World's Largest River Research Articles

MAJULI RAAS MAHOTSAV AND ITS PRESENT-DAY RELEVANCE WITH RESPECT TO SOCIO-ECONOMIC DEVELOPMENT OF MAJULI

Majuli, the world's largest river island (Guinness World Records) situated in the Brahmaputra River in Assam, India, hosts an annual cultural extravaganza in the month of November known as the Raas Mahotsav. This research article explores the historical and cultural significance of the Majuli Raas Mahotsav and analyses its present-day relevance in contributing to the socio-economic development of Majuli. For this study secondary data is collected through review of academic literature, from government reports, government websites and media articles. The study provides a comprehensive analysis of how the festival contributes to local economic growth, cultural preservation, and community cohesion. By examining the impact of the festival on tourism, local businesses, and community engagement, this study aims to shed light on the potential of cultural events like Majuli Raas Mahotsav as catalysts for regional development. This research contributes to the ongoing discourse on leveraging cultural heritage for sustainable development and offers valuable insights for policymakers, researchers, and cultural enthusiasts alike.

Geochemical Weathering Variability in High Latitude Watersheds of the Gulf of Alaska

AbstractHigh latitude regions across the globe are undergoing severe modifications due to changing climate. A high latitude region of concern is the Gulf of Alaska (GoA), where these changes in hydroclimate undoubtedly affect the hydrogeochemistry of freshwater discharging to the nearshore ecosystems of the region. To fill the knowledge gap of our understanding of freshwater stream geochemistry with the GoA, we compile stream water chemistry data from 162 stream sites across the region. With an inverse model, we estimate fractional contributions to solute fluxes from weathering of silicate, carbonate, and sulfide minerals, and precipitation. We assess weathering rates across the region and compare them against global river yields. The median fractional contribution of carbonate weathering to total weathering products is 78% across all stream sites; however, there are several streams where silicate weathering is a dominant source of solutes. Weathering by sulfuric acid is elevated in glacierized watersheds. Finally, cation weathering rates are lower in GoA streams compared to the world's largest rivers; however, weathering rates are similar when compared to a global dataset of glacier fed streams. We suggest that hydrologic changes driven by glacier ice loss and increased precipitation will alter river water quality and chemical weathering regimes such that silicate weathering may become a more important source of solutes and sulfide oxidation may decrease. This contribution provides a platform to build from for future investigations into changes to stream water chemistry in the region and other high latitude watersheds.

Decoding the signals of sediment weathering: Toward a quantitative approach

In order to better interpret the signals of weathering intensity recorded in terrigenous sediment archives, it is essential to appropriately assess the effects of hydrodynamic sorting and sedimentary recycling. To achieve this, we utilized a compiled dataset of major element geochemistry of river sediments from East Asia to develop a quantitative index for evaluating the extent of quartz enrichment related to hydrodynamic sorting and sedimentary recycling. By leveraging the trend of present-cycle weathering products in the CIA-WIP diagram, we proposed the chemical index of quartz-enrichmen (CIQ), which can assess the hydrodynamic sorting and sedimentary recycling effects on the present-cycle weathering products. The CIQ vs. CIA/WIP diagram clearly discriminates the influences of weathering intensity, hydrodynamic sorting and sedimentary recycling on bulk sediment chemistry. In this diagram, representative sediments for the present-cycle weathering product typically cluster around CIQ = 0, and the CIA/WIP (or CIA) of which reflect the recorded weathering intensity. The application of our method to depth profiles from the world's largest rivers provides evidence supporting the utilization of sedimentary records to reconstruct paleo-weathering intensity. This study provides a practical and efficient approach to assess whether or to what extent siliciclastic sediments can accurately depict the present-cycle weathering products. This sheds new light on the quantitative deciphering of weathering intensity signals in terrestrial clastic sediments.

