Sediment Supply Research Articles

Contrasting provenance budgets for suspended load and bedload of the Yarlung Tsangpo, Tibet: Lhasa block or Himalaya?

Sediments from the same river are typically thought to reflect a common provenance, even though they may record notably different, and even opposite, sediment supply and erosion signals. Such discrepancies can introduce significant bias into our interpretation of geomorphological processes. This study focuses on the Yarlung Tsangpo, the Tibetan headwaters of the Brahmaputra River, and contrasts mineralogical and geochemical information on suspended load in transit and on fluvial bars to reveal a major discrepancy in sediment budgets calculated with petrographic and isotopic data for sand and mud fractions. Our results show that in both suspended load and fluvial bars, sand records an overwhelming contribution from the Lhasa block, whereas mud reflects dominant supply from the Himalayan belt. Detritus from the Lhasa block is twice as abundant as Himalayan detritus in sandbars, but Himalayan contribution is 1.4 times that of the Lhasa block in suspended load. Overall, Himalayan sedimentary rocks are estimated to generate as much sediment as Lhasa granitoid and volcanic rocks. Himalayan erosion rates and sediment yields are greatly underestimated if only sand, representing a subordinate part of the total sediment flux, is considered in sediment-budget calculations. Both sand-rich bedload and mud-rich suspended load must be given full consideration in the study of sediment-generation processes. Our results highlight the potential pitfalls of relying solely on sand-sized sediment in provenance analysis and force a reevaluation of how sediment yields and erosion patterns are assessed.

Benthic mud content is a strong indicator of coral cover and ecosystem recovery on turbid coral reefs.

Turbid coral reefs are characterised by high turbidity and sedimentation. However, the impacts of terrestrial sediment inputs on coral communities, as well as their interactions with reef-derived carbonate sediment, remain poorly understood. Here we examine the physical properties of mixed carbonate-siliciclastic benthic sediments from six turbid reefs in southern Singapore, which exhibit coral covers ranging from 15% to 65%. Coral was the dominant contributor to sand and gravel sediment on reefs (mean±SE: 57±1%), followed by molluscan grains (21±1%). Mud content ranged from 0 to 26% between sites and there was a positive correlation (r=0.4) between terrigenous mud and coral-derived sand, while coral cover significantly decreased with increasing mud. Results suggest that losses in coral cover may initially cause an increase in bioclastic carbonate sediment as the reef structure degrades. However, long-term declines in coral cover will ultimately reduce sediment supply, as live coral sources diminish on reefs. Our findings highlight the importance of mitigating high sediment loads on coral reefs, even within naturally turbid environments, to ensure continued bioclastic sand generation and ecosystem functioning. Mud content on reefs was also a strong predictor of reef recovery following major coral bleaching underscoring the need for holistic transboundary management of coral reefs as coastal urbanisation continues to intensify.

Reconstructing Tropical Cyclone Activity from Sedimentary Archives

The brevity of the instrumental record limits our knowledge of tropical cyclone activity on multidecadal to longer timescales and hampers our ability to diagnose climatic controls. Sedimentary archives containing event beds provide essential data on tropical cyclone activity over centuries and millennia. This review highlights the advantages and limitations of this approach and how these reconstructions have illuminated patterns of tropical cyclone activity and potential climate drivers over the last millennium. Key elements to developing high-quality reconstructions include confident attribution of event beds to tropical cyclones, assessing the potential role of other mechanisms, and evaluating the potential influence of geomorphic changes, sea-level variations, and sediment supply on a settings’ susceptibility to event bed deposition. Millennium-long histories of severe tropical cyclone occurrence are now available from many locations in the western North Atlantic and western North Pacific,revealing clear regional shifts in activity likely related to intervals of large-scale ocean-atmosphere reorganization. ▪ Prior to significant human influence in Earth's climate, natural climate variability dramatically altered patterns of tropical cyclone activity. ▪ For some regions (e.g., The Bahamas and the Marshall Islands), earlier intervals of tropical cyclone activity exceeded what humans have experienced during the recent period of instrumental measurements (∼1850 CE–present). ▪ Risk assessments based on the short instrumental record likely underestimate the threat posed by tropical cyclones in many regions. ▪ Changes in atmospheric and oceanic circulation associated with the Little Ice Age (∼1400–1800 CE) resulted in significant regional changes in tropical cyclone activity. ▪ Given the past sensitivity of tropical cyclone activity to climate change, we should anticipate regional shifts in tropical cyclone activity in response to ongoing anthropogenic warming of the planet.

