8. Integrated River Basin Management

For many countries, especially at the stage of their active economic development, multipurposes of IRBM includes many elements, such as use of water for irrigation, municipal and industrial water supply and sanitation; power generation at hydroelectric power plants; navigation and transport; and much more (Worki, 1971). All this undoubtedly requires huge volumes of water. At the same time, it is necessary to maintain a strict balance between all water users.

The modern multifunctional dams are one of the best examples of such management. Firstly, they are called to store water for agricultural, municipal and industrial water use. Secondly, they have a potential to produce environmentally friendly electricity. Thirdly, the cascades of reservoirs regulate the water flow, which makes it possible to avoid floods and droughts since it accumulates a certain volume of water. Fourthly, the water area of ​​the reservoirs is an ecosystem for a number of flora and fauna. Fifth, reservoirs create conditions for social events, for example, tourism, fishery, fishing, hunting, etc. (Branche, 2015).

In order to effectively manage water at the basin level, it is very important to take into account all its properties, and not only positive, but also negative aspects. Accordingly, the positive properties should be improved and multiplied, and the negative ones should be fought and overcome.

When any natural disasters appear, most often it is associated with water in one form or another. Floods, droughts, landslides, avalanches, storms have been increasing and becoming more frequent lately. Water-related disasters have both direct impacts (damage to buildings, crops and infrastructure, loss of life and property) and indirect (reduced productivity and livelihoods, increased investment risk and health impacts). Rising economic and disaster costs should provide a significant incentive for riparian states’ governments to pay more attention to preparedness, prevention and tackling the root causes of vulnerability (UN Water, 2020).

The desire of many countries (especially developing ones) is to manage water as resource to meet the needs of their populations. In this view, the IRBM aims to propose an environmentally sustainable water management approach in which human water needs are met in a way that maintains or restores the ecological integrity of the affected river ecosystems.

The development of waterways is also one of the most important components of IRBM. Apart from their traditional role as a system of travel or transport they serve a variety of functions such as: water supply, transfer and drainage; tourism, sport, cultural leisure and recreational; heritage landscape, open space and ecological services. In other words, waterways act as an agent or catalyst for ecological and social rural and urban areas (British Waterways, 2003).

Water and land use and development can harm water resources if not carefully planned and managed. Therefore, IRBM should aim to integrate water and land use planning in such a way as to support the economic growth of the population and the development of the state, meet environmental needs and ensure a balance of economic, social and cultural benefits.

Changing climatic and water conditions and the increasingly dry conditions have made water conservation a priority for many countries. Landscape professionals can implement simple and smart water saving strategies that will become part of the solution. Appropriate cultural practices are an essential component of efficient water use and management in urban and rural landscapes, reducing waste and maximising the health of landscapes (Wallace & Siegel-Miles, 2017).

Further, forests, grasslands, agricultural land and water resources are closely related. All these issues have been discussed for a long time in isolation from each other. However, in recent years, this connection has been growing and has attracted the attention of experts in the fields of forestry, agriculture, water resources management, landscape conservation and environmental issues. There is an increasing need for different sectors of the economy and countries to communicate and collaborate with each other to facilitate coordination and alignment of plans and instruments, especially in the light of climate change.

For example, in the area of ​​water and forest nexus, there are great opportunities for joint activities that can benefit both forests and water resources. As a result of effective IRBM, forests make the most significant contribution to ensuring water availability. Forests minimise erosion and thus reduce the damage to water quality due to siltation through stabilising the soil. In addition, by trapping sediment and pollutants from other land uses and slope activities, forests can protect water bodies and streams (FAO, 2009).

It is well known that water use and water consumption in upstream countries can substantially affect the economic, social and environmental situation of downstream countries. Moreover, it does not matter how far these countries are from each other—a few kilometers or several hundred kilometers (Rasul, 2014). There are also cases in the world when the well-being of not one, but several downstream countries, and even the entire region as a whole, depends on water use of the upstream country: for example, Amu Darya, Mekong, Nile rivers.

Nepal et al. (2014) classifies the causes of the impact of the upstream country water use on the downstream country into two types: (i) anthropogenic impacts in the form of land use and (ii) natural impacts in the form of a changing climate. Both types of these causes invariably lead to changes in the hydrological regime, which can lead to irreversible consequences.

Undoubtedly, the inappropriate water use by upstream countries can have a negative impact on the statement of downstream countries. Conversely, the careful and wise use of water resources upstream contributes to development of downstream countries and the entire basin region. This raises the dependency ratio of downstream countries (Dukhovny & De Schutter, 2018).

Some literature also contains descriptions of how downstream countries’ activities influence the development of upstream countries. For example, as the downstream countries are located closer to the sea, development of navigation in these countries is helpful to economic development (trade, food delivery, etc.) and social development (transport, tourism, etc.) in upstream countries (Barret, 1994). Another example is the development of fisheries in the lower reaches, where the fish leave for spawning in the countries of the upper reaches of some rivers. In these two cases the situation opposite to upstream–downstream influence is observed—where upstream countries become dependent on downstream countries (Moellenkamp, 2007).

There is one more spatial dimension that requires the obligatory resolution of water disputes: between countries on different banks of the river. Scientists believe that in this case, the country's activities on the right bank may affect the country's activities on the left bank (and vice versa). For the most part, the literature provides two main factors of impact on water resources in this approach: environmental problems and navigation problems, which ultimately can lead to social and economic losses. If a system of joint coordinated actions is introduced, they will entail the same common benefits (ADB, 2013).

With regard to navigation, a joint approach, research and work in the field of maintaining the bottom and banks of the river in a normal functioning state is needed here. For example, it is necessary to take measures to clean the bottom and river banks from sediment, prevent the formation of river islands, joint works to recover from natural disasters, etc. (Wood, 1999).

Taking into account the long tradition of water cooperation on the basin levels, it can be concluded that understanding and considering all of the above circumstances is a prerequisite for IRBM which should be based on:

Influenced by distinct wet and dry climate, climate change and El Nino, floods or droughts of varying degrees frequently happened in the basin (Chen et al., 2020). Since the 1980s, the drought and flood disasters have occurred frequently in the basin due to the interaction of multiple monsoon systems (Hundertmark, 2010).

Between 1970 and 2017, 225 flood disasters occurred in the basin that led to almost 12,000 deaths, over 100 million lives disrupted and more than 50 billion USD in economic damages (CRED, 2017). The more hazardous flood disasters in recent decades include the 2000–2001 and 2011 Mekong Delta floods, the 2011 Central Thailand floods and the 2015 Myanmar floods. Climate change is expected to further increase the frequency and intensity of floods in the region in the coming decades (IPCC, 2012).

In terms of flood risk, it can be noted that the highest risk of flooding in the basin is observed in Cambodia and Vietnam, a high risk in Laos, and medium risk is in Thailand (Sok, 2013).

Floods, on the one hand, cause huge financial losses. Such losses in 2012 amounted to 61 million USD (MRC, 2013a, 2013b). Estimating form the data of Chen et al. (2019), from 2000 to 2017, the average flood fatalities in the riparian 5 countries except China was 253.5 person/a and showed an increase trend. On the other hand, they can be beneficial. The benefits of flooding include maintaining annual fish yields, especially in the Great Lake, maintaining 5.24 million hectares of flooded wetlands in the lower Mekong with associated socio-economic benefits, providing water for irrigation during dry seasons, fertilizing floodplains with annual silt deposition, etc. (MRC, 2013a, 2013b).