A synthesis of the diversity of freshwater fish migrations in the Amazon basin

AbstractIn the Amazon, the world's largest river basin, migrations within freshwater habitats are one of the predominant life history strategies for fishes. The flood pulse and the extensive river network provide aquatic organisms with temporal and spatial accessibility to a mosaic of freshwater habitats. Although migratory fish species are central to freshwater ecosystems and fisheries, the knowledge of species and migratory patterns has traditionally relied on anecdotal and scattered information, lacking a unifying methodological and conceptual framework. We quantitatively synthesize the evidence about this biological phenomenom in the Amazon basin through a systematic literature review. We constructed a reference database of migratory events in the Amazon basin, including species, life stages, purposes, direction, habitats and subbasins. We found that 223 species were documented in 90 references as performing migrations distributed across eight orders and 31 families. Migration is a conserved trait in the evolutionary history of Amazonian fish fauna, suggesting that ~41% of migratory species are likely unreported. We noted a geographical bias in the report of migratory events towards 13 of the 27 major subbasins of the Amazon. We found a significant association between the hydrological timing at the beginning and end of migrations across species, including reproduction as the most commonly reported purpose. However, most species lack the application of robust methods (e.g. telemetry, otolith microchemistry) to classify them as migratory, relying upon secondary sources of information (i.e. reviews or species checklists). Further, we discuss future opportunities and challenges to continue the study of fish migrations in the Amazon basin.

Fishing eDNA in One of the World's Largest Rivers: A Case Study of Cross-Sectional and Depth Profile Sampling in the Yangtze.

The world's largest rivers are home to diverse, endemic, and threatened fish species. However, their sheer sizes make large-scale biomonitoring challenging. While environmental DNA (eDNA) metabarcoding has become an established monitoring approach in smaller freshwater ecosystems, its suitability for large rivers may be challenged by the sheer extent of their cross sections (>1 km wide and tens of meters deep). Here, we sampled fish eDNA from multiple vertical layers and horizontal locations from two cross sections of the lower reach of the Yangtze River in China. Over half of the ASVs (amplicon sequence variants) were detected in only a single combination of the vertical layers and horizontal locations, with ∼7% across all combinations. We estimated the need to sample >100 L of water across the cross-sectional profiles to achieve ASV richness saturation, which translates to ∼60 L of water at the species level. No consistent pattern emerged for prioritizing certain depth and horizontal samples, yet we underline the importance of sampling and integrating different layers and locations simultaneously. Our study highlights the significance of spatially stratified sampling and sampling volumes when using eDNA approaches. Specifically, we developed and tested a scalable and broadly applicable strategy that advances the monitoring and conservation of large rivers.

Coupled primary production and respiration in a large river contrasts with smaller rivers and streams

AbstractAlthough time series in ecosystem metabolism are well characterized in small and medium rivers, patterns in the world's largest rivers are almost unknown. Large rivers present technical difficulties, including depth measurements, gas exchange (, ) estimates, and the presence of large dams, which can supersaturate gases. We estimated reach‐scale metabolism for the Hanford Reach of the Columbia River (Washington state, USA), a free‐flowing stretch with an average discharge of 3173 . We calculated from semi‐empirical models and directly estimated it from tracer measurements. We fixed at the median value from these calculations (0.5 ), and used maximum likelihood to estimate reach‐scale, open‐channel metabolism. Both gross primary production (GPP) and ecosystem respiration (ER) were high (GPP range: 0.3–30.8 g , ER range: 0.8–30.6 g ), with peak GPP and ER occurring in the late summer or early fall. GPP increased exponentially with temperature, consistent with metabolic theory, while light was seasonally saturating. Annual average GPP, estimated at 1500 g carbon , was in the top 2% of estimates for other rivers. GPP and ER were tightly coupled and 90% of GPP was immediately respired, resulting in net ecosystem production near 0. Patterns in the Hanford Reach contrast with those in small‐medium rivers, suggesting that metabolism magnitudes and patterns in large rivers may not be simply scaled from knowledge of smaller rivers.

