Human barriers fragment three-quarters of all rivers in the Mekong basin

Influence of salinity intrusion on the speciation and partitioning of mercury in the Mekong River Delta

The Mekong River, a global freshwater biodiversity hotspot, has suffered from intensive barrier construction, resulting in major challenges in safeguarding its fauna. Here, we provide a comprehensive evaluation of the impacts of river barriers on the distribution of 952 fish species in the Mekong River Basin. Our analysis revealed that 93% of Mekong fish species analysed suffer from habitat fragmentation, and species with larger habitat range requirements experienced higher river fragmentation impacts. Sub-basins along the main channel in the Lower Mekong had high values of species richness but relatively high barrier impacts. Across all migration types, freshwater-resident migratory (potamodromous) fishes were affected by the greatest levels of habitat fragmentation (Fragmentation Index, 42.45 [95% confidence interval, 38.61–46.62]). Among all International Union for Conservation of Nature conservation status categories, Critically Endangered species experienced the highest habitat fragmentation index (34.48 [19.46–53.52]). Among all barrier types in the Mekong, small dams and sluice gates contributed more to habitat fragmentation than large dams. While the effects of existing individual large dams on habitat fragmentation and fish distribution in the Mekong Basin are greater than for small barriers, the cumulative impacts of small barriers are greater; hence, basin-wide connectivity planning is needed for more effective conservation.

This research thesis will explore the relationship between integrated water resource management (IWRM), water conflict and cooperation within the Mekong River Basin and the capacity of the Mekong River Commission (MRC) to influence water governance in a rapidly evolving region. The paper will also analyse the historical evolution of the MRC and its role and influence on regional water governance within a highly complex geopolitical landscape. Based on a comparative analysis of the MRC's response to two large-scale water related infrastructure investments the paper will also assess its institutional capacity to mitigate and mediate transboundary conflict. The paper will then explore ways by which the MRC could become more relevant and effective in its role of mitigating and mediating transboundary water related conflict within the changing Mekong waterscape. The Mekong River starts in Tibet and journeys 4,000 kilometres through Southeast Asia until it reaches the South China Sea in Vietnam. It is of vital importance for national economic development of Upper and Lower Mekong countries whilst also being integral for the subsistence livelihoods of millions of rural poor reliant on the river for existence (Osborne, 2000). The current rate of rapid economic growth in Southeast Asia and China is placing increased pressure on natural resources and the environment. The impact of competing national development objectives between the Mekong riparian states (and also within sovereign borders) has, and will continue to result in sporadic localised tension (Jacobs, 2002). Examples of localised conflict on the Lower Mekong Basin catalysed (or escalated) by competing water resource developments includes disputes over the Nam Theun 2 Hydropower Project, the proposed Thailand Water Grid Project and the Sesan, Sre Pok and Sekong (3S) hydropower projects on the Vietnam - Cambodia tributaries of the Mekong River. This third example (which will be explored in greater detail in Chapter 5) demonstrates the reality of water-related transboundary conflict. As an internationally supported river basin organisation, the MRC will be an important stakeholder in regional water governance especially as the drivers for water-related conflict increase within the Mekong Basin. However, the current institutional capacity of the MRC to mitigate and resolve conflict between riparian countries is weak (Backer, 2007). In a period of increasing economic growth and regional integration among Mekong riparian nations the increasing threat of water-related transboundary conflict has the potential to impede economic growth and compromise geopolitical stability. This thesis analyses water governance mechanisms in the Mekong however it is impossible to decouple water governance from the water, energy and food security nexus. This nexus reflects the interdependencies between water, energy and food by which energy production can influence water demand and access for food production. At the same time, water use can affect food security as well as energy requirements. The choices people make about what food they consume (which are closely linked with demographic and lifestyle changes and economic growth), influence both water and energy demands. For transboundary water governance to be effective it must acknowledge the links between water, energy and food security and the impacts on or relations to environment, climate, people's livelihoods and the economy.