Channel Incising and Sandbar Growth in the Upper Yangtze River Estuary During 1994–2019, China

Anthropogenic activities and climate change have increased the stress on the world’s estuaries over the past decades. Limited knowledge exists about how estuarine receding responds to human interference, particularly the geomorphic dynamics of channels and sandbars. Here, we evaluate the topographic evolution of the upper Yangtze River Estuary (YRE), the largest branch reach with frequently shifting sandbars, from 1994 to 2019. Our results show that a net channel erosion of 9.59 × 108 m3 occurred in the upper YRE, equivalent to an annual erosion depth of 8.67 cm. On the contrary, sandbars with a large area increased from 47.68 km2 to 70.88 km2, showing the opposite development of estuarine channels. Reduced riverine sediment supply may have been responsible for the estuarine channel erosion, and river engineering may have contributed to intense erosion in local areas. Also, the engineering projects were likely the main reason for the stability and growth of the sandbars. This study reveals the branching channel–sandbar system of the upper YRE in response to anthropogenic and climatic change forcing. The knowledge gained from this study can be applied to other similar estuarine systems around the world, helping develop sustainable strategies for the utilization and protection of the world’s estuaries and deltas.

Increased sea level rise accelerates carbon sequestration in a macro-tidal salt marsh.

Salt marshes are known as key ecosystems for nature-based climate mitigation through organic carbon sequestration into their sediment beds, but at the same time they are affected by accelerating sea level rise induced by climate warming. Consequently, an important question is how organic carbon accumulation rates (OCAR) of salt marshes will respond to future accelerating rates of relative sea level rise (RSLR). To date, existing insights are either based on (1) comparison of geographically distant marsh sites, differing in local RSLR rates but also in other environmental conditions that additionally can affect OCAR, or (2) experiments in given marsh sites, in which proxies for RSLR are manipulated, but run over periods of years instead of decades, the latter being the relevant time scale of marsh responses to RSLR. Here we bridge these shortcomings by studying the OCAR over four decades at two nearby salt marsh sites in the Netherlands, with similar environmental conditions, but with one site experiencing an accelerated RSLR rate of 9.7-11.7mmyr-1 (i.e., within the range of projected global mean sea level rise rates by 2100) due to local land subsidence induced by gas extraction, while the other site does not experience subsidence and has a low background RSLR rate of 2.0mmyr-1 (i.e. close to the current global mean sea level rise rate). Our results reveal that the salt marsh site experiencing the accelerated RSLR rates shows OCAR values that are on average twice as high as those found in the marsh site experiencing the low background RSLR rates. Moreover, the increase of OCAR in response to faster RSLR was even more pronounced (i.e. 63% increase) on marsh levees within 10m from tidal creeks, while this was more subtle (i.e. 27% increase) in marsh basins at a distance of 30-40m from the creeks. These observations of increased OCAR are mainly attributed to increased sediment accretion rates (SAR) in response to (1) increased tidal inundation due to accelerated RSLR and (2) larger sediment supply due to closer proximity to creeks, while sediment organic carbon content was relatively little affected. Our findings support expectations that nature-based climate mitigation actions, through salt marsh conservation and restoration, are sustainable on the long term of the coming decades, and are even likely to become more effective with future accelerations in global sea level rise, at least for macrotidal sites not limited by sediment supply.