Animal production predominantly contributes to antibiotic profiles in the Yangtze River

Human-induced antibiotic pollution in the world's large rivers poses significant risk to riverine ecosystems, water quality, and human health. This study identified geophysical and socioeconomic factors driving antibiotic pollution in the Yangtze River by quantifying 83 target antibiotics in water and sediment samples collected in its 6300-km-long reach, followed by source apportionment and statistical modeling. Total antibiotic concentrations ranged between 2.05–111 ng/L in water samples and 0.57–57.9 ng/g in sediment samples, contributed predominantly by veterinary antibiotics, sulfonamides and tetracyclines, respectively. Antibiotic compositions were clustered according to three landform regions (plateau, mountain-basin-foothill, and plains), resulting from varying animal production practices (cattle, sheep, pig, poultry, and aquaculture) in the sub-basins. Population density, animal production, total nitrogen concentration, and river water temperature are directly associated with antibiotic concentrations in the water samples. This study revealed that the species and production of food animals are key determinants of the geographic distribution pattern of antibiotics in the Yangtze River. Therefore, effective strategies to mitigate antibiotic pollution in the Yangtze River should include proper management of antibiotic use and waste treatment in animal production.

Idealized hydrodynamical numerical model dataset with no-river runoff at the western tropical North Atlantic.

The western tropical North Atlantic (WTNA) is a very complex region, with the influence of intense western boundary currents in connection with equatorial zonal currents, important atmospheric forcings (e.g Intertropical Convergence Zone), mesoscale activities (e.g NBC rings), and the world's largest river discharge as the Amazon River runoff. The volume discharge is equivalent to more than one-third of the Atlantic river freshwater input, with a plume that spreads over the region reaching the northwestward Caribbean Sea and eastward longitudes of 30°W, andinfluencing from physical to biological structures. Therefore, in order to enable and encourage more understanding of the region, here we present a dataset based on an idealized scenario of no river runoff of the Amazon River and Par ´a River in the WTNA. The numerical simulations were conducted with a regional oceanicmodeling system (ROMS) model and three pairs of files were generated with themodel outputs: (i) ROMS-files, with the parameters of the ROMS-outputs raw data in a NetCDF format and monthly and weekly frequencies; (ii) MATLAB-files, which contain oceanographic parameters also in monthly and weekly frequencies; and (iii) NetCDF-files, with oceanographic parameters again in monthly and weekly frequencies. For each file, we present the coordinates and variable names, descriptions, and correspondent units. The dataset is available in the Science Data Bank repository (doi: https://doi.org/10.57760/sciencedb.02145).

Hydrological modelling of largest braided river of India using MIKE Hydro River software with rainfall runoff, hydrodynamic and snowmelt modules

Abstract The Brahmaputra River is one of the world's largest river systems, India's largest braided river, and its springtime runoff and downstream streamflow are mostly due to snowmelt processes. This study analysed and used Tropical Rainfall Measuring Mission (TRMM)/Global Forecast System (GFS) rainfall, PET, and snowmelt data as inputs to a Rainfall–Runoff (RR) model, which is based on the MIKE Hydro River NAM software package. The mathematical model was calibrated against the available observed discharge data for the sub-catchments. The model performed reasonably well and simulated discharge in good agreement with observed discharge in terms of timing, rate, volume, and shape of the hydrograph. During the calibration procedure, the optimum values of the nine RR-NAM parameters are obtained. The performance of each model has been checked against measured discharge using a coefficient of determination (R2). It is observed that the value of R2 varies from 0.6 to 0.86. This is deemed acceptable for the purposes of this study. In addition to R2, the overall Water Balance error is also checked. The WBL error is less than 6%. Despite the inherent uncertainties in hydrological modelling, it is determined that the calibrated RR-NAM model can be utilized for the Brahmaputra basin's Flood Forecasting and Early Warning System design, as well as water resource management and planning.