The resource development and changes of hydrological regime, sediment and water quality in the Mekong River basin have attracted great attentions. This research aimed to enhance the study on transboundary pollution of heavy metals in this international river. In this study, eight sampling sites were selected to collect the bed sand samples along the mainstream of the Mekong River. In addition, the contents of 5 heavy metal elements and their spatial variability along the mainstream of the river were analyzed. The geoaccumulation index (Igeo) and potential ecological risk analysis were employed to assess heavy metal pollution status in the mainstream of the Mekong River. The results show that the average content of the heavy metal elements Zn is 91.43 mg/kg, Pb is 41.85 mg/kg, and As is 21.84 mg/kg in the bed sands of the Upper Mekong River, which are higher than those (Zn 68.17 mg/kg, Pb 28.22 mg/kg, As 14.97 mg/kg) in the Lower Mekong. The average content of Cr in the Lower Mekong is 418.86 mg/kg, higher than that in the Upper Mekong (42.19 mg/kg). Luang Prabang has a very high Cr concentration with 762.93 mg/kg and Pakse with 422.90 mg/kg. The concentration of Cu in all of the 8 sampling sites is similar, except for in Jiajiu with 11.70 mg/kg and Jiebei with 7.00 mg/kg. The results of the geoaccumulation index reveal that contaminations caused by Zn and Pb while Pb and As are more than those by Zn in Upper Mekong. Cr is the primary pollutant in the Lower Mekong, especially at Luang Prabang and Pakse. Slight pollution with As also occurs in Pakse. The potential ecological risk index indicates that the potential ecological risk of heavy metals in the mainstream of the Mekong River is low. We argue that the pollution of water quality and contamination of heavy metals in bed sediment caused by mining of mineral resources or geochemical background values in the Mekong is not transmitted from the Upper to the Lower Mekong because of the reservoir sedimentation and dilution along the river.

Satellite observations and modeling to understand the Lower Mekong River Basin streamflow variability

The Mekong River, one of the world’s great rivers, is facing the disruption of its sediment balance with anthropic reductions in its sediment load and resultant impacts on nutrient fluxes, aquatic ecology and evolution of its river channel, floodplain and delta. Using long-term monitoring data from 1993-2018, we estimated the temporal variability of sediment loads in Tonle Sap and Lower Mekong Rivers in Cambodia, assessing the sediment linkage between Tonle Sap Lake and Mekong River, which are connected by a seasonally reversing flow through the Tonle Sap River. We used data from three monitoring stations established in Cambodia in 1993, from the Mekong at Kratie (upstream) downstream to the Mekong at Chroy Changvar (just upstream of the Tonle Sap confluence), and the Tonle Sap River at Prek Kdam (about 40 km upstream of the Mekong confluence). We estimated the annual sediment in the main Mekong River was 72±38 Mt/yr at Kratie and 78±22 Mt/yr at Chroy Changvar from 1993-2018. Our calculated sediment load for the Lower Mekong River is lower than reported in older studies (prior to the 2000s), which is consistent with sediment trapping by dams on Upper Mekong mainstream and major tributaries built since 1993, and consistent with other recent estimates of sediment load on the Lower Mekong. Our analysis of water discharge and sediment concentration indicates that Tonle Sap Lake provided 0.65±0.6 Mt of sediment annually to the Lower Mekong River from 1995 to 2000. However, since 2001, Tonle Sap Lake has become a sink for sediment, accumulating an average of 1.35±0.7 Mt annually. Net storage of sediment in Tonle Sap Lake reduces the annual sediment transport to the Mekong delta, further compounding the effects of sediment delivery to the Delta resulting from upstream dam construction and instream sand mining.