Characterization and Spatial Distribution of Sand Group Architecture and Channel Types in Tight Gas Reservoirs: A Case Study From the Jurassic Shaximiao Formation of the Jinqiu Gas Field in the Central Sichuan Basin of China

There is an abundance of tight gas resources in narrow channel sand‐bodies from the Jurassic Shaximiao Formation of the Jinqiu gas field in the central Sichuan Basin of China. The architecture of sand group in the study area is undefined, and the spatial distribution of channel sand‐bodies is unclear. The complex and inhomogeneous sandstones have a significant impact on the reservoir’s physical properties and the fluid mobility of the reservoir. In this study, data from drilling cores, logs, seismic, and experiment testing were used to investigate the spatial distribution of sand group architecture and the channel types. There are five channel genetic types, including the multiphase superimposed type, deeply incised type, abandoned type, progradational superimposed type, and normal single genetic type. Based on the channel genetic types, the ratio of sandstone and mudstone, the ratio of width to depth, the connectivity ratio of sand‐bodies, and the production dynamic characteristics, the channel sand‐body connectivity is defined into three types. The connected sand‐bodies occur in the multiphase superimposed and deeply incised types of channels, with an average connectivity ratio of 83%, a ratio of sandstone and mudstone larger than 0.9, and a ratio of width and depth larger than 40. Based on the association of sandstone and mudstone and rhythmic structure, the sand group architecture can be divided into three types, including (A) uniform‐grain‐sequence pure sandstone architecture, (B) positive‐grain‐sequence thick sandstone and thin mudstone architecture, and (C) positive‐grain‐sequence thick mudstone and thin sandstone architecture. There is a high content of natural gas in Types A and B of sandstones, with a daily gas production of 29.16 × 104–47.6 × 104 m3/day and pressure coefficients of 0.72–1.08. The sand group architecture of the study area is mainly controlled by the channel sinuosity and the ratio of accommodation and sediment supply, and Types A and B of sand group architectures occur with large channel sinuosity of 1.14–1.36 and a large ratio of accommodation and sediment supply of 0.61–2.92. Based on the connectivity degree of channel sand‐bodies, the sand group architectures, and production data, the channels of the study area can be divided into three types. Type I channels mainly occur in Sand Groups 6, 8, and 9, and Type II and Type III channels occur in Sand Groups 6 and 7 in the western and southern parts of the study area. The technology of fine characterization for channel sand‐bodies on the basis of human–computer interaction and seismic attributes is proposed, and geological modelling of the spatial distribution of sand group architectures and channel types is established. The research results achieve a theoretical breakthrough in the characterization of the sand‐body structure of tight sandstone reservoirs in narrow river channels and assist in the efficient exploration and development of tight sandstone gas.

Sediment dynamics of watershed urbanization and river restoration: Insights from 10 years of research in small gravel‐bed rivers

AbstractWatershed urbanization frequently leads to river channel enlargement and incision. Mitigation strategies such as in‐channel restoration and stormwater management have been developed to counteract these problems, yet alternatives are rarely considered with respect to the sediment dynamics that underlie the process of river degradation. In the current paper, we revisit two heavily urbanized small gravel‐bed rivers where aggressive stormwater management and river restoration projects were completed. The goal is to consider how information from 10 years of research on sediment dynamics might change the adopted mitigation strategies. By synthesizing previous work and novel analyses, we provide a diagnosis of the effects of urbanization and existing restoration efforts on the channel morphology and sediment dynamics and develop a sediment augmentation strategy to restore the dynamic equilibrium of the study reach. Additionally, we model the cumulative impact of various stormwater management strategies on sediment dispersal. We place these river management strategies within a framework that seeks to address the root cause of urban river degradation by rebalancing the sediment supply and capacity of these channels.

Eco-geomorphic modelling response of tidal marshes to sea level rise and changes in suspended sediment supply.

