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The Don Sahong Hydropower Project (DSHPP), located in the Khone Falls area of Khong District, Champasak Province, southern Laos, near the border with Cambodia, has been amongst the most controversial hydropower dams in the Mekong River Basin. Despite considerable regional and international opposition, the dam was finally constructed, becoming commercially operational in 2020. In 2011, I expressed serious concerns about the project's potential impacts on long-distance migratory fish and associated fisheries, especially fish that migrate from Cambodia and Vietnam up the Mekong River to Laos and Thailand. However, my concerns have somewhat changed along with the circumstances, although they continue to focus on dam impacts on migratory fish. I am now particularly concerned about the indirect impacts of the DSHPP on the fishing livelihoods in the Khone Falls area, and the impacts of the dam on fish larvae and fish passing through the dam's turbines when migrating downstream. Using a political ecology approach, this paper considers how infrastructure, in this case a large hydropower dam located on the Mekong River, has altered the spatialities associated with the project in particular ways. [ABSTRACT FROM AUTHOR]

Forecasting flood and drought events requires accurate modeling tools. Hydraulic river models are based on estimates of riverbed geometry which are traditionally collected in situ. The novel Ice, Cloud and Land Elevation Satellite 2 [ICESat‐2] lidar altimetry mission with 6 simultaneous high‐resolution laser beams provides the opportunity to define river cross‐section geometries as well as observe water surface elevation [WSE] and water surface slope spatially resolved along the river chainage. This paper describes a method to utilize terrain altimetry and water surface slope estimates to define complete river geometries from ICESat‐2 data products, using the diffusive wave approximation to calculate depth in the submerged section not penetrated by the lidar. Exemplifying the method, cross‐sections are defined for a stretch of the Mekong River. Hydrodynamic model results of the stretch are compared with ICESat‐2 WSE estimates and in situ gauging station time series. Insights in river characteristics from satellite imagery and the ICESat‐2 slope estimates allow for fine‐tuning of the cross‐sections using spatially varying Manning numbers. The final model achieves a root mean square error against the ICESat‐2 WSE of 0.676 m and average Kling‐Gupta Efficiency against gauging station time series of 0.880. The method is limited by the diffusive wave approximation resulting in inaccurate cross‐section estimates in sections with supercritical flow or significant acceleration. Errors can be identified from ICESat‐2 WSE estimates and reduced with additional cross‐sections. Combined with hydrological models, the method will allow for cross‐section definition without in situ data. Plain Language Summary: The depth and width of the river channel are important factors when seeking to predict floods. To predict water level in a river, computer models for flood forecasting must be informed with river channel geometry at cross‐sections along the river. Such cross‐sections are traditionally measured in the field which is expensive and time consuming. This paper describes a method to estimate river cross‐sectional shape from land and water height measurements from the satellite ICESat‐2. When the satellite path crosses the river, the precise laser instrument onboard outlines the surface to indicate the river channel shape, but it does not penetrate the water. We found that it was possible to estimate the shape of the submerged part of the cross‐section using the water surface slope obtained from the satellite's unique instrument. Evaluating the method on a stretch of the Mekong River, we were able to model water level along the river with accuracy sufficient for flood forecasting. In some sections, where the flow speed changes quickly, a hydraulic model with the defined cross‐sections does not reproduce observed water levels. Using satellite crossings from days with steadier flow reduces the error. The method allows for building hydraulic river models without cross‐section surveys. Key Points: Land elevation and water surface slope estimates from the ICESat‐2 lidar altimetry mission are combined to define river cross‐sectionsRMSE of 0.676 m and KGE of 0.88 is achieved for a hydrodynamic model of a stretch of the Mekong River using the derived cross‐sectionsSections with significant flow acceleration lead to inaccurate cross‐sections, but errors are reduced with additional ICESat‐2 crossings [ABSTRACT FROM AUTHOR]

Information on the feeding habits of the critically endangered Irrawaddy dolphin (Orcaella brevirostris) in the Mekong River and how their diet overlaps with that of the species targeted by fisheries is limited. The information provided herein will be helpful for evaluating and protecting important dolphin food resources. The composition of large prey species of Irrawaddy dolphins and the extent of fish resources were surveyed in the Mekong River, Cambodia, using local ecological knowledge. The results showed that Irrawaddy dolphins fed on 13 species of large fish, with Pangasius conchophilus, Cyclocheilichthys enoplos, and Hemibagrus wyckii the most frequently consumed prey. Twelve of the 13 reported prey species were usually caught by fishers. The results also revealed that the average weight of fish caught daily by fishers was significantly lower in 2019 than about five years before that, regardless of the season. A significant decline in fish resources and a high overlap of dolphin diets with fishery target species indicate that Irrawaddy dolphins may also be threatened by prey depletion, especially during the dry season when dolphins are concentrated in deep pools. [ABSTRACT FROM AUTHOR]

The Mekong River Basin is facing growing challenges in managing water resources, environmental protection and sustainable development whilst fostering rapid economic development within a region of riparian states. The need for cheap and renewable energy is rising, fostering the increase in hydropower development where logistically possible. This case study focuses on what role and impact civil society organizations play regarding water governance in the Mekong region, particularly in regard to hydropower projects. A particular focus is on the Mekong River Commission and its Procedures for Notification, Prior Consultation and Agreement focusing on the Xayaburi and Pak Lay dams. [ABSTRACT FROM AUTHOR]

Tonlesapia tsukawakii Motomura et Mukai, 2006 is an endemic freshwater representative of the family Callionymidae (Teleostei: Perciformes: Callionymidei) living in the Mekong River. According to the materials of studies conducted in 2018–2022, the occurrence of the species in catches in the Mekong Delta averaged 20.7%, the proportion in terms of abundance was 2.6%, in terms of weight, 0.6%. The maximum values of the species parameters were 81.2 ind./effort and 176.2 g/effort, respectively. The abundance of fish significantly decreased in the conditions of low content of dissolved oxygen and on a substrate with plant residues. The sex ratio of T. tsukawakii was 1.0 : 3.1 with a stable prevalence of females, which distinguishes the species from other representatives of the family. Females have larger sizes compared to males and they gain weight faster. The fecundity of the species was determined for the first time; the average number of mature oocytes was 2950 psc. During a low-water period, callanoid copepods of the family Pseudodiaptomidae mainly represented by Pseudodiaptomus dauglishi Sewell, 1932 were the main food items of T. tsukawakii. [ABSTRACT FROM AUTHOR]