Shoreline hardening affects species composition and condition of larval fishes in the Yangtze River: Implications for conservation and inshore habitat restoration

AbstractInshore habitats play a crucial role in the downstream drift of larval fish in large rivers. Natural inshore habitats of most of the world's large rivers have been heavily degraded by shoreline stabilization measures, but the knowledge about consequences for the condition and distribution of larval fishes is extremely limited. Investigations during the peak season of larval drift into species composition as well as into feeding intensity and body injuries of representative taxa were conducted at different inshore mesohabitats along semi‐natural and stabilized shores in a free‐flowing section in the middle Yangtze River. Species were unambiguously identified by a combination of DNA barcoding and morphological analyses. Overall, 37 taxa from seven families were detected from a total of 9,915 fish larvae caught. Earlier developmental stages greatly outnumbered the later stages in diversity and abundance in each investigated mesohabitat. Abundance and assemblage structure as well as the body injury rate differed significantly between mesohabitats. Generally, overall species richness and abundance tended to be lower, and divergence of the assemblage and body injury rates tended to be higher at heavily hardened shores, which could be contributed to the poor riparian vegetation and restricted suitable hydraulic conditions there. Our results may reflect the underestimated impact of inshore habitat degradation on larval fishes and highlight the demand for creating adequate areas of shallow and slow‐flow habitats with good riparian vegetation conditions along the shores. These measures will foster an appropriate connectivity between spawning areas and high‐quality nurseries for larval fishes.

Riverine food environments and food security: a case study of the Mekong River, Cambodia.

Rivers are critical, but often overlooked, parts of food systems. They have multiple functions that support the food security, nutrition, health and livelihoods of the communities surrounding them. However, given current unsustainable food system practices, damming and climate change, the majority of the world's largest rivers are increasingly susceptible to environmental degradation, with negative implications for the communities that rely on them. Here we describe the dynamism and multifaceted nature of rivers as food environments (i.e. the place within food systems where people obtain their food) and their role in securing food security including improved diets and overall health. We also provide a conceptual framework that explain rivers as food environments within the broader food system and describe approaches to characterizing these food environments to better inform our understanding of how they influence food security and nutrition outcomes. Applying this framework to the Mekong River in Cambodia, we describe rivers as multifaceted wild food environments embedded within ecosystems, sociocultural and political environments and sectors of influence. We also explain the ways in which individual factors might influence how communities interact with this food environment. Developing and articulating food-related, ecosystem-specific frameworks and their constructs can guide implementation of policies aimed to improve specific public health or environmental sustainability outcomes. Our conceptual framework incorporates the multiple dimensions of rivers, which will aid future work and public health policy framing to better describe, understand and intervene to ensure protection of rivers' biodiversity and ecosystems as well as food security, health and livelihoods.

Quasi-equilibrium channel metamorphosis in planform of a subtropical river in India in post-dam period

A basic fluvio-dynamics process in the sediment-energy fluxes is channel meandering and braiding. The construction of dams and barrages has primarily altered the meandering and braiding patterns of the world's large rivers. To this end, the present study aims to analyze post-dam planform characteristics of the Damodar River, a subtropical river in India, based on the analysis of meander geometry, Mueller’s sinuosity index, braiding index, channel count index, and channel length index during the last 50 years (1970–2020) using topographical maps and satellite images. The study also intends to forecast channel meandering and braiding characteristics for 2030 and 2050 using an Artificial Neural Network (ANN) model. The study results show that between 1970 and 2020, the standard sinuosity index (SSI) showed a larger tendency to meander in the middle of the river than the upper and lower courses owing to hydraulic control, while the lower and upper courses were controlled by topographic factors. Similarly, channel braiding indices (BI*, BI, and Pt) exhibit swelling in the upper and middle courses followed by a gradual fall in the lower stretch. The building of dams, the presence of lineaments, faults, and other anthropogenic controls primarily regulate the physical planform characteristics of the river. Regarding allometry, the river's upper segments exhibit both positive and negative allometry in certain combinations, whereas the lower segments primarily exhibit negative allometry. The study demonstrates that over the past 50 years, overall channel meandering has slightly decreased while braiding has slightly increased over time, indicating a quasi-equilibrium channel metamorphosis. However, the prediction with higher accuracy (80–95 %) shows that the river meandering and braiding (BI* and BI) will increase in the short run (2030), and decrease in the long (2050). The study findings may be helpful for the various planners and stakeholders involved in the development of the river basin.