Following the MRC (2010b), the average annual direct cost of flooding to agriculture, infrastructure, and buildings in the lower Mekong is 60–70 million USD a year. Where Cambodia and Vietnam account for two-thirds of the total. In turn, the average annual value of flood benefits is 8–10 billion USD, i.e. about 100 times the cost. The challenge for better flood risk management is to reduce the costs and impacts of flooding while preserving the benefits (ADB, 2008).

Following the devastating floods of 2000–2001, the basin member countries have taken decisive steps to combat floods. The aim of these activities was to reduce the damage to infrastructure, economic losses, loss of life among the population, and their livelihoods as a result of extreme floods (Hoang et al., 2018).

The Flood Management and Mitigation Strategy was developed and implemented, followed by the development and implementation of a special Flood Management and Mitigation Program (FMMP) in 2004. The FMMP was developed as an integrated programme, consisting of the following components: (i) Establishment of the Regional Flood Management and Mitigation Center; (ii) Structural Measures and Flood Proofing; (iii) Enhancing Cooperation in Addressing Trans-boundary Flood Issues (iv) Flood Emergency Management Strengthening; and (v) Land Management (Sok, 2013).

In addition to natural causes of floods in the basin, there are also anthropogenic ones. Among them is the widespread construction and use of hydro-electricity plants (HEPs). In order to generate electricity on HEPs, it is necessary to discharge water mainly during winter periods, which entails a change in the flow regime of the river. And the sudden start of the hydropower station and the sudden release of water from the dam cause the downstream river to rise sharply, which may cause flood losses. The situation is aggravated by climate change and the region's susceptibility to these changes. For example, in 2019, the issue of floods was at the head of the annual report of the Mekong River Commission (MRC, 2020). On the other hand, the dams of HEPs are designed to regulate the river flow, which in the future will allow avoiding, or at least minimizing the risk of floods.

According to Chen et al. (2019), from 1900 to 2017, the total death toll of flood, storm and drought was respectively 12,941, 165,830 and 0 person, the total injury toll was 5742, 33,594 and 0 person, the total economic loss was 52.99, 15.29 and 11.26 billion US$. The water-related 3 disasters took up 93% death toll and 98% economic loss of all natural disasters.

For better responding to flood and drought, in addition to hydrological data, other information has been shared among the basin states. A mechanism of data and information sharing on floods, droughts and emergency water-related situations is under continuous consideration and discussion among basin member countries (LMC, 2018).

In order to help downstream countries with flood control and drought resistance, the Ministry of Water Resources of China started to officially provide the whole year of hydrological information of the Lancang River and Mekong River countries from November 1, 2020 (MWR, 2020).

The other most pressing issue for IRBM in the basin is navigation development and its hidden benefits. Waterfalls and dangerous shoals have greatly hindered the development of river navigation. Which in turn through its capacity influence on trade and thus on social and economic development of the entire basin region of.

The most significant event in this direction took place in June 2001. The Chinese government signed an Agreement with Thailand, Myanmar and Laos to improve river navigation. Remarkably, this agreement was the first time the authorities of China and Myanmar were involved in any agreement on use of the Mekong River waters. As part of this agreement, China received the right to clean 33 km of the Mekong River. The ultimate goal of this event was to allow commercial up to 150 tonnes vessels from China (Yunnan Province) to Luang Prabang (Osborne, 2019). This route is restricted for large vessels due to obstacles on the river. Huge investments for the development of cross-border river trade have already been completed in the Chinese territory. Extensive dredging of the river was carried out and ports were built at Simao and Jinghong, as well as on the China-Laos border at Guanlei. For China, this agreement marks an important event called the “Opening of the South Gate”. This implies a policy of expanding economic ties with the countries of the mainland of Southeast Asia (Ratner, 2003).

Thailand's interest in this agreement is mainly of an economic nature. This would provide a direct trade route to China, which could give a competitive edge over other countries using the congested ports of the South China Sea (Goh, 2004).

The prospect of expanding economic and trade ties through the development of navigation has great implications for political and social relations between downstream and upstream Mekong river countries. On this base, China has established a research group at the state level to develop and plan development cooperation with other basin countries.

There is no doubt that any development requires certain sacrifices in the short term. But mutual agreement will give undeniable development results in the long term. Therefore, development often requires some concessions in protection.

The river basin provides a huge potential for cooperation in view of HEPs construction and electricity generation. Most of the existing HEPs as well as those under construction are located in China. The construction and use of HEPs is extremely important for the region. For China, this is an opportunity to cover the needs of its large population in electricity at the expense of cheap and environmentally friendly energy. For Laos, it is an opportunity to achieve the title of “Battery of Southeast Asia”, to develop its exports, and thereby improve its economic situation and eradicate the poverty of its population (OECD et al., 2020).

During the construction of HEPs large areas are flooded, and people are sometimes relocated far from their original homes. There were even cases where people’s costs from forced resettlement in Laos was not recompensed (Belay et al., 2010). Moreover, not only settlements are flooded, but also agricultural lands. The appropriate protection of people is needed in order to regulate the development of the country.

In addition to the problems associated with the construction and operation of HEPs, the basin has a number of other factors that negatively affect it. This includes over-intake water for irrigation in Thailand during dry seasons, such as observed in 2016. This caused damage to the Mekong delta in Vietnam. This, along with the constant discharge of drainage water from agricultural fields, wastewater discharge from industrial and municipal enterprises, as well as mining, already leaded to water pollution and environmental problems, especially in the downstream of the river (Xing, 2017).

As described above, river shoals have greatly hindered the development of river navigation and this in turn is a cause of conflict between China's desire to use the Mekong as a transport corridor and Thai activists desire to maintain the environment. In order to start up heavy vessels on the river, China needs to carry out dredging works. Thai environmentalists are against this, because they fear that these procedures may harm the fragile ecology of the Mekong in their territory (Ratner, 2003).

The uncontrolled extraction of sand and gravel from the river bed for construction purposes in Vietnam leads to erosion and the collapse of river banks, as well as the release of suspended particles into the water and hence sediment deposition in the Mekong delta.

All described problems are still existing in LMRB. But again, in order to achieve the economic development success, many difficulties must be overcome.

Despite the pace and levels of modern development, there are a number of obstacles to its successful implementation. One of these barriers is security. The description of this problem begins with a negative example on the Mekong River.

In October 2011, the Chinese river vessels Heping and Yuxing-8 were attacked on the Mekong River. The attack took place in the Golden Triangle, where the borders of Myanmar, Laos and Thailand converge.

Soon after the seizure of vessels, the bandits were attacked by the Thai military, who recaptured vessels. At the end of the military operation, large consignments of drugs were found on the sides of both vessels, which the bandits planned to transport downstream of the Mekong.

According to the Thai military, the Nor Kham group, including more than 400 militants in its ranks, attacked Chinese ships and destroyed their crews because the Chinese refused to pay the gang for protection. Vessels in turn were hijacked to transport drugs from Myanmar to Thailand. In the past, the group has tried to levy payments from Chinese vessels on the Mekong River.