Surface water quality (SWQ) has been degraded in the Mekong River Basin under increasing pressures of population growth, economic development, and global climate change. This study employed the comprehensive water pollution index (CWPI) to assess the spatio-temporal variation of SWQ in the downstream Mekong River estuaries. Eight water quality parameters were measured between 2005 and 2021 at 21 sampling sites downstream of the Mekong River. These parameters included total suspended solids (TSS), biological oxygen demand (BOD5), chemical oxygen demand (COD), ammonia (N-NH4+), nitrate (N-NO3-), phosphate (P-PO43-), iron (Fe), and total coliform. Most of the monitoring locations in the estuaries of Ham Luong, Cua Dai, Ba Lai, and Co Chien exhibited slightly to moderately polluted conditions, as indicated by the CWPI values ranging from 0.67-2.91, 0.41-2.20, 0.27-3.02, and 0.37-2.95, respectively. TSS and Fe concentrations consistently exceeded the allowable limits, while the majority of values for N-NH4+, N-NO3-, P-PO43-, and coliform remained within acceptable thresholds. Additionally, parameters indicative of organic pollution, namely BOD5 and COD, displayed a noticeable upward trend between 2005 and 2021. SWQ exhibited significant spatial and temporal variations with TSS, organic matter, nutrients, and iron being the main areas of concern. These findings can provide guidance to policymakers involved in the assessment and enhancement of water quality in the presence of pollutant compounds that lead to a decline in water quality.

Tectonic subsidence on rifted, passive continental margins are largely controlled by patterns of extension and the nature of strain partitioning in the lithosphere. The Sunda Shelf, adjacent to the SW South China Sea, is characterized by deep basins linked to regional Cenozoic extension associated with propagating seafloor spreading caused by slab pull from the south. Analysis of seismic reflection profiles and drilled sections crossing the Nam Con Son and Cuu Long basins highlight Oligocene extension, with most of the thinning concentrated in the ductile mid-lower crust. Upper crustal extension was modest and ductile flow is inferred to be directed northwestwards, towards the oceanic crust. Basin inversion occurred in the Mid Miocene, associated with the collision of the Dangerous Grounds Block and Borneo. Subsequent accelerated tectonic subsidence exceeded predictions from uniform extension models assumed to relate to extensional collapse after inversion. We correlate this to a period of faster erosion onshore driven by strong monsoon rains in Indochina and Peninsular Thailand at that time. Erosion of the onshore basement, inducing rock uplift and coupled with loading of the basins offshore, drives ductile mid-lower crustal flow, likely to the northeast under Indochina, and/or to the west where Plio-Pleistocene subsidence of the shelf is very slow. Significant sediment delivery from the Mekong River into the Cuu Long Basin began in the Late Miocene and migrated seawards as the basin filled. Mass balancing suggests that the basins of this part of the Sunda Shelf are filled through erosion of bedrock sources around the Gulf of Thailand. There is no need for sediment delivery from a major river draining the Tibetan Plateau to account for the deposited volumes. [ABSTRACT FROM AUTHOR]

This study aimed to determine relationship between Kai algae and water quality using the Applied Algal Research Laboratory-Physical and Chemical Score (AARL-PC score) and inverse distance weighting (IDW) in the Mekong River. Kai algal habitats were discovered at three sampling sites during a survey of the Mekong River, Chiang Khong district, Chiang Rai Province, Thailand in January and February 2022. Kai algae were collected in the field and identified by microscope. Kai morphology and environmental factors including length, dry weight, water conductivity, DO, BOD, nitrate nitrogen, soluble reactive phosphorus and ammonium nitrogen were investigated. The results showed two Kai genera, Cladophora sp. and Rhizoclonium sp., were presented in the sampling sites. In January, BOD had a strong positive relationship with the length of Kai algae while ammonium nitrogen had a significant relationship with water conductivity (p < 0.01), and DO had a significant relationship with dry weight (p < 0.05). In February, BOD had a strong positive relationship with the length of Kai algae (r = 0.9804), while ammonium nitrogen had a significant relationship with water conductivity (p < 0.05). IDW derived from the AARL-PC-score revealed that clean-moderate water quality was directly related to environmental factors in January and February and can be used to determine Kai algal growth in the Mekong River. [ABSTRACT FROM AUTHOR]

In the past several decades, drought events have occurred frequently around the world. However, research on the propagation of drought events has not been adequately explored. This study investigated the drought propagation process from meteorological drought to agricultural drought (PMAD) and from meteorological drought to hydrological drought (PMHD) using a 72-year reanalysis dataset in the tropical Lancang–Mekong River Basin. Firstly, we used a new method—Standardized Drought Analysis Toolbox—to construct drought indices. Then, a linear method (Pearson correlation analysis) and a nonlinear method (mutual information) were used to investigate the drought propagation process. Cross-wavelet analysis and wavelet coherence analysis were employed to explore the statistical relationship among the three drought types. Finally, the random forest method was applied to quantify the major factors in drought response time (DRT). The results revealed the following: (1) both linear and nonlinear methods exhibited strong temporal and spatial consistency for both PMAD and PMHD, with linear relationships being stronger than nonlinear ones. (2) The DRTs of PMAD and PMHD were around 1–2 months and 3–5 months, respectively. Significant differences existed in the DRT between the dry season and the rainy season. (3) A divergent spatial pattern of the proportion of DRT was observed between PMAD and PMHD. (4) Significant statistical correlations between meteorological drought and agricultural drought and between meteorological drought and hydrological drought were observed in specific periods for each sub-region; (5) Hydrometeorological factors contributed the most to DRT, followed by terrain factors and the land cover types. The findings of this study deepened our understanding of the spatial–temporal relationship of multiple drought propagation types in this transboundary river basin. [ABSTRACT FROM AUTHOR]