Numerous studies have examined the changes in streamflow in the Mekong River Basin (MRB) using observations and hydrological modeling; however, there is a lack of integrated modeling studies that explicitly simulate the natural and human‐induced changes in flood dynamics over the entire basin. Here we simulate the river‐floodplain‐reservoir inundation dynamics over the MRB for 1979–2016 period using a newly integrated, high‐resolution (~5 km) river hydrodynamics‐reservoir operation model. The framework is based on the river‐floodplain hydrodynamic model CaMa‐Flood in which a new reservoir operation scheme is incorporated by including 86 existing MRB dams. The simulated flood extent is downscaled to a higher resolution (~90 m) to investigate fine‐scale inundation dynamics, and results are validated with ground‐ and satellite‐based observations. It is found that the historical variations in surface water storage have been governed primarily by climate variability; the impacts of dams on river‐floodplain hydrodynamics were marginal until 2009. However, results indicate that the dam impacts increased noticeably in 2010 when the basin‐wide storage capacity doubled due to the construction of new mega dams. Further, results suggest that the future flood dynamics in the MRB would be considerably different than in the past even without climate change and additional dams. However, it is also found that the impacts of dams can largely vary depending on reservoir operation strategies. This study is expected to provide the basis for high‐resolution river‐floodplain‐reservoir modeling for a holistic assessment of the impacts of dams and climate change on the floodpulse‐dependent hydro‐ecological systems in the MRB and other global regions.

The Mekong/Lancang River Basin is undergoing a period of rapid hydropower development. Construction of over 20 dams is currently underway just in the Lower Mekong Basin, with at least an additional 42 dams planned for construction in the next 20 years. The resulting alteration of the basin’s natural flow and sediment regimes, geomorphologic makeup, and fish migration patterns could significantly impact the exceptional biodiversity and food production characteristics of the basin’s ecosystems. This study is devoted to assessing the improvement in sediment passage (compared to current and expected conditions) that could be achieved at various locations in the basin by implementing reservoir sediment management practices such as sediment bypasses, drawdown flushing, seasonal drawdown routing, and sluicing. Specific sediment management options are discussed for selected reservoirs in the Sre Pok, Se San and Se Kong sub-basins (called the “3S” basins). These sub-basins are particularly important with respect to biodiversity and ecological productivity, and serve as an important source of flow, sediment and nutrients to the mainstream Mekong River. Simulations are performed with the Sediment Simulation Screening (3S) Model. Using a daily time-step, this model simulates the mass-balance of flow and sediment to predict in relative terms the spatial and temporal accumulation and depletion of sediment in river reaches and in reservoirs under different sediment management policies. It also identifies the relative tradeoffs between hydropower output and reliability and sediment regime alteration for these sediment management options. INTRODUCTION In the recent past, wars have kept the Mekong Basin natural, and as such it has supplied the fish that support the livelihoods of more than 50 million people living in the basin. The production of fish results from the natural hydrological and sediment (and nutrient) regimes as the water flows for 4,750 km from the upper Mekong (called the Lancang) River basin of China through portions of Myanmar, Laos, Thailand and Cambodia on its way to the Vietnam Delta. Given the current absence of military conflict in the basin the river has become attractive to hydropower developers and those who will benefit economically from that development, at least in the short run. The Mekong River and its tributaries form one of the world's most diverse ecosystems, containing over 560 endemic fish species and over 300 globally threatened vertebrate species. The basin’s productivity and biodiversity are driven by an annual flood pulse, which carries most of the annual 1297 World Environmental and Water Resources Congress 2013: Showcasing the Future © ASCE 2013