Tidal marshes are coastal systems that provide valuable ecosystem services, highlighting coastal protection and carbon burial. For centuries, these dynamic ecosystems have kept pace with sea level rise through organic and mineral matter accumulation. In the current situation of accelerated sea-level rise and changes in suspended sediment concentrations, the evolution of these systems has gained special attention across scientific fields. Several methodologies like process-based models and machine learning algorithms have been applied to assess the evolution of tidal marshes in different sea level rise and suspended sediment concentration scenarios. However, up to now, these methodologies have not been integrated to assess and model the evolution of marshes. In this study, we have successfully combined a machine learning algorithm with a dynamic process-based eco-geomorphic model to assess and evaluate potential distributions of three Spanish marshes, under a selection of potential sea-level rise and suspended sediment concentrations which may unfold during this century. Results obtained from this study have proven that through the integration of existing methodological approaches and their adaptation to test contexts, we can better simulate the potential evolution of marsh systems on a local scale considering potential sea level rise and suspended sediment concentration changes. Under current sediment supply and public land availability, marshes in Oka estuary, Bay of Santander, and Cadiz Bay could lose 6.7-28.9%, 33.1-87.5%, and 41-86.4% of their area, respectively. The integrated marsh evolution model presented in this work can be extrapolated and/or customised to other coastal and marine systems, fostering its reusability.

Characteristics and mineralogy of sediments in the Hongsa lignite deposit, northwestern Laos

Two sediment cores from the central part of the Hongsa lignite deposit in northwestern Lao PDR (Lao People’s Democratic Republic; Laos) have been analysed in order to understand their sedimentary characteristics using grain-size analysis, petrography, X-ray diffraction and scanning electron microscopy. This analysis has revealed that the deposit is primarily composed of fine-grained sediments, mainly silt and clay, with quartz as the dominant mineral and trace amounts of other minerals such as kaolinite, illite and montmorillonite. Gypsum and chlorite have also been found in some layers. Scanning electron microscope analysis has revealed a card-house structure of clay minerals, suggesting sedimentation from suspension driven by physico-chemical reactions influenced by pH and water chemistry. This arrangement increases porosity and water retention, significantly affecting the permeability and mechanical properties of sediments. Petrographic analysis has documented angular quartz and poorly sorted sediments, indicating minimal sediment reworking or short-distance sources. The palaeoenvironment of the Hongsa Basin, reconstructed from various rock units, suggests low-energy water conditions for the Underburden and moderate sediment supply in a wet forest swamp or bush moor environment for the Lower Lignite Zone Formation. The Middle Lignite Zone Formation indicates a more limited sediment supply in a similar environment, while Interburden Formation 1 suggests overbank deposits or stagnant water deposits. In summary, the Neogene Hongsa lignite deposit is characterised by fine-grained sediments, indicating low-energy water currents in mire environments. Occasional flood events brought coarser grains, although movement of facies should also be taken into account. The mineral composition suggests the presence of components derived from recycled sedimentary rocks along the northern border of the Hongsa Basin.

Fine sediment-associated contaminants in gravel bed rivers: Evaluating storage times and turnover using fallout radionuclides (FRNs).

Excess fine sediment supply and its associated contaminants can have detrimental effects on water quality and river ecology with sediment deposition on, and subsequent infiltration in, streambeds impacting riverine habitats. Fallout radionuclides (FRNs) are used as tracers in aquatic systems, and the 7Be/210Pbex ratio is a useful indicator for sediment residence/storage time. Suspended and submerged mid-channel bar sediments were collected during five surveys within a 5km reach of a typical temperate lowland agricultural river system. Solids were analysed by gamma (7Be and 210Pbex) and inductively coupled plasma (ICP, trace metals and phosphorus) spectrometry, and analysed for total nitrogen and organic carbon, to assess sediment dynamics and associated contaminant and nutrient storage. Significant spatial and temporal variation in 7Be/210Pbex activity ratios was observed, indicating changes in sediment sources closely related to contaminant inputs from legacy mining and agriculture. Storage times and the proportion of recently deposited sediment (RDS) varied between sampling sites and seasons in response to local channel characteristics and floods, which also influenced particulate contaminant distributions. This study demonstrates that FRN technology offers improved understanding of fine sediment and contaminant storage and turnover in river channel systems, which is vital to aid sediment management, river restoration and to tackle the global challenge of siltation and associated pollution in riverine habitats.