Remote sensing of broad-scale controls on large river anabranching

River patterns reflect complex geomorphological processes and affect ecosystems and human development along floodplains. Physical controls on anabranching development have been studied primarily at local scales on relatively small rivers (i.e., discharge <1000 m3 s−1). However, there has been little systematic quantification of river anabranching (development of stable multiple channels) for large rivers globally. Here, we use remote sensing, cloud computing, and geospatial analysis to explore the importance of water surface slope, floodplain topography, sediment supply, substrate lithology, and permafrost to anabranching throughout 20 of the world's largest river basins. We use Landsat-derived surface water extent to compute an anabranching index (Ai) for ∼1 M km of river reaches, and compare it with global datasets of water surface slope, available floodplain extent, substrate lithology, and permafrost. We find that ∼49 ± 19% of large-river mainstems are anabranching, ranging from at least ∼17% (Mekong) to as much as ∼84% (Ob’). At the basin scale, anabranching channels comprise at least ∼17% (Yenisey) to as much as ∼55% (Kolyma) of all Landsat-observable river reaches, with a mean global value of 35 ± 11%. Anabranching channel patterns are most commonly associated with low water surface slope (normalized slope < 0.2; absolute slope < 0.2 m km−1) accompanying wide floodplains (normalized floodplain width > 0.6; absolute width > 42 km). Cross-sectionally averaged channel width increases in the most intensely anabranching reaches, suggesting net sediment storage due to reduced stream power. Unconsolidated sedimentary substrates promote both the prevalence and intensity of anabranching, whereas the presence of permafrost enhances intensity only. Overall, our results identify pervasive channel anabranching and some important control mechanisms throughout the world's large river basins, with strong implications for satellite-based discharge retrievals. We conclude that global analyses afforded by remote sensing and cloud computing offer additional, complementary approaches to traditional field-based approaches for understanding large river geomorphic processes and channel forms.

Difference in riverbed micromorphology of two world large lowland rivers: Implication of natural and human effects

Dunes of the lowermost Mississippi River (LmMR) in USA and the lowermost Yangtze River (LmYR) in China have been recently investigated in some reaches. Yet, our understanding of dune formation in large lowland alluvial rivers is still limited. This study compared characteristics of dunes in an 8-km long reach in the LmMR and a 35-km long reach of the LmYR to discern similarity and difference in dune development. The primary goal of the study was to assess morphological responses of dunes to natural factors and human interventions in these two world's large rivers. Multi-beam bathymetric measurements in the river reaches were used to quantify dune length, height, stoss and lee angles. The results showed that both the LmMR and LmYR had large (10–100 m in length), medium (5–10 m in length), and small dunes (0.6–5 m in length), but very large dunes (dune length >100 m) only developed in the LmYR. Large dunes were predominant, amounting to 88.4% in the LmMR and 54% in the LmYR. There was a close correlation between dune height and dune wavelength in both the LmMR and LmYR. Significant differences (p < 0.0001) in dune size, geometry (height/wavelength), and symmetrical characteristic (stoss slope/lee slope) were found between the LmMR and the LmYR. On average, dunes in the LmYR were longer and taller (20.08 m and 0.91m) than those in the LmMR (17.62 m and 0.87 m), which was likely owing to the human activities in the LmYR and the backwater effects in the LmMR, as well as the influence of river discharge and flow strength. Little difference was found in dune roughness between the LmMR and the LmYR, indicating that the flow resistance is similar in the two regions. The findings suggest that human activities are the dominant factor affecting dune morphology in large lowland rivers.