On the Mekong River, north of Stung Treng town in northeastern Cambodia, and below the border with Laos, lies an area of riverine seasonally flooded forest designated as an internationally significant Ramsar wetland site because of its exceptional biodiversity and importance to livelihoods. This article reports on the cumulative and cascading impacts of numerous upstream hydropower dams in China and Laos on this vital ecosystem due to the release of water during the dry season, which prevents the flooded forest from undergoing its critically important drying out period. In particular, we investigate the damage being wrought on these flooded forests and on the various species dependent on them. Different species have been variously affected, but some have been largely destroyed. Others are being increasingly impacted. This habitat loss is negatively affecting fisheries, especially for a number of Pangasiidae catfish and Cyprinid carps, which is having an adverse effect on local livelihoods. New dams upriver, and continued high dry-season water from existing dams, are likely to lead eventually to the increased degradation and possibly the eradication of the flooded forests along the mainstream Mekong River, unless measures are taken to address the problem. [ABSTRACT FROM AUTHOR]

The South China Sea (SCS) is a hydrographically complex and physically dynamic marginal sea of great economic importance. Primary production in the SCS experiences strong seasonal forcing through the monsoon cycle, which affects both riverine runoff and circulation within the basin. The summer monsoon in particular produces a mix of waters affected by the Mekong outflow and coastal upwelling embedded within a dynamic wind-driven surface circulation. Here, we discuss the distribution, abundance, and symbiotic state of a suite of host diatoms and Diatom-Diazotroph Associations (DDAs) in different habitats defined in terms of the physical and biological characteristics of the SCS during the early stages of the SW Monsoon of 2016. DDA host diatoms were broadly distributed throughout our study region, and we found intact symbioses in all of the habitats sampled, though infection rates (abundance of hosts bearing symbionts) and infection intensities (number of symbionts per host) were lowest in waters affected by coastal upwelling. Host infection rates tended to be highest in offshore waters, and DDA host diatoms generally varied widely in size and infection intensity both within and among defined habitats. These differences may reflect different optimal strategies for allocating biomass and energy between host and symbiont. [ABSTRACT FROM AUTHOR]

Food environments are a critical place within the food system to implement interventions aimed at enabling sustainable diets. In this perspective article, we argue for the need for food environment research to more comprehensively examine the different types of food environments that people access within their communities to ensure that interventions and programs are better aligned with people’s lived experiences. We highlight the potential ways in which participatory mapping (PM) can be leveraged to better design food environment research by: (1) identifying the different food environment types that are accessed within a given community; (2) providing insight into the timing for data collection; (3) informing the prioritization of where to conduct food environment assessments; and (4) highlighting the dynamism of food environments over time (e.g., across a given day or across seasons). We provide a case study example of the application of PM and the lessons learned from it in Cambodia. By conceptualizing food environments in a more comprehensive way, from the perspective of the people living within a given community, we will be able to measure food environments in a way that more closely aligns with people’s lived experiences.

Riverbank erosion is a major hazard for riparian communities in the Mekong River Basin. This study aims to (1) assess the livelihood vulnerability of two communities residing along the Mekong River, namely, Kaoh Soutin (KS) and Ruessei Srok (RS), by using the livelihood vulnerability index framed within the Intergovernmental Panel on Climate Change vulnerability framework (LVI-IPCC) and (2) identify the coping strategies of the communities based on semi-structured interviews. The results show that KS is slightly more vulnerable to riverbank erosion than RS, as indicated by LVI-IPCC values of 0.49 and 0.46 for KS and RS, respectively. RS exhibits high adaptive capacity and low sensitivity, but its exposure level is relatively high. Majority of the respondents in KS (62 %) and RS (93 %) were affected by riverbank erosion. In KS, approximately 48 % of the respondents experienced displacement, and 39 % of them relocated once. Meanwhile, in RS, 81 % of the respondents experienced displacement, with 46 % displaced at least three times. The affected households have coped with riverbank erosion by reducing expenses, diversifying their income sources, seeking support from others, and receiving assistance from local authorities, NGOs, and government interventions. Despite such efforts to mitigate the effects of riverbank erosion, the high level of exposure and external factors, such as high living costs and low profits from agriculture, have weakened the ability of the people in both communities to cope with disasters. Moreover, the social ties among households, especially in KS, have declined, thereby making low-income households highly vulnerable to riverbank erosion.

The Mekong River is the 10th largest river in the world. It is recognised as the most productive river in Southeast Asia and economically essential to the region, with an estimated 60-65 million people living in the lower Mekong Basin. The Mekong Delta within Vietnam is considered a highly vulnerable ecosystem under threat from increasing anthropogenic pressure, such as dam construction and, as a consequence, the Delta is sinking and altering the natural hydrological cycle. Dams also lead to eutrophication and pollution of downstream water from regulated water flux and water stagnation. Another threat is climate change coupled with the lower rainfall, which could lead to an increased risk of drought in the Mekong Delta Basin. Thus, these project data represent an important baseline reference. The ecological health of the Mekong Delta's environment, as indicated by the quality and availability of its water and biological resources, largely determines the economic and social development of the region, which produces about half of the agriculture and aquaculture products of Vietnam. New information This paper reports quantitative data on the biodiversity of six groups of aquatic organisms: bottom and pelagic fish, macrozoobenthos, microorganisms, phyto- and zooplankton in the Mekong Delta within Vietnam, as well as data on the physicochemical parameters of water and bottom sediments. The data were collected during 2018-2022 as part of the Ecolan E-3.4 programme within the framework of the research plan of the Joint Russian-Vietnamese Tropical Research and Technological Center. All presented datasets are published for the first time. [ABSTRACT FROM AUTHOR]

A new species of the wholly freshwater hymenosomatid crab genus Limnopilos Chuang & Ng, 1991, is described from Tonle Sap River in the lower Mekong drainage, Cambodia. This is the fourth species known, and the second one from Indochina. The type species, of Limnopilos , L. naiyanetri Chuang & Ng, 1991, was from the Chao Phraya drainage to the west in central Thailand. The new species is characterized by the presence of a rostrum, relatively shorter ambulatory legs and a different structure of the male first gonopod. Limnopilos sumatranus Naruse & Ng, 2007, is also recorded from southern Peninsular Malaysia for the first time. [ABSTRACT FROM AUTHOR]