New data on the structure of the Laptev Sea flank of the gakkel ridge (Arctic Ocean)

The article provides new data on the structure of the Laptev Sea flank of the Gakkel Ridge. The intensive supply of clastic material from the shelf of the Laptev Sea leads to the development of a thick alluvial cone at the continental foot, which determines the structure of the bottom topography. In the northwestern direction, the influence of the fan decreases and tectonics becomes the main relief-forming factor. The bathymetric survey traced the asymmetrical rift valley of the Gakkel Ridge, the western side of which is complicated by terraces. The presence of fault structures, bottom subsidence and intensive sediment supply, and the widespread development of landslide processes indicate high neotectonic activity of the Laptev Sea flank of the Gakkel Ridge. For the first time in this region, numerous carbonate formations have been discovered, the authigenic cement of which is represented by magnesian calcite or aragonite with an admixture of terrigenous material. Palynological and micropaleontological analysis of carbonate formations indicates the Quaternary formation of authigenic carbonate cement. An important role in the formation of authigenic carbonates was played by diagenetic solutions coming from the sedimentary cover together with methane and products of oxidation of gases and organic matter. Authigenic carbonates were deposited mainly in isotopic equilibrium with bottom water at a temperature of about 0°C. The negative correlation between 87Sr/86Sr and δ13C indicates the presence of at least two different sources of carbonate-forming solutions.

Baseflow and Coupled Nitrification-Denitrification Processes Jointly Dominate Nitrate Dynamics in a Watershed Impacted by Rare Earth Mining.

Mining activities cause severe nitrogen pollution in watersheds, yet our understanding of the transport pathways, transformation processes, and control mechanisms of nitrate (NO3-) in these areas is limited. Based on nearly 4-year observations of groundwater and river in China's largest ion-adsorption rare earth mining watershed, we revealed the dynamics of NO3- and its drivers using stoichiometry-based load model, molecular biological, and multi-isotope approaches. Results indicated that the NO3- dynamics were jointly controlled by sources (precipitation, terrestrial inputs, and sediment supply) and processes (hydrological and biological). The monthly NO3- export load from the 444.4 km2 watershed was 3.72 × 105 kg. Groundwater (36 ± 26%) and soil nitrogen (25 ± 17%) were the primary exogenous sources of NO3-. Baseflow was the main hydrological pathway for legacy nitrogen into the river, contributing 66.8% of the NO3- load. Coupled nitrification-denitrification were key biological processes affecting the NO3- transformation, with denitrification contributing 58%. Burkholderia were most associated with NO3- transformation. Dissolved organic carbon and oxygen were major drivers affecting the NO3- production and consumption. This study highlights effective control and management strategies for nitrogen pollution in mining-affected watersheds, considering not only reducing nitrogen inputs but also integrating hydrological pathways and nitrogen transformation mechanisms.

A Holocene history of high bluffs, strandplains, terraces, and dunes along the southeastern margin of Lake Superior (Michigan, USA) with reference to fluctuating lake levels