Mapping the long‐term influence of river discharge on coastal ocean chlorophyll‐a

AbstractAlthough the potential of river discharge to support ocean productivity and marine ecosystems is known, the specifics of this relationship are poorly understood in many regions of the world. Global estimates of river flow indicate that river discharge is decreasing due to the increasing fragmentation, extraction and regulation of rivers. This likely means that the contribution of river flow to coastal productivity and water quality is changing, potentially leading to fewer and smaller magnitude ocean fertilisation events. We developed a simple analysis method, based on Earth observation data, to investigate where coastal ocean chlorophyll‐a is most strongly influenced by river discharge. The per‐pixel spatiotemporal correlation technique (implemented using Python) correlates chlorophyll‐a concentration (a proxy for phytoplankton biomass and indicator of primary productivity) from MODIS ocean colour data with river discharge data. The method was tested globally on 11 different rivers discharging into coastal ocean regions. Our findings suggest some of the world's largest river systems, such as the Amazon River, have zones of elevated coastal chl‐a that extend hundreds to thousands of km from the river mouth. These findings suggest the influence of river discharge may have been underestimated in many coastal regions of the world. The method appears more effective for larger river systems discharging to ocean waters with less complex nutrient dynamics and weaker seasonal productivity patterns, most notably in temperate regions. Increasing our understanding of the specific areas influenced by river discharge, and the degree of influence over space and time, is an important step towards the improved river and coastal management. This method will increase the capacity of researchers to monitor how, when and where coastal waters are affected as river discharge continues to change into the future.

Merchants of the north: Infrastructure and indebtedness along Brazil's Amazon estuary

Drawing on fieldwork with riverine householders in the Amazonian municipality of Abaetetuba, this article recounts an ongoing struggle against the construction of a fluvial grain terminal by Cargill, the global grain‐trading giant. Over the past two decades, grain‐trading firms have engaged in a publicly subsidized bidding war to expand export infrastructure in the Amazon, harnessing the world's largest river system to flush agricultural commodities on the global market. The so‐called Northern Arc consists of a regional network of private grain terminals, ports, roads, and railways connecting disparate places within a broader landscape of value. This article overlays ethnography with historical analysis to examine the confrontation between a regional project of commodity chain integration and a set of valuations derived from the long‐standing role of commodity exchange in the Amazon. I suggest that the Northern Arc articulates with the sociological legacy of the aviamento: a system of debt‐peonage that structured the circulation of commodities in the basin during the nineteenth and twentieth centuries. By connecting local understandings of power, circulation, and authority to a new structural position within the global food system, the Northern Arc has galvanized old antagonisms and reflects the history of social conflict along the Amazon estuary.

Dissolved Fe in the East China Sea Under the Influences of Land Sources and the Boundary Current With Implications for Global Marginal Seas

AbstractThe East China Sea (ECS) lies on a wide shelf and embraces one of the world's largest rivers, Changjiang (Yangtze River), on the west and strong boundary current, Kuroshio, on the east, which can act as potentially huge sources of dissolved iron (dFe) to the overlying water. However, their magnitude, distribution, and transformation in the ECS are not yet well characterized. Here, we reported the results from the first large scale investigation of dFe concentrations in the water column from the ECS collected with an X‐Vane assemblage in August 2013. Samples from the Changjiang and other coastal rivers were also measured to constrain terrestrial fluxes. In surface waters, dFe concentrations show a strong gradient with patchy nature from the coast adjacent to the river mouth to the mainstream of the Kuroshio Current. The behavior of dFe in the ECS is closely linked to nutrient cycling and development of the sub‐surface chlorophyll maximum. Our dFe budget reveals that mass transport over the East China Sea Shelf (ECSS) is dominated by the input from the Taiwan Strait Warm Water, atmospheric deposition, and exchange with the open boundary current further offshore. The incursion of the Kuroshio Current across shelf break is a source of natural fertilization. The export of dFe from the ECSS nourishes other marginal seas (e.g., Japan Sea/East Sea) and the Kuroshio further downstream. Comparison with the literature data provides evidence that marginal seas play a critical role in bridging terrestrial and oceanic spheres in terms of Fe biogeochemistry.