The transboundary Mekong River spans China, Myanmar, Laos, Thailand, Cambodia, and Vietnam, and provides critical ecosystem services that support millions of people across the river basin. However, the exploitation of its water resources for state‐led development, especially in the realm of hydropower development, not only threatens the livelihoods and food security of communities across the river basin, but also reveal the challenges of governing the Mekong River as a transboundary commons. In focusing on how environmental injustice is produced through hydropower development and the power dynamics within hybrid governance arrangements in the Mekong River Basin, this paper seeks to examine how a grounded perspective of environmental justice may be understood in this context by linkages between the principles of environmental justice and the Mekong literature in three ways. First, the production of, and challenges against distributive injustice in the Mekong River Basin must engage with a strong tradition of scalar analysis in the field of transboundary water governance, particularly in understanding how a politics of scale underlies contestations around the Mekong River. Second, the principle of justice as recognition can be situated within a body of literature that interrogates the politics of knowledge that runs through Mekong water governance, although the co‐production of knowledge types must be acknowledged. Finally, a recent body of literature questioning the legality of dam‐building and public participation around Mekong hydropower dams are closely tied to issues of procedural justice, and reveal the importance of recognizing plurality in ideas around transparency and accountability. This article is categorized under:Human Water > Water GovernanceHuman Water > Water as Imagined and RepresentedHuman Water > Value of Water [ABSTRACT FROM AUTHOR]

A stone stele code-named K-127 was discovered in 1891 on the east bank of the Mekong River in Sambaur district of Kratié province, Cambodia by the French governor of the province. A recent search to relocate the ancient temple where the stele was found showed only brushes and a piece of plain rock of similar colour to K-127. French epigraphist, George Coedès translated its inscriptions in 1930 and was astonished to find that it bears the number 604 of the śaka era which is equivalent to 682 CE. This is the first Khmer inscription that shows the date in numeral form. The top right corner of the stele had been broken when it was found, with the first eight lines of the inscriptions partly broken off on the right side. The zero on K-127 is two centuries older than the internationally accepted oldest Indian zero found in the city of Gwalior. In 1931 Coedès published an article to debunk the belief that our modern decimal number system with the numeral zero as place holder originated in Europe from the Greeks. The origin of the Indian zero comes from the Buddhist logic of catuskoti or tetralemma. To underline the logic, a brief discussion between the wandering ascetic Vacchagotta and Buddha is presented. During the Cambodian civil war in the 1970s, the short-lived Cambodian republican government moved many ancient artefacts to warehouses of Angkor Conservation in Siem Reap for safe keeping. When the Khmer Rouge came to power in April 1975, nobody knows what happened to K-127. It was presumed lost. In 2013, an American mathematics professor who had been travelling the world in search of the origin of the numeral zero in our decimal number system, learned about K-127. He set off again on his search through Southeast Asia, and eventually found K-127 in a shed of Angkor Conservation. He organized with the ministry of Culture and Fine Arts to have the stele brought back to the National Museum in Phnom Penh. In July 2018, the Bodleian Library at Oxford University in England announced in newspapers and through YouTube the results of their C-14 analysis of the Bakhshali manuscript found in 1881 in Bakhshali village near Peshawar in today's Pakistan. The age of the manuscript from the C-14 test spans from the third to the tenth century CE. The results were immediately rejected by five experts in Indian mathematics, historians of Indian math, and a specialist in Indian palaeography. So long as the age of the Bakhshali manuscript remains unsettled, K-127 can rightfully claim to bear the world's oldest zero. [ABSTRACT FROM AUTHOR]

Freshwater mussels belonging to the tribe Pseudodontini (Bivalvia: Unionidae: Gonideinae) play a keystone role in riverine and lacustrine environments throughout Southeast Asia. Many of them are narrowly endemic and habitat specialists, which need special conservation efforts. Unfortunately, the systematics of this group is rather poorly understood. Here, we show that Pseudodon inoscularis, the type species of the genus Pseudodon, belongs to the eastern clade of the tribe (Southeast Asia, east of the Salween Basin), and, more certainly, clusters with members of the genus Monodontina. Hence, the latter genus becomes a synonym of Pseudodon, while the subtribe Pseudodontina (=Pilsbryoconchina syn. nov.) should be used as a name for the eastern clade. In turn, the genus Indopseudodon stat. rev. and the subtribe Indopseudodontina subtr. nov. must be applied as valid names for the western clade of the Pseudodontini (Ayeyarwady to Salween). We present a revision of the genus Indopseudodon with an updated synonymy and a description of a new species, I. indawgyiensis sp. nov. We also link four species-level genetic lineages from the Mekong to available taxonomic names in the genera Pseudodon and Sundadontina (Pseudodon cambodjensis, P. vagulus, Sundadontina harmandi, and S. sulcata). Based on morphological features, we revise the taxonomic position of several recent and fossil species that were traditionally placed in the Pseudodontini and transfer them to the genera Parvasolenaia, Pseudodontopsis, Ptychorhynchus, Simpsonasus nom. nov. (a new name for Nasus Simpson, 1900) (Gonideinae: Gonideini), Pseudobaphia (Unioninae: Unionini), Balwantia and Lamellidens (Parreysiinae: Lamellidentini). [ABSTRACT FROM AUTHOR]

Coastal upwelling zone hosts most fishing grounds worldwide and nitrogen dynamics in those systems mainly have been addressed through the nutrients replenished the ecosystem. Compared with coastal and upper-layer ecosystems, relatively limited knowledge has been explored by nitrogen isotope imprints through the food webs of mesopelagic fishes and their links to upwelling characteristics. Here, we collected several species of mesopelagic fishes in the mid-west region of the South China Sea (SCS) and analyzed them by biochemical methods (fatty acids, δ13C, δ15N, and δ13C of fatty acids). Due to the occurrence of upwelling, we want to evaluate the variation of nitrogen isotope compositions among these mesopelagic species and their links to the upwelling nitrogen supply. The δ15N of mesopelagic fishes in the non-upwelling area was depleted by 2‰ when compared with those in upwelling area, which indicated different nitrogen sources impacting for fishes in the two areas. A combination of multi-biochemical proxies was used to divide mesopelagic fishes into six groups and indicated that the feeding behaviors and upwelling were the main factors to affect the biological composition of mesopelagic fishes, even in such small regions. The differences in δ15N values between the upwelling and non-upwelling areas allowed us to estimate that N2 fixation supported about 37% of the N-demand of food sources in the non-upwelling area, which was larger than the 10% in the upwelling area. These results illustrate that nutrients from the deep layers in upwelling systems might be the main factors that cause differences in δ15N in mesopelagic fishes between upwelling and non-upwelling areas. Thus, more studies of N dynamics in mesopelagic fishes should be conducted in the upwelling systems to illustrate the impact of climate changes on fish biomass in the future.