The strandplains of the southeast Lake Superior coastline between Au Sable Point and Whitefish Point, Michigan, USA, record a response to Holocene lake-level variation (∼75 m), changing sediment supply, and accommodation space, and are good analogues for future shoreline behaviors associated with water-level changes along lacustrine and marine coastlines. A variety of datasets were used to explore these thick and extensive coastal deposits and to extend further back in time the available relative lake-level curve for the Lake Superior basin. Little is known about the coastal history prior to the Nipissing phase (6−2.8 ka) and what the geomorphic effect was from high rates of water-level rise (∼2−4 cm/yr) during the Nipissing transgression (ca. 8−4.5 ka). Rising and peak water levels eroded into glaciofluvial deposits, which resulted in eastward littoral transport of massive amounts of sediment that led to aggradation and basinward progradation of strandplains during the Nipissing transgression and Nipissing phase. Strandplains developed sequentially eastward as discreet littoral cells ending at the large (∼88 km2) Whitefish Point strandplain complex. High rates of sediment supply drove a shoreline behavior of depositional transgression and depositional regression. In post-Nipissing time, erosion likely continued into glaciofluvial deposits, but falling lake levels likely resulted in decreased sediment supply. With less sediment and less accommodation space, there was an increase in west-to-east longshore sediment transport that contributed to the growth of Whitefish Point during the Algoma (2.8−2.0 ka), Sault (2.0−1.0 ka), and Sub-Sault (1.0 ka to modern) phases. Lake-level rise after 1.0 ka has resulted in retrogradation (erosional transgression) along most of the shoreline between AuSable Point and Whitefish Point.

Seismic stratigraphic units, palaeogeography and their effect on hydrocarbon distribution in the upper Neogene of the Pannonian Basin

The upper Neogene lacustrine sedimentary succession of the Pannonian Basin in Hungary was divided into 40 sequences (clinothems) by seismic stratigraphic methods. The geographic position of the clinothems reflects the gradual infilling of the basin. Different patterns of progradation, aggradation and retrogradation can be observed within the same clinothem sets along the northern shelf of Lake Pannon, demonstrating that lake level was controlled by local factors, such as tectonics and sediment supply, not solely by climate. The distribution of discovered hydrocarbon resources within the clinothems by time, by depositional environment and by geographical regions was investigated for the first time through a systematic reservoir-level analysis. The reservoir sands are increasingly younger from the northwest towards the southeast, following the progradation of the shelf. Within each region, the oldest reservoir layers appear in the deep basin, whereas the second or third reservoir peaks are associated with deltaic environments. There appear to have been no general time intervals with basin-wide reservoir peaks when sands in anomalously large quantity or with specifically good reservoir quality were deposited. The petroleum accumulation was influenced by local factors in each sub-basin, such as inherited structure, syn- and post-sedimentary deformation, sediment source and maturity, and topographic confinement.

Decadal Morphological Evolution and Governance Measures of the South Branch, Changjiang Estuary

Estuaries and deltas hold significant socioeconomic importance and immense ecological value due to their dynamic geomorphic processes and unique geographical advantages. However, in recent decades, delta recession and the instability of river regimes have become global challenges, driven by intensive human interventions in upstream river basins and local regions. This study examines the South Branch of the Changjiang Estuary as a typical case to investigate its morphological evolution over the past decades and project future trends, offering suitable solutions to enhance the river regime stability. Analysis of bathymetric data reveals substantial channel–shoal adjustments in the South Branch from 1958 to 2016, characterized by significant erosion and deposition on a decadal scale. After 1997, reduced fluvial sediment supply has led to widespread erosion in the South Branch. Further disturbances at the Baimao Shoal and Biandan Shoal have exacerbated the instability of the river regime. Numerical predictions indicate continued erosion in the South Branch over the next 20 years, accompanied by further channel–shoal pattern adjustments. Hydrodynamic modeling of proposed measures demonstrates an improved flow ratio for the North Baimao Shoal Channel, contributing to enhanced channel–shoal system stability. These integrated governance measures have been incorporated into the latest renovation plan for the Changjiang Estuary. The findings provide valuable scientific guidance for the comprehensive management of the Changjiang Estuary and offer insights applicable to other large estuaries facing similar challenges.

Fluvial erosion in a catchment cultivated with no-tillage at the edge of the southern Brazilian plateau.