Adaptation of urban distribution to the fluvial geomorphic environment and the reconstruction of the river system structure by urban distribution in Haihe Plain in the past 2,200 years

AbstractThe alluvial plain of many of the world's large rivers is generally an area with concentrated urban distribution and a long history of development. In eastern China, the alluvial plains downstream of large rivers such as the Haihe River are the most densely populated and economically developed regions. Beijing, the capital of China, is located on the Haihe Plain. It is a matter of great significance that the region's capacity for livability and sustainable development be improved. Using the river geomorphic data and historical town information, methods of spatial statistical analysis and historical literature research, this paper analyzes the spatial relationship between the distribution of urban centers and the fluvial geomorphology of the Haihe Plain over more than 2,200 years. The research finds that: first, different levels of cities and towns have led to varying choices regarding the fluvial geomorphic environment. Second, in different periods in which different transportations were dominant, the distribution of cities and towns has adapted to the fluvial geomorphology. In the land transportation period, urban settlements mainly concentrated on the diluvial‐alluvial fans at the foot of the Taihang Mountain; in the inland shipping period, they focused on gathering at the confluence of rivers on the central plain and along the canals; in the period of global trade, Tianjin, a trade center, established itself at the mouth of the Haihe River. Third, humans intend to rebuild water system structures (such as the Grand Canal) in the context of major water conservancy projects in the basin to meet the needs of the expanding regions and towns. The research shows that the urban development of the Haihe Plain should adapt to the natural regional pattern and establish a solution based on respecting the natural conditions of the basin, to make the regional sustainable development more resilient.

Carbonate weathering dominates magnesium isotopes in large rivers: Clues from the Yangtze River

Carbonate weathering regulates the short-term carbon (C) cycle and global climate due to its fast response to hydrological processes. The carbonate weathering flux needs to be well constrained to better understand the climate change at short time scale. Riverine magnesium (Mg) isotopes are sensitive to primary mineral dissolution and so have great potential to trace carbonate weathering. Global large rivers draining continental crust dominate weathering flux to the oceans, but how riverine Mg isotopes respond to carbonate weathering remains unclear. The Yangtze River drainage basin (YRDB) was selected to test the robustness of riverine Mg isotopes (δ26Mg) in tracing continental carbonate weathering because it spans a wide range in lithology, geomorphology and climate. The riverine δ26Mg values within the YRDB show a decreasing trend from the headwater to the mainstream ranging from −1.36‰ to −0.59‰. The dissolved δ26Mg have strong negative correlations with carbonate weathering rate and intensity within the YRDB, indicating a sensitive response of riverine δ26Mg to the carbonate weathering flux. In a compilation of Mg fluxes and δ26Mg in the world's largest rivers, there is similar dominance of carbonate weathering on riverine Mg fluxes and isotopes. Therefore, we propose that riverine δ26Mg in large rivers are a robust tracer of carbonate weathering intensity. Intensifying carbonate weathering under global warming tends to increase riverine Mg and C fluxes to the oceans and thus the atmospheric CO2 sink at the millennial time scale.

Occupancy modelling confirms the first extirpation of a freshwater fish from one of the world's largest river systems

Abstract Numerous human‐induced changes to freshwater ecosystems are implicated in the loss of biodiversity, including fish extinctions. Dams and weirs have profoundly altered the ecological character of the Murray–Darling Basin (MDB), one of the world's largest river systems, which discharges at the mouth of the River Murray in South Australia. These impacts are evident in the Ramsar‐listed final reaches of the river, where regulation, drought, and flow reductions over recent decades have led to the collapse of freshwater fish populations, including Yarra pygmy perch (Nannoperca obscura), which is Endangered on the International Union for Conservation of Nature Red List of Threatened Species. Nannoperca obscura occurs in several major catchments in south‐eastern Australia, but the genetically unique population in the MDB only inhabits wetlands near the terminus of the River Murray where wild populations were last recorded in 2008 approaching the end of a prolonged drought. This study involved repeated surveys targeting N. obscura to evaluate its occupancy in the MDB while accounting for imperfect detection. The surveys failed to find N. obscura despite regular detection of the closely related southern pygmy perch (Nannoperca australis), which provides a proxy for the congener. This study presents strong evidence that N. obscura is absent in the survey region, thereby inferring the first freshwater fish extirpation from the MDB. The outcomes of this study should prompt responses required to aid the species' population recovery and present a reliable approach to confirm the status of freshwater fishes for conservation management.