Study region: Lancang River Basin (LRB) Study focus: Previous studies provided different results in terms of the roles played by climate change and human activities, leaving a knowledge gap in the complex streamflow variability and its physical causes in LRB. In the study, a Budyko framework-aided analytical approach was applied to explore the streamflow decrease in LRB and its physical causes, by especially clarifying the different performances of eight Budyko equations and the influences of diverse baseline periods. New hydrological insights for the region: Results revealed that the four parametric Budyko equations can provide proper parameters to reflect changes in catchment characteristics (including human activities) and their influences, and thus, they performed better than the four non-parametric Budyko equations. It was further found that more reasonable attribution results of streamflow change were obtained when a longer baseline period (at least 20 years) was used. Results also indicated that human activities (accounting for −158.9%) dominated the streamflow decrease during the transition period (1986–2004), by offsetting the positive effects of climate change (accounting for 58.9%). During the impact period (2005–2015), human activities (accounting for −65.6%) still dominated the streamflow decrease, and climate change (accounting for −34.4%) further aggravated the streamflow decrease in LRB.

Illustrated descriptions of two new free-living nematode species, Amphimonhystrella tropica sp. n. and Oncholaimus mekongis sp. n., found in the Mekong River delta (Vietnam) are given. A.tropica is similar to A. major Gagarin, Nguyen Dinh Tu, 2021 and A. paramonovi (Phan Ke Long, Nguyen Dinh Tu, Gagarin, 2020) in body size. It differs from first species in pharyngostoma shape, thicker body, longer tail and spicules, and by the absence of posterior uterus sac. It differs from second species in its thicker body, longer spicules, and different gubernaculum structure. Daptonemaparamonovi Phan Ke Long, Nguyen Dinh Tu, Gagarin, 2020 is transferred to the genus Amphimonhystrella Timm, 1961: A. paramonovi (Phan Ke Long, Nguyen Dinh Tu, Gagarin, 2020) comb. nov. Dichotomy and pictorial keys for the identification of males of valid species of the genus Amphimonhystrella are given. Oncholaimus mekongis sp. n. is similar to O. balli (Kreis, 1934) and O. menzeli Schneider, 1973 in body size and spicule length. It differs from both species by the presence of big spiny seta in the precloaca area, the position of ventral papillae on tail in males, a thicker body, shorter outer labial setae, and a longer and more slender tail in males. [ABSTRACT FROM AUTHOR]

Glyptothorax irroratus, a new species of sisorid catfish from the Mekong River drainage in Laos and China, is described. It differs from its Indochinese congeners in having both large and small tubercles arranged irregularly on the lateral surfaces of the body and by combinations of colour pattern, morphometry (with particular regards to the eye, body depth, adipose fin and caudal peduncle) and thoracic adhesive apparatus morphology. [ABSTRACT FROM AUTHOR]

In the past several decades, drought events have occurred frequently around the world. However, research on the propagation of drought events has not been adequately explored. This study investigated the drought propagation process from meteorological drought to agricultural drought (PMAD) and from meteorological drought to hydrological drought (PMHD) using a 72-year reanalysis dataset in the tropical Lancang–Mekong River Basin. Firstly, we used a new method—Standardized Drought Analysis Toolbox—to construct drought indices. Then, a linear method (Pearson correlation analysis) and a nonlinear method (mutual information) were used to investigate the drought propagation process. Cross-wavelet analysis and wavelet coherence analysis were employed to explore the statistical relationship among the three drought types. Finally, the random forest method was applied to quantify the major factors in drought response time (DRT). The results revealed the following: (1) both linear and nonlinear methods exhibited strong temporal and spatial consistency for both PMAD and PMHD, with linear relationships being stronger than nonlinear ones. (2) The DRTs of PMAD and PMHD were around 1–2 months and 3–5 months, respectively. Significant differences existed in the DRT between the dry season and the rainy season. (3) A divergent spatial pattern of the proportion of DRT was observed between PMAD and PMHD. (4) Significant statistical correlations between meteorological drought and agricultural drought and between meteorological drought and hydrological drought were observed in specific periods for each sub-region; (5) Hydrometeorological factors contributed the most to DRT, followed by terrain factors and the land cover types. The findings of this study deepened our understanding of the spatial–temporal relationship of multiple drought propagation types in this transboundary river basin.

The South Vietnam Upwelling (SVU) develops off the Vietnamese coast (South China Sea) during the southwest summer monsoon over four main areas: the northern coastal upwelling (NCU), the southern coastal upwelling (SCU), the offshore upwelling (OFU) and the shelf off the Mekong River mouth (MKU). An ensemble of ten simulations with perturbed initial conditions were run with the fine-resolution SYMPHONIE model (1 km inshore) to investigate the daily to intraseasonal variability of the SVU and the influence of the ocean intrinsic variability (OIV) during the strong SVU of summer 2018. The intraseasonal variability is similar for SCU, MKU and OFU, driven to the first order by the wind variability. MKU and SCU are induced by stable ocean dynamics (the northeastward then eastward boundary current) and have very little chaotic variability. The OIV has a stronger influence on OFU. In July, OFU mainly develops along the northern flank of the eastward jet. The influence of OIV is strongest and related to the chaotic variability of the meridional position of the jet. In August this position is stable and OFU develops mainly in the area of positive wind curl and cyclonic eddies north of the jet. The influence of OIV, weaker than in July, is related to the organization of this mesoscale circulation. NCU shows a completely different behavior from the other areas. In the heart of summer, the large-scale circulation formed by the eastward jet and eddy dipole is well established with an alongshore current preventing NCU. In early and late summer, the large-scale circulation is weaker, allowing a mesoscale circulation of strongly chaotic nature to develop in the NCU area. During those periods, the OIV influence on NCU is very strong and related to the organization of this mesoscale circulation: NCU is favored (annihilated) by offshore-oriented (alongshore) structures. [ABSTRACT FROM AUTHOR]

The Mekong region experienced severe droughts in 2019–20, generating alternative explanations of whether Chinese dams, climate or other drivers are the main causes. Reviewing academic journals, news outlets and online public discussions, we argue that it is important to address these differences in explanations not only between China and downstream countries but also including non-state actors, regional and external non-governmental organizations, media, think tanks, and research institutes to reveal the politics that frame the scientific debate on drought in the region. Data-sharing and collaborative research are needed to shift the discourse on drought towards a more systematic approach rather than focusing on politically expedient causal factors. [ABSTRACT FROM AUTHOR]