The incident led to the organisation in Beijing by ministerial representatives of China, Laos, Myanmar and Thailand of joint patrols on the Mekong River. There, it was decided that from the end of 2011, the four countries would begin joint patrols on the Mekong River.

At the meeting, it was agreed that a headquarters would be set up in China to manage joint patrols on the Mekong River, and points of contact would be set up in the other three countries. The concerned authorities of these countries would be provided with a round-the-clock information channel. In addition, China pledged to provide Laos and Myanmar with the necessary patrol equipment and training support (Shabalina, 2012).

Cross-border criminal economies often develop in parallel with the integration of legitimate economies between countries. For centuries, there have been opportunities and channels for the trade of illegal goods across borders in East Asia and elsewhere.

Unfortunately, the opening of borders between partner countries, the organisation of economic and transport corridors, the opening of transnational trade river, land and railways entails the activation of transnational criminal supply chains (Luong, 2020). This requires consideration of safeguards and careful training of public security agencies.

The Mekong River has always been an important channel for people and goods between the many cities along its banks. The traditional forms of small craft trade that bind communities continues today. But the river is also becoming an important link in international trade routes.

Likewise, the social, economic and developmental importance of the Mekong River also creates opportunities for criminal activity to flourish. Improvements in infrastructure have exacerbated criminal threats to prosperity and peace, and many types of organised crime are expected to benefit from easier access and wading through the Mekong. Trafficking in drugs and precursors is a key problem that needs to be addressed. Recognising this, the basin states signed the Memorandum of Understanding (MOU) on Drug Control, together with UNODC, as key to illicit drug production and trafficking (UNODC, 2016).

Moreover, realising the seriousness of the problem in October 2015, the Ministerial Meeting on Law Enforcement and Security Cooperation along the Mekong River was held in Beijing and the Joint Statement on Strengthening Cooperation on Law Enforcement and Security Cooperation along the Mekong River was approved.

This joint statement comprehensively addresses security cooperation along the Mekong River in the LMRB countries; methods of combating crime, drug trafficking, terrorism, cybercrime; key areas of cooperation and the fight against smuggling, illegal migration and the detention and repatriation of criminals (Lu, 2016).

As has been repeatedly described in various sources, water is an essential resource that plays a special role in sustainable development of every nation. Public security is a prime concern in every country in the world. And in transboundary river basins such as the LMRB, it largely depends on international cooperation between countries.

Despite this, it was China that initiated the Lancang-Mekong Cooperation Mechanism, which improves political and economic ties in the basin. Both of these formats of riparian relations will be discussed in detail in subsequent chapters.

The countries that the Lancang-Mekong River flows through have different social development and economic systems. The utilisation of water in these countries mainly includes domestic water use, agricultural irrigation, hydropower, and waterway transportation, among which agricultural irrigation results in the most loss and consumption of water in the basin (Wen, 2016). Currently, the Lancang-Mekong countries have different development and utilisation status of water resources, which is described as below.

Since the 1990s, the Chinese government has attached great importance to the development of the Lancang River. The economic benefits brought by its development are not only conducive to the harmony and stability of ethnic minorities in the frontier areas but also influential to the lower reaches. The Lancang River Basin has many mountains and canyons, which are not suitable for agricultural development and farming. As a result, agricultural irrigation only accounts for minor use of water resources. On the other hand, the development of hydropower, shipping, and tourism requires high water use intensity (Wen, 2016).

In Laos, agriculture plays a dominant role in its economic structure, accounting for two-thirds of its annual GDP. The country is highly dependent on the Mekong River since agriculture feeds nearly 75% of its population, and the Mekong River provides water for about 60,000 hm² of land (Liu, 2013). At present, Laos hopes to promote its farm economy by developing and utilising the water of the Mekong River while developing irrigation and water conservancy to push agricultural development. In Myanmar, the Mekong River only flows through a small part of the country. The water development issues related to agricultural irrigation and hydropower are relatively insignificant, and thus apart from a few small-scale agricultural irrigation projects, it has low participation in the water development in the basin. Its primary concern lies in the development and protection of its forest vegetation (Liu, 2013).

Compared with countries dependent on agricultural development, Thailand’s economic structure is relatively diversified. Although the development of agriculture only accounts for about 20% of Thailand’s GDP, the export of agricultural products makes up a high proportion of around 60% of the total, and farming provides 70% of job opportunities in Thailand. However, since Thailand has continued its extensive use of the traditional slash-and-burn, many problems have emerged, including a significant reduction in forest area, severe soil erosion, decreased irrigation efficiency, land salinisation, uneven distribution of water resources, etc. Thailand mainly hopes to use the water of the Mekong River to irrigate the largest arid area in the northeast of the country and expand the irrigation area by the “Northern Mekong-Loei-Chi-Mun River Management and Artesian Water Diversion” project (Qi & Long, 2011). Thailand also wants to promote the regional economy by developing agriculture, so the Mekong River is of great significance to Thailand's economic and social development.

More than 85% of Cambodia’s territory lies in the Mekong River basin. In 1993, agricultural output accounted for about 50% of Cambodia's gross national product (GNP), and about 80–85% of the labour force was engaged in agricultural production. The development and utilisation of the water resources of the Mekong River in the Cambodian basin, including the largest freshwater lake in Southeast Asia-Tonle Sap Lake (also known as the Great Lake), and the irrigation of agriculture, are irreplaceable for Cambodia's economic and social development. Cambodia's primary demand is that the Mekong River's upper reaches would collect a considerable amount of floodwater during the rainy season to ensure the fertility of the soil in the floodplain of the Tonle Sap Lake. Therefore, Cambodia opposed the plan that Thailand diverts water from the Mekong River to increase its irrigation in the low water season because it affects the lower Mekong countries’ water volume (Zhao et al., 2017).

Vietnam is also on the opposing side. Among the four member states of the Mekong River Commission (MRC), Vietnam has the largest area to be irrigated, and it also has the highest dependence on agriculture. The use of water in Vietnam heavily relies on the volume of water in the upper reaches. Therefore, Vietnam hopes that the development and utilisation of water in the Mekong River can ensure its agricultural irrigation, prevent seawater intrusion, and protect its ecological environment. The water intake and storage project on the upper mainstream will impact the natural flow of the Mekong River, especially in the dry season. If the upstream water continues to decrease and the water level continues to drop, there will be an increase in the possibility of a drought delta and seawater inflow. Large-scale seawater intrusion will lead to salinisation of the fertile farmland in the area. Therefore, Vietnam strongly opposes the construction of dams and consequent diversion of water in the mainstream of the Mekong River.

In recent years, Mekong countries have continued to accelerate the Mekong River's development of water resources at different levels. Due to the water's cross-border mobility, lower Mekong countries have realised the economic potential of Mekong water resources in promoting economic growth, reducing poverty, and creating job opportunities. However, without proper planning and management, its rapid development will have a negative impact on the health of the river, resulting in increased pollution, floodplain changes, and habitat destruction. In order to ensure the sustainable use of water resources, a plan for the Lower Mekong River Basin was proposed. This plan set goals and provided measures for the optimal and sustainable development of the resources in the basin, and meanwhile, it also put forward a mechanism to offset the adverse effects.