The dynamics of fluvial erosion responds to soil erosion and surface runoff on hillslopes due to land use and environmental fragility, conditioned by the soil, geology, relief, and rainfall rate. Despite the increasing problems associated with fluvial erosion in Brazil, little information is available on bedload transport in headwater catchments under intense agricultural activity. Therefore, this study sought to characterize the fluvial erosion processes and bedload dynamics in an experimental catchment in southern Brazil located at the edge of the Brazilian Meridional plateau, which is representative of a large area of high environmental fragility and intense agricultural activity in Southern Brazil. The Guarda Mor River drains a headwater catchment (18.5 km2) with undulating and hilly terrain with fragile soils and intense agricultural activity controlling fine and coarse sediment supply downstream. During 11 major rainfall-runoff events, monitoring was conducted to measure streamflow, bedload transport rates, sediment size, and hydraulic parameters, such as Manning's n values and viscous layer thicknesses. A rating curve was established based on 40 streamflow and bedload discharge measurements taken at different water levels and stages along the hydrograph. In addition, a river portion (gravel bed) was characterized as well as the granulometric characteristics of its surface and subsurface layers. The results showed that the transport pattern is influenced by factors other than hydraulic parameters alone, including the interdependence between successive events, armoring effects, and hysteresis. These factors are strongly related to the surface runoff and erosion observed on the hillslopes, which define the streamflow energy and the supply of sand fraction, respectively. A discussion is held on the bedload transport dynamics under non-equilibrium conditions in the modeling of fluvial erosion processes.

Bedload response to dam removal: Results from a 6-year particle tracking survey in the Leitzaran River (Basque Country)

Dams, weirs and transverse barriers to rivers interrupt sediment continuity and reduce sediment supply downstream. In this regard, dam removal is an increasingly used river restoration measure to recover longitudinal connectivity of sediment, among many other river processes. In this work we present a 6-year (from 2016 to 2022) monitoring of bedload transport before, during and after the removal of the 7-meters high Olloki dam in the Leitzaran River (Basque Country). The removal process started in 2018 with the upper 3 m and was completed in 2019 with the remaining 4 m of the dam. To monitor bedload transport, we seeded RFID-tagged stones in three reaches: a control reach unaffected by the dam, a reach immediately upstream of the dam, and a reach downstream of the dam. We deployed 300 tagged stones each year (100 by reach), i.e., 1800 in total. We measured important mobilization and displacement of tracer stones (with maximum travel distances of ∼8.8 km of tracers seeded upstream the Olloki dam) during an active hydrological year following the complete removal of the dam, with some tagged particles even travelling across a downstream weir. We also reported changes in the progression of tagged stones in the dam-affected reaches (upstream and downstream) with the removal, with further and faster dispersal of sediments once the dam was removed. In addition, in these reaches we estimated larger volumes of mobilized bedload in the three years following removal than in the previous years, especially in the upstream reach. In this regard, the relationship between bedload and cumulated energy suggests that less energy was expended in the upstream reach for mobilizing bedload once the removal of the dam was completed. Conversely, in the control reach no major changes were observed before and after the removal of the dam; this reach showed only an increase in sediment mobilization during the last hydrological year, which was the most hydrologically active of the whole monitoring period. In summary, our tracer observations document that travel distances and mobilization volumes are considerably increased with dam removal, especially once the dam was completely removed.

Waterfalls Alter Reach‐Scale Fluvial Erosion Rates: Evidence From Field Data and Process Modeling