Freshwater diaptomid copepods in Cambodia are poorly studied, and only seven taxa were known previously. From February to October 2007, 255 samples were collected from 237 freshwater sites in nine provinces representing four regions (northwest, Cardamom and Elephant Mountains, Mekong Lowlands, and east) of Cambodia. Diversity, seasonal occurrence, geographical distribution of each species, and a checklist of diaptomid copepods are documented. In this case, 24 species were recorded, including two undescribed species belonging to the genera Phyllodiaptomus and Tropodiaptomus. In this case, 14 species are recorded for the first time in Cambodia, and Mongolodiaptomus formosanus has been recorded for the first time in Southeast Asia. One species appears to be endemic to Cambodia and nine to the lower Mekong River Basin. Mongolodiaptomus malaindosinensis was the most common, frequently recorded throughout the four regions (40.5% of the sampled sites), followed by Vietodiaptomus blachei (39.2%), Eodiaptomus phuvongi (38.8%), Mongolodiaptomus formosanus (31.2%), and Eodiaptomus draconisignivomi (27.4%). Very rare species such as Eodiaptomus sanoamuangae, Tropodiaptomus vicinus, Mongolodiaptomus calcarus, and Tropodiaptomus sp. were only recorded at 0.4% to 0.8% of the sampled sites. The species diversity of Cambodia is most similar to that of Thailand, where 22 species (91.6%) occur both in Cambodia and Thailand, and 12 (50%) of the species have been recorded in both Cambodia and Vietnam. [ABSTRACT FROM AUTHOR]

Glacial and proglacial erosion are important sediment sources in river basins. The retreat of many glaciers on the Tibetan Plateau has important implications on the supply of fresh water and sediment dynamics for downstream river basins. Despite the importance of water and sediment dynamics at these catchments, existing quantification of suspended sediment fluxes from glacial catchments on the Tibetan Plateau is limited due to poor accessibility and challenging environments. This study presents the results of in‐situ investigations of water discharge and suspended sediment fluxes from the Mingyong Glacier catchment in Yunnan, Southwest China, between August 2013 and July 2017. The results show that the variation in water discharge and suspended sediment was highly seasonal. The variation of average suspended sediment concentration was large—69 ± 45; 119 ± 104; and 94 ± 97 mg/L in 2013, 2015, and 2016, respectively. We estimate that the sediment yield from the Mingyong catchment was highly variable ranging from 1104 t/km2/year in 2013 to 2281 t/km2/year in 2016, with 65%–78% of the total annual sediment load occurring during the summer (June to August). These annual variations in sediment yield can be attributed largely to precipitation patterns and extreme melting events. This study has provided a benchmark dataset that can be used for further works that investigate the impact of climate change on sediment dynamics in glacierized catchments in the Tibetan Plateau. Subsequently, the study helps us to better understand the increasing sediment supply to the Upper Mekong River from glacierized headwater catchments. [ABSTRACT FROM AUTHOR]

Oreoglanis brevicula, a new species of sisorid catfish, is described from a single specimen collected in northern Laos. It belongs to the Oreoglanis delacouri group in lacking a distinct median notch on the posterior margin of the lower lip and can be distinguished from congeners by the following combination of characters: nasal barbel length 31% of head length, pectoral fin not reaching pelvic-fin origin, body depth at anus 10% of standard length (SL), adipose fin with base 36% SL and posteriorly separate from dorsal procurrent caudal-fin rays, caudal peduncle length 19% SL, caudal peduncle depth 4% SL (= 4.3 times in peduncle length), lunate caudal fin, and 42 vertebrae. [ABSTRACT FROM AUTHOR]

Freshwater mussels belonging to the tribe Pseudodontini (Bivalvia: Unionidae: Gonideinae) play a keystone role in riverine and lacustrine environments throughout Southeast Asia. Many of them are narrowly endemic and habitat specialists, which need special conservation efforts. Unfortunately, the systematics of this group is rather poorly understood. Here, we show that Pseudodon inoscularis, the type species of the genus Pseudodon, belongs to the eastern clade of the tribe (Southeast Asia, east of the Salween Basin), and, more certainly, clusters with members of the genus Monodontina. Hence, the latter genus becomes a synonym of Pseudodon, while the subtribe Pseudodontina (=Pilsbryoconchina syn. nov.) should be used as a name for the eastern clade. In turn, the genus Indopseudodon stat. rev. and the subtribe Indopseudodontina subtr. nov. must be applied as valid names for the western clade of the Pseudodontini (Ayeyarwady to Salween). We present a revision of the genus Indopseudodon with an updated synonymy and a description of a new species, I. indawgyiensis sp. nov. We also link four species-level genetic lineages from the Mekong to available taxonomic names in the genera Pseudodon and Sundadontina (Pseudodon cambodjensis, P. vagulus, Sundadontina harmandi, and S. sulcata). Based on morphological features, we revise the taxonomic position of several recent and fossil species that were traditionally placed in the Pseudodontini and transfer them to the genera Parvasolenaia, Pseudodontopsis, Ptychorhynchus, Simpsonasus nom. nov. (a new name for Nasus Simpson, 1900) (Gonideinae: Gonideini), Pseudobaphia (Unioninae: Unionini), Balwantia and Lamellidens (Parreysiinae: Lamellidentini).