Abstract. Koompoot K, Watiroyram S. 2025. Freshwater copepods in the tributaries of the lower Mekong River Basin in Thailand. Biodiversitas 26: 1846-1859. The study of copepod species diversity exclusively in the Mekong River Basin of Thailand has been sporadic, with no relevant publications for over a decade. Additionally, cyclopoid taxonomy remains a serious issue. This study investigated the species composition and distribution of copepods in three river basins of the Mekong River in Thailand: the Mekong, Songkhram, and Mun River Basins. Samples were qualitatively collected using a 60-µm plankton net during the monsoon rainy season by dragging the net from the limnetic zone to the littoral zone. A total of 47 species were identified from 480 samples collected across various waterbodies between 2020 and 2023 in seven provinces within the Mekong River Watershed, including Nong Khai, Bueng Kan, Udon Thani, Sakon Nakhon, Nakhon Phanom, Amnat Charoen, and Ubon Ratchathani. Among 27 calanoid and 20 cyclopoid species, one new cyclopoid species was named and published in prior research, and one new calanoid species is currently being prepared for the nomenclature process. Two cyclopoid copepod species were newly recorded in Thailand: Mesocyclops woutersi and Microcyclops pachyspina. We confirmed the presence of Microcyclops karvei in the country after its previous status had been uncertain. Furthermore, at least three calanoid species previously recorded in the study area: Mongolodiaptomus uenoi, Neodiaptomus songkhramensis, and Phyllodiaptomus christineae were not observed, suggesting their rarity, potential disappearance, or possible taxonomic misidentification. Similar to cyclopoid copepods, additional sampling and taxonomic study are required for confirmation. The species diversity and species list have been updated, and a brief morphological description along with taxonomic notes are provided for the two newly recorded species and one rare species of cyclopoid copepods.

The Mekong River is one of the world’s largest rivers and has an annual captured fish production of about 2.3 million tonnes, equivalent to around 11 billion USD. Although the Mekong provides important ecological and socioeconomic benefits to millions of people, it is facing intensive change due to anthropogenic stressors. Therefore, it is necessary to understand the changes to the spatiotemporal fish communities to inform sustainable fisheries management. Here, we aimed to characterize patterns of the fish communities and identify the ecological status of each fish community using daily catch data from 2007 to 2018 at 25 monitoring sites in the Lower Mekong Basin (LMB). The collected data were classified by a self-organizing map into four main groups. Group 4 represented the lower Vietnam Mekong Delta (VMD), while groups 1, 2, and 3 were subdivided into subgroups 1a (upper LMB), 1b (upper and middle LMB), 2a (Mekong River below the Khone Falls and Sesan River), 2b (Mekong River below the Khone Falls and Sekong, Sesan and Srepok (3S) Rivers), 3a (Floodplain-Tonle Sap and Songkhram) and 3b (upper VMD). Among the 571 species recorded, 119 were identified as indicator species. Based on the abundance and biomass comparison curves, the fish community of 2b was in a healthier condition with a positive W-statistic value while the rest had a negative W-statistic value. The highest species richness and diversity were observed in 3a and 2b, so these subgroups deserve high management and conservation priority. Likewise, 1a should also be considered as a high priority area since it harbors several endangered and long-distant migratory fishes. It was also noticed that the fish communities of groups 3 and 4, located far from the hydropower dams, remained mostly unchanged compared to those of groups 1 and 2, close to the mainstream and tributary dams in the upper LMB and 3S Rivers.

Global river systems are experiencing rapid changes in sediment transport under growing anthropogenic and climatic stresses. However, the response of sediment discharge to the coupled influence of anthropogenic and natural factors and the associated impacts on the fluvial geomorphology in the Yangtze and Mekong rivers are not comprehensively assessed. Here, we recalibrated a seamless retrieval algorithm of the total suspended sediment (TSS) concentrations using in situ data and concurrent satellite data sets to analyze spatiotemporal patterns of the TSS concentrations in the lower Yangtze and Mekong rivers. Combined with soil erosion rates estimated by the Revised Universal Soil Loss Equation for the past 20 years, we examined the contributions of different factors to TSS trends. The results show that TSS concentrations in the Yangtze River decreased from 0.47 g L−1 in 2000 to 0.23 g L−1 in 2018 due to the construction of the Three Gorges Dam (TGD), especially in the Jingjiang reach, with a declining magnitude of 0.3 g L−1 (∼56%) since the TGD began operating. The Mekong River experienced increasing TSS concentration trends upstream and decreasing trends downstream from 2000 to 2018, possibly attributed to increased upstream soil erosion and decreased downstream water discharge. Declining TSS concentrations in both rivers have driven varying degrees of river channel erosion over the past two decades. This study investigated long‐term changes in the TSS concentrations and soil erosion in the Yangtze and Mekong rivers, and the results provide baseline information for the sustainable development of river sediment delivery.