The plan for LMB can be traced back to 1957, when Laos, Thailand, Cambodia, and South Vietnam formed the Committee for Coordination of Investigations of the Lower Mekong Basin. The initial cooperation focused extensively on data collection and carrying out research on agriculture, fishery, navigation, and education. In 1970, the MRC formulated the Indicative Basin Plan (IBP) and proposed a 30-year development plan that included the expansion of irrigation, navigation, hydropower, and flood control infrastructure. Later in 1987, the MRC cut the plan to twenty-nine projects. However, due to regional military conflicts, most of the projects have not been implemented. With the continuous increase of investment in water infrastructure projects, people have recognised the strong impact of the Mekong River's water resources on society and the environment. Therefore, the four lower Mekong countries (Cambodia, Laos, Thailand, and Vietnam) reached an agreement on cooperation for sustainable development for the Mekong River Basin, which facilitated the establishment of the MRC and opened a new era for taking coordinated approaches to the planning of the LMB. Based on the 1995 Mekong Agreement, the MRC served as the coordination center for cooperation, provided shared information and technical guidance, and assisted member countries in achieving their basins’ goals. The MRC was also required to formulate a basin development plan to classify the basin's first-level projects and programs. Consequently, it established a Basin Development Plan (BDP), focusing on developing principles, tools, research, and strategic directions. The plan was divided into two phases. From 2001 to 2006, the first phase (BDP1) laid the foundation for cooperative planning, bringing four member states together to build relationships, and developed planning processes, assessment tools, and knowledge bases. From 2007 to 2010, the second phase (BDP2) was implemented, but there was increasing pressure on the development of water resources in the basin during this process. After recognising the limitations of the data and the knowledge, BDP2 focused on transnational water issues to eliminate the long-term obstacles that affect the Mekong River Basin's sustainable development. By 2010, BDP2 had provided sufficient data and information for member states to develop and evaluate their interests, development plans, and positions related to water resources. The assessment of the whole basin's prospects in BDP2 was used to formulate a basin development strategy (BDS). The strategy, which was based on the integrated water resources management mechanism, further promoted cooperation among member states to ensure the Mekong River's sustainable development and management. The emergence of the first BDS between 2011 and 2015 has become an essential milestone in the history of Mekong cooperation. The MRC shifted its focus from acquiring water management knowledge and the best practices to focusing on how the Mekong countries share, use, and manage the resources to sustain the Mekong River's economic growth. During the third phase of BDP (BDP3), the MRC started to draft a status report and supported the implementation of MRC procedures. The “2016–2020 River Basin Development Strategy” continues to address the first BDS priorities, focusing on the sustainable development of the LMB and the management of plans and projects brought up by various countries (https://​www.​mrcmekong.​org/​our-work/​functions/​basin-planning/​). The “2016–2020 River Basin Development Strategy” elaborated on implementing the updated Mekong River Basin development strategy based on integrated water resources management by the MRC at the regional level. It also presented the institutional reform measures brought up by the MRC in the decentralisation roadmap. The strategy guided the Mekong River Commission Secretariat (MRCS) to support the MRC member countries to promote and coordinate the sustainable development of the Mekong River Basin in the next five years, and also pointed out the direction of cooperation among the MRC, the implementing agencies of member countries, the dialogue partners (China and Myanmar), the development partners and the stakeholders on a larger scale (MRC, 2016). In order to better promote and coordinate the sustainable management and development of water and other related resources in the LMB, the MRC launched the 2021–2030 Basin Development Strategy (BDS) and the 2021–2025 MRC Strategic Plan (SP) in April 2021 (MRC, 2021).

In addition, each country is carrying out integrated water resources management based on their conditions. With more complete systems and management frameworks, each country will explain its national water policies and strategies and define the responsibilities for water resources management in a more precise way. All LMB countries now have a dedicated agency responsible for water resources management. These agencies establish river basin organisations and carry out water management in a participatory approach at the sub-basin level. Water resources management should be further strengthened between and within the Mekong countries.

China, Laos, Myanmar, and Thailand have paid close attention to the Lancang-Mekong River Basin since the 1990s. The governments of China and Laos signed an agreement to jointly investigate and develop Mekong international navigation in 1989, which marked the official start of developing Lancang-Mekong international navigation. In May and October 1990, experts from China and Laos inspected the 701-km channel from Jinghong to Luang Prabang and the 1,177-km channel from Jinghong to Vientiane, the capital of Laos. Based on extensive investigation, China and Laos carried out the cross-border shipping trial in 1991. Later from January to May in 1993, China, Laos, Myanmar, and Thailand jointly inspected the boundary pile No. 234 between China and Myanmar—the 361-km section in Bankelong, Thailand, and gave a inspection report on Mekong navigation (Wang, 2001). At the same time, China approved Simao Port and Jinghong Port as national first-class ports (Zhao, 2019). China and Laos signed the “Agreement on Passenger and Cargo Shipment on the Lancang-Mekong River” in 1994, and later China and Myanmar also signed the agreement in 1997. In 1995, the “Agreement on the Cooperation for the Sustainable Development of the Mekong River Basin” between Laos and Thailand provided theoretical and political basis for the development of container traffic in the Lancang-Mekong River Basin. In 2000, China, Laos, Myanmar, and Thailand signed the “Agreement on Commercial Navigation on the Lancang-Mekong River” and carried out cross-border shipping trials, with volume increasing from 500 tons per year to 200,000 tons per year. Based on the agreement, the four countries signed the “Memorandum of Understanding of the Implementation of the Quadripartite Agreement on Commercial Navigation on the Lancang-Mekong River”, formulated eight technical supporting rules, including the “Guidelines on the Maintenance and Improvement of the Navigability of the Lancang-Mekong River”, and established the Joint Committee on Coordination of Commercial Navigation on the Lancang-Mekong River. Myanmar also opened Sole Port in 2001. The official navigation of the Lancang-Mekong River marks the official entry of international shipping into the fast lane of development (Li & Xiao, 2019).

With the success of the maiden voyage on the Lancang-Mekong River, China, Laos, Myanmar, and Thailand formally opened channels for navigation in 2001. From 2002 to 2004, the four countries jointly improved the upper Mekong waterway, making sure the upper Mekong basin was navigable all year around. The improvement also promoted the rapid growth of transnational traffic and guaranteed the safe transportation of 200–300 t cargo ships. In Laos, a natural waterway with a total length of about 300 km from Houayxay to Luang Prabang and a river channel of about 459 km from Vientiane to Savannakhet were built, and the channel from Luang Prabang to Vientiane was approximately 476 km long. In 2002, Thailand opened the Chiang Saen Port, and the total volume of cargo in and out reached 500,000 tons per year. The construction of the class V waterway of the Lancang River was completed in 2006. Later in 2007, the Lancang-Mekong international navigation was open all year around, with an annual shipping volume of over 396,000 tons, which marked a relatively complete shipping system of Lancang-Mekong River. In 2011, China, Laos, Myanmar, and Thailand held the 10th Meeting of the Joint Committee on Coordination of Commercial Navigation on Lancang-Mekong River and discussed the maintenance and improvement of the waterway, the navigation safety, and the implementation of the “Charge Rules”. In 2012, Myanmar carried out the construction of a 31-km class V waterway on the Lancang River at the China-Myanmar boundary. Myanmar set up a company with overseas partners to open up the shipping market in 2013. In 2015, Laos built power stations, including the Nam Mae Lai hydro-power station Unit 2, to generate electricity. Later in the same year, China, Laos, Myanmar, and Thailand passed the “Development Plan of International Navigation on the Lancang-Mekong River (2015–2025)”, which aimed to enhance the overall shipping capacity through cooperation. In 2018, Thailand opened the channel for transporting frozen goods from Guanlei Port to the new Chiang Saen Port, and in the same year, the overall planning and design of waterway transportation in the Mekong River Basin was completed. The development of navigation has deepened win–win cooperation, strengthened regional economic exchanges and tourism development, promoted regional prosperity, and increased the number of ships, the types of cargo, and the total volume of foreign trade among China, Laos, Myanmar and Thailand. The comprehensive utilisation of water resources has brought benefits to the countries and improved regional transportation networks in South Asia and Southeast Asia.