Fish often migrate to feed, reproduce and seek refuge from predators and prevailing environmental conditions. As a result, migration tactics often vary among species based on a diversity of life history needs, although variation within species is increasingly being recognised as important to population resilience. In this study, within‐ and among‐species diversity in life history migratory tactics of six Mekong fish genera was examined using otolith microchemistry to explore diadromous and potamodromous traits. Two species were catadromous and one species was an estuarine resident, while the remaining three species were facultative in their migration strategies, with up to four tactics within a single species. Migrant and resident contingents co‐existed within the same species. Management, conservation and mitigation strategies that maintain connectivity in large tropical rivers, such as effective fishway design, should consider a diversity of migration tactics at the individual level for improved outcomes. [ABSTRACT FROM AUTHOR]

The Mekong River Basin comprises approximately 38% of Southeast Asia. Our study area comprises the right-bank tributaries, which drain a substantial portion of Northeast Thailand. This study aimed to estimate the evaporative losses from streams during the 2013–2015 period. The normal and warm El Niño–Southern Oscillation (ENSO) phases caused higher temperatures and low rainfall in the 2014–2015 period. The results show that the local meteoric water line for precipitation isotopes had seasonal variation due to variable precipitation. The enrichment of river isotopes indicated that streams lost an average of 4% of their water through evaporation. During the cooling ENSO phase, significant evaporation occurs due to the deep convection that typically occurs in tropical regions. In contrast, evaporation was low during the warm ENSO phase because of its geographic location. The El Niño year's isotope values were significantly more enriched than the La Niña year's, showing that precipitation and positive temperature anomalies affected the isotopic compositions in the continental basin. Furthermore, the deuterium excess helped distinguish the relative contributions of the wet and dry seasonal sources to the moisture origin, indicating that the predominant source of moisture is inland evaporation, with a small contribution from the ocean. [ABSTRACT FROM AUTHOR]

The offshore Cuu Long (CLB) and Nam Con Son (NCSB) basins of SE Vietnam are two important Cenozoic hydrocarbon-bearing basins in the southern South China Sea (SCS), which can contribute to understanding the evolution of major SE Asian river systems, in particular the Mekong River. The Oligocene to Early Miocene basin fill of the Cuu Long Basin is dominated by sediment sourced locally from the Da Lat Zone basement on land. Sandstones have abundant Cretaceous detrital zircons and heavy mineral assemblages dominated by apatite and epidote. The Bach Ho Unconformity at c. 16 to 20.5 Ma marks a major provenance change and the overlying Middle to Late Miocene Con Son and Dong Nai formations were sourced by a large river system, which drained Indochina or even the Himalaya, resembling the present-day Mekong River. These formations have heterogeneous detrital zircon populations dominated by Triassic ages and zircon–rutile–tourmaline-dominated heavy mineral assemblages. The Oligocene Cau and Early Miocene Dua formations of the Nam Con Son Basin have a similar provenance to the CLB Con Son and Dong Nai formations, indicating a comparable drainage history of a large proto-Chao Phraya. At the Dua Unconformity at c. 15.4 to 17 Ma the Indochina provenance signature changes to a predominant Sibumasu signature. The early Mekong River evolution is suggested to have involved two river systems. A proto-Mekong 1 initially filled the CLB, while at the same time a proto-Chao Phraya River with a capture area into the Himalaya filled the Malay Basin and/or the NCSB. At the end of the Early Miocene drainage was completely reorganised, and the proto-Mekong 1 captured the headwaters of the proto-Chao Phraya and became the proto-Mekong 2, while the NCSB was filled by sediment from the Malay–Thai Peninsula. This major change marks a very important drainage reorganisation in the southern SCS region. [ABSTRACT FROM AUTHOR]

This study proposes a conceptualisation of transboundary environmental publics (TEPs) to interrogate the distinctive pathways through which contestations over public participation and 'stakeholder engagement' in transboundary environmental governance have emerged. While much attention has been paid to how the discourse and practices of public participation have been co‐opted by powerful actors to serve vested interests, fewer studies have questioned how the contingent and fragmented nature of 'publics' entangled with a politics of representation have contributed towards the failings of public participation. This paper examines how state and nonstate TEPs have emerged as multi‐dimensional, material and discursive entities within hybrid hydropower governance arrangements in the Mekong River Basin. Public participation in relation to hydropower dams on the Mekong River's mainstream is intertwined with the contingent formation of TEPs through the Mekong River Commission's Procedures for Notification, Prior Consultation and Agreement (PNPCA). Contestations over public participation may be understood by examining the specific ways in which 1) 'publics' were assembled, 2) notions of 'place' were interpreted, 3) scalar constructions were influenced by centre‐periphery power dynamics and 4) temporality was defined. To understand how this convening of 'publics' fundamentally influences the potential for meaningful public participation, this paper contrasts the TEPs formed through the PNPCA and the Save the Mekong Coalition (STMC), a transnational activism network. The STMC TEP provided reimaginations of scalar constructions and broadened definitions of publics, places, and temporality in participation, reflecting how the topographical and discursive practices that underlie the creation of TEPs have important implications for their legitimacy and effectiveness in influencing decision making. [ABSTRACT FROM AUTHOR]

The prevalence of Schistosoma mekongi in humans in the Lao People’s Democratic Republic (Lao PDR) has been relatively well monitored and has decreased due to effective interventions such as preventative chemotherapy with mass drug administration of praziquantel and community awareness programs. However, the prevalence among potential domestic reservoir animals remains broadly unclear, except for a few villages in the endemic area. Therefore, we conducted S. mekongi surveys for the domestic animals that had contact with Mekong River water. We conducted a cross-sectional study of the domestic animals in the seven sentinel villages in the Khong and Mounlapamok Districts of Champasak Province in southern Lao PDR in 2018 by random sampling with a statistically reliable sample size. Stool samples of the five predominant domestic animal species, cattle (n = 160), pig (n = 154), buffalo (n = 149), dog (n = 143), and goat (n = 85), were collected and examined using parasitological FECT method and the LAMP technique. The microscopic analysis did not detect any eggs of S. mekongi in the stool samples of any animal species. However, S. mekongi DNA was detected by the LAMP test in dog stool samples (0.7%; 1/143). On the other hand, the prevalence of other helminths was quite high and heterogeneous among animal species and sentinel sites by the microscopic analysis. These findings suggested that an intervention for S. mekongi infection should focus solely on human populations. However, periodic surveillance for S. mekongi infection among dogs should be conducted to monitor a possible resurgence of S. mekongi infection in the domestic animal population.

