagery. We quantify errors in the common width‐based method and test alternative partitioning techniques to find that width‐based discharge partitioning is universally applicable, suggesting that absent any site‐specific information, discharge partitioning by average channel width is an appropriate approach. We also provide networks, streamflow measurements, and flux partitioning estimates for 28 delta networks as the Discharge In Distributary NeTworks (DIDNT) dataset. Plain Language Summary: River deltas are home to hundreds of millions of people worldwide, yet their sheer size and a high number of channels mean it is not feasible to measure flow in all individual channels. However, knowledge of flow in river channels is critical for understanding the evolution of these systems at the time and space scales relevant to humans, as the flow of water mediates the transport of solutes and sediment. Researchers have proposed methods to estimate the division of flow in delta channels, but the accuracy of these methods has not been quantified. We estimate flow in the channels of 15 river deltas by conceptualizing channels as links and junctions as nodes in a network graph and compare these estimates to reported flow observations. We quantify the error in our estimates and test alternative methods of flow estimation. We find that the division of flow based on average channel width is appropriate. We extract channel networks and apply this method to estimate flow through 28 delta channel networks to form the Discharge In Distributary NeTworks (DIDNT) dataset. Key Points: Channel width‐based flow partitioning through delta networks is tested by comparing estimates based on channel widths to reported valuesPartitioning discharge by average channel width in distributary networks is generally an appropriate strategyThe accompanying Discharge In Distributary NeTworks (DIDNT) dataset provides discharge partitioning estimates for 28 delta networks around the world [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]

Catastrophic drainage of glacial Lake Missoula through the Columbia River Gorge, USA, produced some of the largest floods ever known. However, erosion of the gorge during flooding has not been quantified, hindering discharge reconstructions and our understanding of landscape change by megafloods. Using a neural network and geomorphic observations, we reconstructed the gorge topography and found ∼7.4 km3 of rock was eroded from gorge walls. Accounting for a narrower canyon and matching flood high‐water marks resulted in peak‐flood discharge reconstructions of 6 × 106–7 × 106 m3 s−1, which are 30%–40% lower than prior estimates based on the present‐day topography. Sediment transport modeling indicated that more frequent intermediate‐sized floods transported most of the eroded rock. Thus, similar to alluvial rivers, discharge magnitude‐frequency tradeoffs may also govern canyon formation by repeated megafloods. Plain Language Summary: Some of the largest floods in Earth's history occurred 20–14 thousand years ago from the catastrophic drainage of glacial Lake Missoula. The floodwaters coalesced near the Columbia Gorge, where the water flowed through a single channel. Prior work focused on the Columbia Gorge to reconstruct the magnitude of these floods. However, these estimates did not account for erosion by the massive floods, which changed the topography. In this study, we used a machine learning computational technique to reconstruct the topography prior to megaflood erosion. Our results show ∼7.4 km3 of rock was eroded from gorge walls. Accounting for a narrower gorge and matching flood high‐water marks resulted in a peak‐flood discharge that is 30%–40% lower than prior estimates, thus downsizing the largest floods in Earth's history. Modeling sediment transport over a series of floods indicated more frequent intermediate‐sized floods transported most of the eroded rock, rather than the largest floods. This finding is similar to observations in rivers with sediment banks where tradeoffs exist between the size and frequency of floods in governing channel shape. Thus, it is possible that the same principles that govern the form of alluvial rivers also govern bedrock canyon formation by repeated megafloods. Key Points: Geomorphic observations and topographic reconstruction indicate the Missoula floods caused widening of the Columbia GorgePeak discharge estimates decline by 30%–40% when accounting for a narrower canyonMegafloods of moderate frequency and magnitude, rather than the largest flood, transported most of the eroded rock from the Columbia Gorge [ABSTRACT FROM AUTHOR]

Alluvial fan sediments contain valuable information on environmental change and anthropogenic influences, and function as an archive of landscape development. The purpose of this paper is to describe the analysis of the sedimentary information from core samples taken from two alluvial fans in the Saskatchewan prairies. The analysis assessed grain size, organic matter content, and microfossil presence to identify periods of accumulation and stabilization. Parker, Beavers, and Product weathering indices were used to analyze the modern source soil samples and the borehole cores to determine weathering/leaching conditions. The bed‐load transport sediments showed a higher leaching intensity whereas lower leaching was revealed for periods of stability, silt sheet flows, and sand sheet flows. The Beavers Index was not only a strong indicator of a bed‐load transport layer, but also of calcium accumulation zones marking alluvial fan surface stability. The large Dolomite/Calcite ratio marking the calcium accumulation zones was used for correlation between the two alluvial fan sites on a regional scale. A comparison of the stratigraphy of the two alluvial fans documents that a relationship exists between agriculture and an increase in erosion. Key Messages: When faced with a lack of 14C data, sedimentary information can provide insight into processes of alluvial fan development.The small number of microfossils found did not allow microfossils analysis.Weathering indices and EDX analysis were able to identify phases of weathering/leaching, indicating periods of accumulation and stability. [ABSTRACT FROM AUTHOR]