Cambodia, Laos, Vietnam, and Thailand signed the “Agreement on the Cooperation for the Sustainable Development of the Mekong River Basin” and established unified coordination and comprehensive management agency—MRCon April 5, 1995. Later, in 1996, China and Myanmar became dialogue partners of the MRC, and the MRC was the only water resources management organisation that China participated in then. As a dialogue partner of the MRC, China has extensively exchanged experience, planned technical training, and organised field visits with member countries (Hao, 2018). The MRC has already become an agency for regional cooperation and international coordination for 12 development projects in the Lower Mekong River Basin, including water resources utilisation, hydropower development, agricultural irrigation, and navigation. Article 9 of the “Agreement on the Cooperation for the Sustainable Development of the Mekong River Basin” grants the freedom of navigation on the mainstream of the Mekong River and stipulates that “on the basis of equality of right, freedom of navigation shall be accorded throughout the mainstream of the Mekong River without regard to the territorial boundaries, for transportation and communication to promote regional cooperation and to satisfactorily implement projects.” In order to promote the development of navigation on the mainstream of the Mekong River, the MRC issued the “Navigation Strategy” in 2003, the “Navigation Programme (NAP) 2013–2015” in 2012, and the first “Dangerous Goods Management Manual (DGGM)” in 2013 (Li, 2017).

To promote the development of international navigation in the LMB, the member states of the MRC have signed a series of bilateral and multilateral agreements, including the “Hanoi Agreement between Cambodia and Viet Nam on Waterway Transportation” on December 13, 1998, the “Agreement among the Lao PDR, Thailand and Viet Nam for Facilitation of Cross-Border Transport of Goods and People” on November 26, 1999, “Phnom Penh Agreement between Cambodia and Vietnam on the Transit of Goods” on September 7, 2000, and the “New Agreement on Waterway Transportation between Vietnam and Cambodia” on December 17, 2009. Although China is not a member of the MRC, it has also actively participated in and promoted the development and cooperation of international navigation on the Mekong River. The agreements it has signed with Mekong countries include the “Agreement between China and Lao PDR on Freight and Passenger Transport along the Lancang-Mekong River” in November 1994 and the “Agreement on Commercial Navigation on the Lancang–Mekong River among the governments of China, Laos, Myanmar and Thailand” on April 20, 2000. China, Laos, Myanmar and Thailand also agreed to establish the Joint Committee on Coordination of Commercial Navigation on Lancang-Mekong River to negotiate and resolve international navigation issues (Chen & Liao, 2008). In November 2014, China, Laos, Myanmar, and Thailand polished and finalised the “Development Plan of International Navigation on the Lancang-Mekong River”. The four countries reached an important consensus on the second phase of improving the navigation channel in the Mekong River and stated that by 2025, 890 km of the river from the Nandeba Area of Simao Port to Luang Prabang of Laos would be built and upgraded for vessels up to 500 DWT. Some passenger and cargo ports will also be built along the channel (Meng & Liu, 2015). The four countries also agreed on the preliminary work of the second phase of improving the navigation channel in the Mekong River, covering a 631-km channel from the China-Myanmar Boundary to Luang Prabang in Laos in September 2015. The work included channel improvement, port construction, and the building of a support and security system (Zhang et al., 2016).

China has proposed cooperation initiatives such as the Belt and Road Initiative and the construction of the China-Indochina Peninsula Economic Corridor, which safeguards the development of Lancang-Mekong international navigation. Moreover, a series of international cooperation funds, such as the Silk Road Fund, China-ASEAN Maritime Cooperation Fund, Asia Regional Cooperation Special Fund, and Lancang-Mekong Special Cooperation Fund, also provide favorable financial options for its development. China and ASEAN have formed a new cooperation pattern in 9 areas, including bilateral, sub-regional, and “10 + 1” relationships. China has already developed comprehensive strategic partnerships with Indonesia, Vietnam, Laos, Cambodia, Myanmar, Thailand, and Malaysia, and has continued to promote political mutual trust, economic cooperation, and people-to-people exchanges, which has laid a solid foundation for the development of Lancang-Mekong international navigation.

With the signing of the Treaty of Amity and Cooperation (TAC) in Southeast Asia, Thailand has established strategic partnerships with other ASEAN countries. Open economies in the Mekong basin have formulated a ten-year development strategy and a sub-regional action plan for tourism. Thailand has seized the historical opportunity, and actively opened up a new phase in the development of Lancang-Mekong navigation by cooperating with other countries. The Golden Triangle Tourism Cooperation Plan has been passed, which combines the development of navigation with tourism and other related industries, and boosts the international tourism industry. In addition, Vietnam has also established many tourism programmes through navigation, such as the development of tourism in the triangle area of Cambodia-Laos-Vietnam and the Red River Delta.

The cooperation on cross-border shipping has been set up. The North–South Economic Corridor, the East–West Economic Corridor and the Southern Coastal Economic Corridor have helped Vietnam maximise its economic benefits from transportation hubs and promoted trade and investment among different regions in the area. On the East–West Economic Corridor, Vietnam, Thailand and Laos have signed an agreement to open a route connecting three capital cities and two large ports: Laem Chabang Port and Hai Phong Port.

The “navigable section of the Mekong River runs from Gongguo Bridge in Yunnan Province to the estuary of the river, with a total distance of 3646 km, passing through more than 100 towns and 20 cities including Vientiane and Phnom Penh.” The Lancang-Mekong international navigation enables the transport of large amounts of copper, gold, silver mines, abundant oil and natural gas in Myanmar, as well as 10 billion tons of potash and forest resources in Laos. The international navigation will boost the economic development of Mekong countries, which have all set the development of the Lancang-Mekong navigation as a priority for regional economic cooperation.

In addition, based on some analysis reports on the economic impact of ports, it can be found that every 10,000 tons of throughput contributes 1.1 million yuan to a country's GDP and creates 20 jobs. The development provides more employment opportunities for people in the basin, which does not only accelerate the development of the regional economy, but also increase the living standard of local people.

Regions around the Lancang-Mekong River Basin are relatively underdeveloped in economy and navigation. The economic development mainly relies on agriculture, which is not beneficial for the development of navigation. Besides, the upper Mekong Basin is mountainous, with turbulent water flow. There are numerous shoals and reefs in navigable channels and the hydrological conditions are complicated, which requires a large sum of money for upgrading. There is a little infrastructure of ports and wharves that are open for navigation, and most of the channels are not managed or maintained.