AbstractWaterfalls are often interpreted as transient, upstream‐propagating features that mark changes in external conditions. Thus, waterfalls are commonly used to infer past tectonic and climatic forcing, making understanding the controls on waterfall erosion central to predicting how external perturbations move through landscapes. Surprisingly, there exist few direct field measurements of waterfall erosion, and existing waterfall retreat measurements are rarely paired with measurements of waterfall morphology and frequency, which, theory suggests, modulate retreat rates. This lack of data limits our ability to test existing theory and explore how waterfalls alter reach‐scale bedrock erosion rates. Here, we use cosmogenic 10Be accumulated in bedrock riverbeds to measure erosion rates in fluvial reaches with varying waterfall frequency and morphology. We find that waterfall‐rich reaches erode one to five times faster than the landscape average, and that reach‐averaged erosion rates increase with increasing waterfall frequency. We develop a new, process‐based model combining waterfall and planar‐channel erosion to explore mechanistic controls on the relative erosion rate between waterfall‐rich and waterfall‐free reaches. This model predicts that reach‐averaged erosion rates increase with waterfall frequency at low sediment supply, consistent with our field measurements, but that waterfalls can also slow reach‐averaged erosion rates for high sediment supply, large grain sizes, low water discharge, or large plunge pools. Our work is consistent with previous suggestions that waterfall erosion rates may decrease in low drainage areas and can influence long‐profile morphology.

Comparison between thermal models across the Middle Magdalena Valley, Eastern Cordillera, and Eastern Llanos basins in Colombia

Abstract In this research, we compared thermochronological and traditional one-dimensional thermal models across the Middle Magdalena Valley (MMV), Eastern Cordillera, and Eastern Llanos Basins in Colombia. These models allow us to relate cooling/heating histories to exhumation events, subsidence, fault activities, and fluid flow. The Eastern Cordillera and Eastern Llanos Basins are discriminated as prospective zones for hydrocarbon (HC) generation. The De Armas Syncline at the MMV basin shows high thermal maturity (127°C), describing rocks completely depleted in terms of HC generation. The eastern zone of this syncline is buried until it reaches temperatures above 150°C. The contrast between thermal models in the Eastern Cordillera basin suggests a possible sediment supply scenario from the west to the east of the Soapaga Fault. In the Eastern Llanos basin, the thermal histories between both models explain the latest uplift and sediment supply processes from the hinterland zone of the Eastern Cordillera to the foothill zone of the Eastern Llanos basin. In terms of thermal evolution, the three basins are different, reaching different levels of burial and HC maturity. However, in terms of cooling events related to exhumation, the basins have similarities as they are controlled by the mountain building of the Eastern Cordillera of Colombia.

Low average shoreline change rate in 51 years on the raised Aldabra Atoll

Atolls are at risk of losing their ability to physically adapt due to rising sea levels and coral reefs’ reduced sediment supply, resulting in faster erosion of reef islands. This research examines Aldabra, a raised atoll and UNESCO World Heritage Site in the Indian Ocean with diverse coastal ecosystems, to track shoreline changes against a regional sea level rise of 2–3 mm yr−1. Aerial and satellite images in 1960 and 2011 were used to study 85% of the atoll’s shoreline through a Digital Shoreline Analysis System. Over 51 years, 61% of the shoreline remained unchanged, while 24% changed at an average rate of 0.25 ± 0.36 m yr−1, a low rate compared to global atoll changes. Among the areas that did change, rates of accretion and erosion in absolute values were nearly balanced and affected similar percentages (12%) of the shoreline. However, localized changes were pronounced: for example, part of the lagoon shoreline transformed from a sandy beach to a mangrove habitat, accreting by 214 m over the period. Erosion occurred at crucial turtle nesting sites and the research station. The lagoon shoreline underwent more rapid changes than the erosion-resistant ocean shoreline, particularly in areas exposed to wind and waves. Despite its dynamic shoreline, Aldabra maintained its net shoreline and likely total land area over the past 51 years, akin to other Indo-Pacific atolls—underscoring its adaptive capacity. Our research suggests that current knowledge of geomorphological processes of low reef islands is transferable to the raised Aldabra Atoll, reconfirming similar mechanisms of island-building processes at the island crest. These insights highlight an urgent need to minimize local impacts on sediment availability and transfer that might alter the natural dynamics of the shoreline of reef islands and hence limit adaptation potential. Ongoing shoreline monitoring will remain crucial for informing timely adaptation strategies for the conservation of Aldabra’s unique ecosystem.