Crystal lattice defects in quartz have long been exploited for age determination, yet also show potential for sediment provenance studies. Here, we introduce a novel method for tracking aeolian dust provenance by utilizing the natural accumulation of E1' and peroxy defect centers in quartz. Our approach is based on the previously observed premise that E1' and peroxy centers arise from Frenkel defect pairs, and that their concentration increases with the age of the quartz‐bearing source rock. We propose that these defect centers can be utilized as a characteristic feature of the source rock and consequently, for fingerprinting sediments derived from it. We successfully apply our new protocol to distinguish fine‐grained quartz extracted from loess deposits from two regions in Central Asia which are known to derive from different source material of differing age. Our method offers strong potential for identifying variability in source, both spatially and through time down sedimentary sequences. Plain Language Summary: Identifying the origins of dust deposits allows us to reconstruct sediment transport pathways which are essential for understanding past atmospheric circulation patterns. Here, we propose to exploit the characteristics of two naturally occurring defect centers in crystalline quartz, the E1', and peroxy centers, as a means to distinguish sediment deriving from different origins. These centers occur as pairs and are hypothesized to increase with the age of the quartz‐bearing rock. By this logic, the E1' and peroxy centers can be used to determine the lithic origins of sedimentary quartz in a similar way to detrital zircon‐based provenance techniques, while analyzing a more ubiquitous mineral (quartz). We apply our approach, which uses a simplified protocol for measurement in contrast to earlier studies, to successfully distinguish between loess (wind‐blown dust deposits) from two different basins in Central Asia. Our new method holds great potential in its application to loess sequences as well as other sedimentary archives. Key Points: New sediment provenance tool exploits E1' and peroxy paramagnetic defects in quartzNew proxy successfully differentiates quartz in loess from two different basins in Central AsiaPotential applications for identifying climate‐driven source change through time in loess and other sedimentary sequences [ABSTRACT FROM AUTHOR]

The Cenozoic history of the Tibetan Plateau topography is critical for understanding the evolution of the Indian‐Eurasian collision, climate, and biodiversity. However, the long‐term growth and landscape evolution of the Tibetan Plateau remain ambiguous, it remains unclear if plateau uplift occurred soon after the India‐Asia collision in the Paleogene (∼50–25 Ma) or later in the Neogene (∼20–5 Ma). Here, we reproduce the uplift history of the southeastern Tibetan Plateau using a 2D landscape evolution model, which simultaneously solves fluvial erosion and sediment transport processes in the drainage basins of the Three Rivers region (Yangtze, Mekong, and Salween Rivers). Our model was optimized through a formal inverse analysis with 20,000 forward simulations, which aims to reconcile the transient states of the present‐day river profiles. The results, compared to existing paleoelevation and thermochronologic data, suggest initially low elevations (∼300–500 m) during the Paleogene, followed by a gradual southeastward propagation of topographic uplift of the plateau margin. Plain Language Summary: When and how was formed the Tibetan Plateau which includes the highest mountains in the world? The answer is critical to decipher between competing models trying to explain how the Indian and Asian tectonic plates behaved when they collided. It is also a major constraint on the evolution of Asian mountain biodiversity, Asian monsoons, and even global climate. However, current estimates of past Tibetan elevation are contradictory, some arguing for a high plateau since the onset of the India‐Asia collision over 50 million years ago, while others favor the region remained low until a sudden rise 25 million years ago. Here, we use a numerical model of landscape evolution to test various hypothesis focusing on the southeastern Tibetan Plateau. We compared the results to see how they fit with the observed river profiles of the Three Rivers. The best fits were obtained with low elevations between 50 and 25 million years ago, and a more gradual growth toward the southeast of the high‐elevated plateau until present day. Key Points: We constrained the uplift history of the southeastern (SE) Tibetan Plateau by performing inversion of the Three Rivers profilesThe inversion suggests that SE Tibet was at a low elevation until 50–25 Ma, followed by gradual outward growth until 0 MaOur modeling does not support Paleogene formation of the SE Tibetan Plateau with a major subsequent degradation via upstream fluvial erosion [ABSTRACT FROM AUTHOR]

River dunes are wavelike features in sandy riverbeds that move slowly in a downstream direction. These bedforms are shaped by the interaction between flowing water and sand grains at the riverbed. Understanding river dune dynamics is essential to determine sediment transport and optimize river maintenance. Earlier research mainly focused on river dune dynamics during high flows; therefore, low‐flow dune dynamics are hardly understood. In this study, we analyzed dune dynamics in a unique data set of 10 years of biweekly bed elevation profiles of the Rhine River, focusing on low‐flow conditions. Here, we show that dunes migrate and become lower and longer during low flows, while current literature states that dunes should become shorter. This dune migration and lengthening at low flows is a new finding and is linked to diffusive dune migration processes that become more relevant when sediment transport is just above incipient motion. Plain Language Summary: River dunes are wavelike features on sandy river beds. They are shaped by the interaction between the flowing water and the sand grains at the river bed and move slowly in the same direction as the river flow. Better understanding of the changes in shape, especially when water depths are small, can help to maintain the river bed since high dune peaks can hinder shipping. We studied the dune shape from bed elevation measurements of the Rhine River covering 10 years, focusing on a period with small water depths. We found that the dunes become lower in height at smaller water depths, which we expected based on earlier studies. We also found that the dunes become longer in length and keep moving, contradicting the current knowledge of dunes. Key Points: River dunes are mobile during low flowCrest‐to‐crest length of river dunes increases when sediment transport capacity approaches incipient motionDuring low flow dunes migrate diffusively [ABSTRACT FROM AUTHOR]