The governance capabilities and governance systems of some Mekong countries are relatively backward. Apart from China and Thailand, which have strict rules and complete institutional mechanisms, the public governance capabilities and the service systems of Laos and Myanmar are relatively underdeveloped. There is no independent department responsible for navigation in Laos, and so it relies on the relevant departments of the central government to manage the situation “remotely and indirectly”. The laws, regulations, and rules for the management of international navigation are incomplete, and at the same time, the human and material resources are insufficient. In Myanmar, the political situation is unstable and the whole country is in the hands of three parties, including the central government, the military, and the armed ethnic minorities. The fourth special zone of Shan State in the North of Myanmar is controlled by armed ethnic minorities. The central government’s inability to take over has led to severe illegal smuggling and environmental damage in the region. Although the Wan Pong Port is under the management of the central government, it does not have a sufficient supply of goods.

Based on the volume of passenger traffic provided by the four countries from 2007 to 2017, the current status of development, and the future economic trends, the freight volume and the passenger volume are expected to reach 1.0305 million tons and 3.0533 million respectively in 2025; the numbers are expected to increase to 1.8487 million tons and 4.1856 million respectively in 2035. Both can see gradual development.

The ports of the four countries are in different development stages. In Laos, the infrastructure construction of ports along the river will continue to maintain the current status, and the situation is not likely to be improved unless there is external investment. According to the Ministry of Public Works and Transportation of Laos, 10 ships will be reduced each year. Laos will try to maintain the momentum and demand for upgrading port facilities, but the situation may go south. In Myanmar, the ports directly managed by the central government will benefit from government policies and funds, and the infrastructure will be improved to build the connection between Myanmar and China. In Thailand, the government will build and develop special economic zones (SEZ) in Chiang Saen, Chiang Khong and other areas to improve port facilities and services. It will give full play to logistics and trade, increase the efficiency and convenience of delivery, and promote regional economic development. China will consolidate the construction of infrastructure along the Lancang-Mekong River, and work with neighboring countries to facilitate international navigation. It will also work with Laos, Myanmar and Thailand to build a major international transportation channel connecting upper and lower Mekong countries (Li & Xiao, 2019).

In recent years, there has been an increase in the construction of hydropower stations in major river systems in China’s Yunnan Province. The new aquaculture areas in the reservoir provide favourable conditions for cage culture and fence aquaculture (Qiu & Du, 2008). Each year, the central government, the government of Yunnan Province, and the construction company of hydropower stations will invest in stock enhancement in the Lancang River Basin. Xishuangbanna Prefecture had nine fisheries enhancement in the Lancang River and its main tributaries, including the Luosuo River, the Puwen River, the Daluo River, and the Nanrun River in 2015. These practices strengthened the supervision and management of implementing the compensation of hydropower projects for aquatic organisms in China and guaranteed that the projects such as fish catch over dams, artificial spawning grounds, fish hatcheries, and large-scale stock enhancements will be implemented as required to minimise the impact of dam construction on fish. The total fishery production in the basin was 41.96 \(\times\) 10⁴ t in 2016. The fish farms covered an area of 3.66 \(\times\) 10⁴ ha, and the aquaculture output was 36.51 \(\times\) 10⁴ t, accounting for 87% of the total fishery production. The output of capture fisheries was 5.45 \(\times\) 10⁴ t, accounting for 13% of the total (Yunnan Department of Agriculture & Rural Affairs, 2016).

The Lower Mekong River Basin (LMB) has the world’s largest inland captured fisheries. In 2015, Laos's total fishery production was 15.86 \(\times\) 10⁴ t, and the aquaculture output was 9.6 \(\times\) 10⁴ t, accounting for 60% of the total (Lao Statistics Bureau, 2016). The output of capture fisheries was 6.61 \(\times\) 10⁴ t, accounting for 40% of the total. Around 594,000 households in Laos were engaged in fisheries, 68,000 of which worked in aquaculture and 526,000 of which worked in capture fisheries (Lao Agricultural Census Office, 2012). Fisheries produced 50% intake of protein for local people. The aquaculture and capture fisheries in the Laos basin covered an area of 79.17 \(\times\) 10⁴ ha, 7.2 \(\times\) 10⁴ ha of which was used for aquaculture. Capture fisheries included Nam Ngum, Nam Theun No. 2 reservoirs, small wetlands, irrigation reservoirs, and weirs, taking up 17.48 \(\times\) 10⁴ ha (Phonvisay, 2013).

Cambodia has the richest fishery resources among the Mekong countries. The Tonle Sap Lake, the Mekong River, and the Tonle Sap River are the country's natural freshwater fisheries. Therefore, the development of fisheries is regarded as the priority of the Mekong River's water resources development in Cambodia. The Tonle Sap Lake has a unique flood pulse system, and it is the most productive freshwater fishery around the world. Before 2000, the fisheries management of Tonle Sap Lake had been following French colonists’ management, restricting the area for commercial fisheries and stipulating commercial fishing seasons. As a result, it greatly limited the fishing area of fishermen in the community. However, the fishery policy has changed since 2000. The Cambodian government has cancelled all commercial fisheries and divided fish protection areas and community fishing areas. Under the supervision of the township government council, a fishermen management committee has been established to formulate management rules. By 2013, 516 community fisheries had been established, and many farmers in the community had actively participated in capturing fish (Lv & Wang, 2021). In 2016, the total output of freshwater fisheries in Cambodia was 68.42 \(\times\) 10⁴ t, 51.51 \(\times\) 10⁴ t of which came from the output of capture fisheries, accounting for 75% of the total, while the output of aquaculture was 16.91 \(\times\) 10⁴ t, accounting for 25% of the total. About 530,000 households in Cambodia were engaged in capture fisheries and aquaculture, 460,000 of which worked in capture fisheries, and 80,000 of which worked in aquaculture (Cambodia National Institute of Statistics, 2015).

In the Mekong River's Vietnam basin, aquaculture took up an area of 78.51 \(\times\) 10⁴ ha, most of which was in the Mekong Delta, covering an area of 77.13 \(\times\) 10⁴ ha. There was only a little aquaculture in the central highlands, which covered 1.38 \(\times\) 10⁴ ha. The fishery production totalled 304.72 \(\times\) 10⁴ t in 2016, with 84% aquaculture and 16% capture fisheries. In the same year, the total fishery production in the Vietnamese Mekong Delta region was 300.78 \(\times\) 10⁴ t, 253.66 \(\times\) 10⁴ t of which was from aquaculture, and 47.12 \(\times\) 10⁴ t was from capture fisheries. Aquaculture in the Mekong River played an essential role in Vietnam in 2016, accounting for 70% of the national aquaculture output (Vietnam General Statistics Office, 2018). By contrast, there was only 3.94 \(\times\) 10⁴ t of fishery production in the central highlands.

In Thailand, the output of capture fisheries in the Mekong River Basin was 90 \(\times\) 10⁴ t, 70 \(\times\) 10⁴ t of which was from fish, and 20 \(\times\) 10⁴ t of which was from other aquatic organisms. The output of aquaculture was 6.19 \(\times\) 10⁴ t (Mahasarakarm, 2007), and the area for aquaculture in the northeast took up 1.88 \(\times\) 10⁴ ha, 1.11 \(\times\) 10⁴ ha of which was used to produce fishery products for household consumption, and 0.77 \(\times\) 10⁴ ha of which was used to produce fishery products for sale. 88,000 families worked in freshwater aquaculture, accounting for 3.2% of the area's total agricultural population (Thailand National Statistical Office, 2014).