The diversity of freshwater calanoid copepods from different habitats in Vietnam is investigated based on our findings from a field expedition in 2012–2013 and literature reviews. We collected 160 samples from 87 sites, including lakes, ponds, roadside canals, rivers, and rice fields in eight provinces of southern Vietnam. A total of 13 species belonging to eight genera and three families were recorded. Among these, four were recorded for the first time in Vietnam (Mongolodiaptomus malaindosinensis, Mongolodiaptomus mekongensis, Vietodiaptomus blachei, and Pseudodiaptomus siamensis). One unidentified taxon (Tropodiaptomus sp.) probably belongs to an undescribed species. Both Eodiaptomusdraconisignivomi and M. malaindosinensis were the most frequently encountered species (28.74% of the sampled sites), followed by Mongolodiaptomus botulifer (24.14%), while Neodiaptomus yangtsekiangensis, Tropodiaptomusoryzanus, and Tropodiaptomus sp. are rare species found in a single locality. To date, 40 calanoid species (33 in the family Diaptomidae) have been recorded from Vietnam, and an updated list is presented. Seven species are potentially endemic to Vietnam. At the same sampling dates, the species richness of the calanoids was a range of 1–5 species per locality. The results of the Canonical Correspondence Analysis showed that pH and conductivity tended to be positively related to the calanoid distribution. [ABSTRACT FROM AUTHOR]

The law of international watercourses consists mainly of a series of bilateral, multilateral, regional, and global agreements that establish binding rules through which state parties jointly manage transboundary water resources. China similarly manages its shared freshwaters through a series of bilateral agreements. Increasingly, however, it relies on non-binding soft law instruments to manage these resources with its riparian neighbours. An important example of this is the Lancang-Mekong Cooperation, a branch of the Belt and Road Initiative. Its use of soft instruments, which recognize international law and promote projects, displays evidence of merging and emerging normativities, ensuring that it is capable of playing both a supporting and a developmental role in the law of international watercourses. [ABSTRACT FROM AUTHOR]

The large-scale monitoring of riverbank erosion is challenging because of human, equipment, and financial limitations, particularly in developing countries. This study aims to detect riverbank erosion and identify riverbank erosion hotspots along the Mekong River in Cambodia. A riverbank erosion rate map was developed using satellite images from Landsat 5, 7, and 8 (1990–2020) using the modified normalized difference water index (MNDWI) at a resolution of 30 m and Sentinel-2 (2016–2021) using the normalized difference water index (NDWI) at a resolution of 10 m. Detecting riverbanks in satellite images using a water index depends greatly on image resolution and water threshold. The riverbank lines were validated using Google Earth images. In the data used in December 2017, the root mean square error (RMSE) of Sentinel-2 was 6.00 m, while the RMSE of Landsat was 6.04 m. In the data used in January 2019, the RMSE of Sentinel-2 was 4.12 m, while the RMSE of Landsat was 5.90 m. The hotspots were identified by overlaying the riverbank erosion rate map and the exposure map of population density and land cover. Field surveys and interviews were conducted to verify riverbank erosion hotspots in the Ruessei Srok and Kaoh Soutin communes. The results showed that within the last 30 years (1990–2020), the riverbank eroded more than 1 km in a direction perpendicular to the river in some segments of the Mekong River in Cambodia. The highest average annual erosion rate was in the Ruessei Srok Commune in Kampong Cham Province, at approximately 43 m/yr. Most eroded areas were farmland and rural residential areas. The riverbank hotspots are situated mainly in the lower part of the Mekong River, where the population is dense, and the erosion rate is high. Riverbank erosion hotspots with a very high impact level and ongoing active erosion include the Peam Kaoh Sna, Kampong Reab, Kaoh Soutin, and Ruessei Srok communes in Kampong Cham Province. This study provides an efficient tool for using satellite images to identify riverbank erosion hotpots in a large river basin. The riverbank erosion hotspot map is essential for decision-makers to prioritize interventions to reduce the risk of riverbank erosion and to improve the livelihood of the people residing along the Mekong River. [ABSTRACT FROM AUTHOR]

The Mekong region has witnessed 'old' and 'new' players in the development of the Mekong river basin since the conclusion of the Cold War. Of specific interest is the participation of the three Northeast Asian countries, China, Japan, and South Korea. Two interesting questions come into play and shall be answered in this exploratory research note. First, why and how do China, Japan and Korea participate in the hydro power projects in Laos? Second, what are the implications of their participation in the geo-politics in the Mekong region? This paper contends that the three countries are not only taking advantage of the economic benefits they can derive from developing Laos but also carving their own respective geopolitical influence in the country. Of the three, sources reveal that China exerts its own dominance in the Mekong region as the hydro-hegemon by 'overpowering' its challengers through its overwhelming hydropower projects in the country and through non-cooperation with Japan and Korea. [ABSTRACT FROM AUTHOR]

Managing transboundary river basins proves a challenge for China when encountering disagreements with its neighbors that experience different political and social conditions. This paper analyzes what happens when China characterizes water as a security issue, examining China's fluid securitization practices, where changes can be identified indicating that the Chinese government values the various water security concerns differently. Two cases are adopted for comparison. In the case of China sharing the Mekong River, the Chinese government has shown a willingness to incorporate more issues found both inside and outside of the water sector. In contrast, in the case of the sharing of the Ganges-Brahmaputra-Meghna, China's security agenda has been limited to the consideration of water availability and has led to military security concerns. The findings indicate that China's water security agenda is not only driven by a concern for water management over specific rivers, but also judgments that incorporate strategic military consideration with regard to countries that they are involved with. The case of China thus suggests that water security is a complex domain that demonstrates competing values and concerns in (de)politicizing water. Therefore, water-related security issues cannot be understood solely from an environmental policy perspective. [ABSTRACT FROM AUTHOR]

The Khorat snail-eating turtle (Malayemys khoratensis) is a recently described (2016), poorly studied freshwater turtle known from the Khorat Plateau in Thailand and a small area near Vientiane, Lao People's Democratic Republic. We investigated the occurrence and natural history of M. khoratensis during field surveys of Xe Champhone Ramsar Site (XCRS) and Nong Louang Wetland Complex (NLWC) in Savannakhet Province, and also examined museum specimens and published and unpublished photographs of Malayemys spp. from Lao. Our field surveys confirmed the occurrence of M. khoratensis in the XCRS and NLWC, where populations remain subject to harvest at levels unlikely to be sustainable. We examined 7 museum specimens from Vientiane, Savannakhet, Khammouan, and Champasak provinces. Museum specimens from Vientiane, Savannakhet, and Khammouan provinces were referable to M. khoratensis. Published photographs confirm the occurrence of M. khoratensis in Vientiane Province, and Malayemys subtrijuga in Champasak Province. Collectively, our records extend the geographic distribution of K. khoratensis by > 300 km from previously reported localities. We posit the existence of a biogeographic barrier in southern Lao and adjacent Thailand that separates the allopatrically occurring M. khoratensis and M. subtrijuga. [ABSTRACT FROM AUTHOR]