We present a framework for strategic dam planning under uncertainty, which includes GHG emissions mitigation as a novel objective. We focus on the Mekong River Basin, a fast‐developing region heavily relying on river‐derived ecosystem services. We employ a multi‐objective evolutionary algorithm to identify strategic dam portfolios for different hydropower expansion targets, using process‐related and statistical models to derive indicators of sediment supply disruption and GHG emissions. We introduce a robust optimization approach that explores variations in optimal portfolio compositions for more than 5,000 state‐of‐the‐world configurations, regarding sediment origins and trapping and GHG emissions. Thus, we can rank dam projects' attractiveness based on their frequency of inclusion in optimal portfolios and explore how uncertainty affects these rankings. Our results suggest that developing dams in the upper Mekong would be a more robust option for near‐term development than, for example, the lower Mekong and its tributaries, for both environmental and energy objectives. Our work presents a novel approach to better understand the basin‐scale cumulative impacts of dam development in high‐uncertainty, data‐scarce contexts like the Mekong Basin.

Over the last few decades, considerable concern has been expressed about the threat of Mekong River Basin hydropower dams to a range of important freshwater riverine fisheries, particularly for fish that seasonally migrate long distances. However, much less attention has been given to the threat of hydropower dams to fish biodiversity in the high-diversity Mekong River Basin, the focus of this paper. Through reviewing the existing state of knowledge regarding Mekong River Basin fish biodiversity, and threats to it, we argue that even though no species are definitively known to have been extirpated from the Mekong River Basin to date, hydropower dam development and various other developments nevertheless pose a serious threat to fish biodiversity. Indeed, dams typically significantly block fish migrations and fish larvae distribution, cause river fragmentation, fundamentally alter river hydrology, and change water quality, all factors that have the potential to intersect with each other and lead to significant species extirpation and extinction, or in some cases, functional extinction, when a small population remains but the important larger population is lost permanently. The circumstances are further exacerbated by the lag time between impact and when that impact becomes evident, cumulative impacts, a lack of consistent data collection, including the collection of base-line data, and insufficient post-project research related to biodiversity. We contend that much more could and should be done to ensure that the Mekong River basin’s exceptional fish biodiversity is not variously diminished and destroyed during the coming years.

Balancing competing interests in the Mekong River Basin via the operation of cascade hydropower reservoirs in China: Insights from system modeling

ABSTRACTThe Mekong Basin in southeast Asia is facing rapid development, impacting its hydrology and sediment dynamics. Although the understanding of the sediment transport rates in the Mekong is gradually growing, the sediment dynamics in the lower Mekong floodplains (downstream from Kratie) are poorly understood. The aim of this study is to conduct an analysis to increase the understanding of the sediment dynamics at the Chaktomuk confluence of the Mekong River, and the Tonle Sap River in the Lower Mekong River in Cambodia. This study is based on the data from a detailed field survey over the three hydrological years (May 2008–April 2011) at the two sites (the Mekong mainstream and the Tonle Sap River) at the Chaktomuk confluence. We further compared the sediment fluxes at Chaktomuk to an upstream station (i.e. Mukdahan) with longer time series. Inflow sediment load towards the lake was lower than that of the outflow, with a ratio on average of 84%. Although annually only a small amount of sediment load from the Tonle Sap contributes to the delta (less than 15%), its share is substantial during the February–April period. The annual sediment load transport from the confluence to the delta in 2009 and 2010 accounted for 54 and 50 Mt, respectively. This was on average only 55% of the sediment fluxes measured at Mukdahan, a more upstream station. Furthermore when compared to sediment loads further downstream at the Cambodia–Vietnam border, we found that the suspended sediment flux continued to decline towards the South China Sea. Our findings thus indicate that the sediment load to the South China Sea is much lower than the previous estimate 150–160 Mt/yr. Copyright © 2014 John Wiley & Sons, Ltd.