Comparing the differences in fishery development, Vietnam has the highest fishery production among the five Mekong countries, accounting for 58% of the total in the basin, and it mainly focuses on aquaculture and commercialised fishery products. Thailand ranks second in fishery production in the basin, accounting for 18% of the total. However, Thailand mainly relies on capture fisheries, so most of its fishery production is consumed by local people. Cambodia ranks third, accounting for 13% of the total in the basin. Since it mainly depends on capture fisheries, the production can barely meet its own needs, and sometimes it even has to import from other countries. China's fishery output ranks fourth, accounting for 8% of the total in the basin. Aquaculture accounts for most of the output, and most fishery production is commercialised. Laos has the lowest fishery output, accounting for only 3% of the total in the basin. The proportion of aquaculture and capture fisheries output is similar to Cambodia. The production can barely meet its own needs, and sometimes it has to import from other countries. In general, Vietnam has taken full advantage of fishery resources in the basin, followed by Thailand, Cambodia, China, and Laos (Wang, 2019).

The LMB cooperation of sustainable development is based on the Mekong River Basin Sustainable Development Agreement signed by Laos, Thailand, Cambodia, and Vietnam. The unified management agency is MRC, which was established in 1995. The MRC abides by the agreement of sustainable development and formulates plans and management of the Mekong fisheries. Although fishery products are the primary source of protein for people in the basin, the industry has not received much attention due to its considerable input and low output. Mekong countries have different needs for the development of fishery resources due to their different geographical locations and economic development, and thus very little cooperation in fisheries has been carried out among Mekong countries. Moreover, the adverse impact of hydropower development on fisheries is a concern for lower Mekong countries, so most cooperation was seen in 2015 between China and Laos. The two countries signed an agreement on protecting fishery resources and worked together on stock enhancement and ecological conservation. As two major hydropower developers, China and Laos’s cooperation shows respect for other countries in the basin (Yu, 2017). In addition, the technical advisory agency of the Lower Mekong fisheries management, which was established in 2000, formulated the 2018–2022 Mekong fisheries management system, including “the Mekong Basin-wide Fisheries Management and Development Strategy (BFMS) 2018–2022”. This regional strategy focuses on inland capture fisheries and prioritises the sustainable use and protection of fish resources.

At present, the Mekong River's freshwater ecosystems (lakes and rivers) are being severely impacted. The aquatic organisms, especially fish resources, face severe challenges, including overfishing, large-scale hydropower development, invasion of alien species, and water pollution. The entire basin has shown a trend of a significant decline in fish species and population. Some species are endangered, and some have even become extinct (Liu et al., 2008).

The most negative impact is the construction of hydropower dams, which results in the block of the upstream channel of spawning, the death of downstream broodstock after breeding, and the deaths of juvenile fish when they go down through the dam, or when they are in the downstream of the dam and in the hydropower system. During the slow migration and periodic entry into the estuary, the continuous fatal impact on the juveniles when they pass through dams will change the original growth pattern of juveniles in the reservoir. The Mekong hydropower dams have become an insurmountable obstacle for the species that need to migrate to complete their life cycle, thus seriously changing and destroying the habitat of fish, along with their migration and reproduction cycle. The populations of short-distance and long-distance migratory fishes have declined, leading to destroyed biodiversity. In addition, the impoundment of the dam has also increased the nutrients in the submerged zone, which would affect water productivity. The construction of water conservancy and hydropower projects has changed the river's hydrological conditions to a certain extent, affecting and destroying the conditions for migration, habitat, feeding, and reproduction of some species (Ferguson & Xu, 2012). The development of hydropower will not only adversely affect the integrity of the Mekong ecosystem but also threaten the economic income, sources of nutrition, and social benefits brought by it. The LMB has the world's largest inland fisheries, with the total fish catch estimated at 3 million tons, 80% of which comes from capture fisheries, and the annual wholesale transaction volume is about US$20–30 billion. According to a report by the International Center for Environmental Management (ICEM) in 2010, the construction of dams on the mainstream of the Mekong River led directly to US$4.76 billion economic losses for fisheries. Moreover, the hindrance of fish migration led to a decline in fish production, which was undoubtedly a heavy blow to the residents of the Mekong River who relied on traditional fishing for their livelihoods and the fragile inland fishery economies, such as Laos, Cambodia, and Vietnam, pushing local low-income families further to extreme poverty (Wei & Zhang, 2015).

In addition, due to the unique geographic location and characteristics of the Lancang-Mekong River Basin, there are incredibly diverse species of fish. Therefore, it is a great challenge to fully understand the diversity and distribution of fish in the basin and explore the variation of diversity and its attribution on the basin scale (Li et al., 2019). Apart from the difficulties in obtaining information, the existing cooperation mechanisms lack sufficient political mutual trust. Several mechanisms are overlapping, and the laws are incomplete. Moreover, the intervention of countries outside the basin has made the initially complex cooperation in the basin more complicated and increased the uncertainty of cooperation (Yang, 2017a, 2017b). The fishery group under the MRC is mainly responsible for organising scientific investigations, information sharing of fishery resources, improving fisheries management skills and capacity in communities, and coordinating and managing fisheries among countries. However, the people in the group have not done their best in their work, and many scientific management methods and close joint-management mechanisms have not yet been established.

To address the current and emerging fisheries management issues, riparian governments have focused their policy priorities on improving productive capacities, protection and conservation of critical habitats and resource enhancement, modernisation of the traditional systems of extensive resource use and their equipment and techniques, fostering of community-based approaches, and promotion of the shift from subsistence to commercial production by professional fishers and fish farmers producing for the market. To complement the national measures, the MRC strives to foster regional efforts towards sustainable management and development of the Mekong fisheries, including through the sharing of technical know-how on fisheries management, raising awareness on the sector’s significance for the Mekong’s environment and its people, and promoting an integrated approach with other sectors. The MRC has also developed monitoring programmes and technical guidelines to track fisheries’ status and trends and promote participatory fisheries management or co-management by members of fisheries agencies and user communities, leading to better management results and sustainable use of fisheries resources. Under the MRC SP 2021–2025, through which the Basin Development Strategy (BDS) 2021–2030 will be implemented, the MRC utilises its fisheries expertise to support the member countries via its regional Expert Group on Environmental Management in managing risks to food security from excessive pressure on fish stocks and in improving understanding of the gender and vulnerability aspects of basin-wide fisheries management (MRC, 2017).

Environmental flow describes the quantity, time, and quality of water flows required to sustain freshwater and estuarine ecosystems and the livelihoods and well-being of the people who rely on them, which is also known as ecological water demand, environmental water demand or ecological water demand in watercourse (Dong et al., 2020; Pastor et al., 2014). It is the core problem of sustainable water resources management in the river basin to scientifically estimate and sustain the environmental flow.

Significantly affected by tropical monsoon climate, the precipitation distribution through a year in the basin is quite uneven. About 80% of the precipitation and 75% of the annual runoff occur during the rainy season (Liu et al., 2020a, 2020b). Influenced by distinct wet and dry climate, climate change and El Nino, floods or droughts of varying degrees frequently happened in LMRB (Chen et al., 2020). Since the 1980s, the drought and flood disasters have occurred frequently in the LMRB due to the interaction of multiple monsoon systems (Hundertmark, 2010). In this context, the security of environmental flow has become the major ecohydrological problem confronted by LMRB.

Over the past 30 years, LMRB basin has had 4 severe droughts, one of which was in 1991–1994, it was a period longer than 38 months. The driest one was in 2015–2016 with a drought area of up to 75.6% (Guo et al., 2017). From the end of 2014 to 2016, countries around the basin suffered drought of varying degrees affected by the El Nino phenomenon. Back then, the water level of the Mekong River dropped to the lowest in nearly 90 years and a large amount of salt water invaded the Mekong Delta (Larson, 2016). It is generally of long duration and high intensity the droughts appeared in the Delta area (Guo et al., 2017). The droughts can result in unprotected environmental flows, impairing the irrigated agriculture, destructing the river ecological balance, and polluting the (drinking) water.

A special flood adaptive ecosystem has been created by the frequent floods. For the past few years, a great number of dams have been constructed along the basin, effectively decreasing the flood risk. However, the yearly flood pulse of the Mekong River has been altered, compromising the ecological system that is sensitive to the flood pulse. The drop of flood pulse amplitude is estimated to reduce the transportation of silt and nutrient substances and exert negative influence on aquatic habitats that depend on large seasonal water level fluctuations (Matti & Juha, 2008). Furthermore, the yearly pulse change of flood and short-term water level change also affect the agricultural activities on flood plains and riverbanks, for instance, the Tonle Sap River, Tonle Sap Lake and its floodplains, Mekong Delta and so on (Arias et al., 2012; Binh et al., 2020; Matti & Juha, 2008). Studies show that the flood peak will be delayed by one month if the flood pulse reduces 50%, which would prevent the Mekong River from flowing into Tonle Sap Lake (Pokhrel et al., 2018).

In general, the water quality in the basin is good. From the perspective of space, in the upstream is better than in the downstream and the in main stream is better than in the tributary.

The basin is naturally advantageous in geographical location with abundant mineral resources. With the exploitation of nonferrous metal ore, the surface heavy metal elements after exploitation, were washed into the river by precipitation, leading to the increase of heavy metal concentration in the river sediment. The majority of the middle and lower reaches of the Lancang River have good water quality, but some reaches (near Gongguoqiao) have enrichment coefficient of heavy metal elements such as Cu, Pb and Zn more than 2, representing mild or moderate pollution (Zhang et al., 2014).

Domestic sewage, industrial waste water and residual nitrogen and phosphorus from farmland drainage are the key factors of the basin chemical pollution. The average annual total phosphorus content in the Langcang River Basin is 1.6 × 10⁴ to 3.9 × 10⁴ tons, among them, soil erosion accounts for 60%. Influenced by agricultural production and other human activities, the spatial distribution of phosphorus content shows an increase trend from the upstream to the downstream (Zhang et al., 2020). In three sub-basins of the Mun River, the tributary of the Mekong, the maximum parameter concentration of NH3-N, fecal coliform bacteria (FCB) and colibacillus have exceeded the surface water quality standards permitted by the Pollution Control Department (PCD) of Thailand (Yadav et al., 2019). Furthermore, the invasion of large amounts of suspended solids and silt has further made the increase of the total phosphorus content in water (MRC, 2018).

With economic and demographical development, infrastructure construction and sustainable increasing of mining and energy demands, large quantities of factory sewage and domestic waste water are discharged in to the rivers in the basin, which make the water pollution more severe and, as a result, the water quality shows a deteriorating trend (Chea et al., 2016; Su et al., 2011).

While the erosion is strengthening in the basin, the sediment transportation to the delta is decreasing because of land use and coverage change, dam construction and flow regime change.

Over the past 50 years, with the rapid development of social economy, demographic growth and land use change, the average erosion ration in the basin has reached 5000 t/km²/year (Anthony et al., 2015), which is the medium erosion level. In addition, the erosion intensity is now increasing due to the change of climate and land use (Chuenchum et al., 2020). The sediments generated during the erosion process make the river turbidity of the Lancang-Mekong River higher. On the other hand, in the Mekong Delta, the amount of sediment inflow is decreasing and some shorelines are eroding at a rate of 50 m per year (Marchesiello et al., 2019). Since 2015, the sandbanks downstream has obviously shrunk. The direct factor for such a phenomenon is the increased erosion force and decrease of sediment inflow, which is mainly because of flow velocity increasing and shear runoff stress, as well as human activities such as dam building, sand mining and urbanisation (Guo et al., 2020).

As the increase of sand demands in foreign market, a large gravel export business has begun to exploit the sediment in the Mekong basin. In 2011, Lao People's Democratic Republic, Thailand, Cambodia and Vietnam exploited 34.48–55.20 million tons (density is dry 1.6 tons per cubic meter) of sediment in the Mekong mainstream, among which, Cambodia was the largest one for such extraction in 2011–2012 (60%), followed by Vietnam (22%) and Thailand (13%) (Bravard et al., 2013). As the sand mining amount is greater than the sediment flux of the river channel, the elevation of the riverbed will decrease, making the riverbank less stable (Hackney et al., 2020; Jordan et al., 2019). According to the Mekong River Commission Report 2018, the sediment load flowing into the Mekong Delta was 143 million tons in 2007, however, it is predicted to be less than 500 million by 2040 due to the influence of the main stem and tributary water conservancy projects (MRC, 2018). The Mekong Delta is the third largest delta plain in the world and sediment has brought in abundant nutrients and huge economic benefits. This development will certainly compromise the production of fishery and planting industries in the delta, as well as the navigation, hydroelectric power generation and ecological security in the basin (Hou et al., 2020).

The Lancang-Mekong River is one of three rivers with the highest freshwater biodiversity in the world. According to the estimation, there are at least 890 freshwater fish (Rainboth et al., 2012), which is only second to the Amazon. Dams that have been built and are being planned in the river basin have changed the eco-hydrological conditions by regulating the natural flow of the river, which may significantly affect the fisheries development, wildlife habitat and biodiversity. These dams will affect more than 50 species of fish and dozens of millions of individuals which spawn and migrate in the basin. Fish migration may be disrupted and they may be trapped in the dams at larval stage, making impact on eggs and larvae, which flow downstream to sustain fisheries replenishment. The mitigation technologies, such as existing fishway, fish lock and fish hoist are incapable of coping with the large biomass of fish migratory scale (Dugan et al., 2010). A total of 78 dams that have been built in the tributary which will immensely reduce fish reproduction and biodiversity. Among these, Lower Se San 2 has had the greatest impact on fish biomass (LSS2, the fish biomass declined 9.3% in the river basin), followed by Se Kong 3d (2.3%), Se Kong 3u (0.9%) and Se Kong 4 (0.75%). By 2030, the additional biomass loss caused by a further 27 dams planned on the tributary will be about 39% (Ziv et al., 2012). A total of 13 species of fish in the Lancang River system are listed in the China Red Book of Endangered Animals (Fish), accounting for 14.13% of the total number of endangered fish (Kang & He, 2007). In the reservoir area above the dam, exotic fish, such as cyprinidae, Taihu new whitebait fish, etc. have been imported and the changes of water environment are more suitable to the dispersal and reproduction there (Havel et al., 2005), in this way, they can be propagated rapidly. In the 1960s, the production of fishing (indigenous) fish was 7 times that of farmed (exotic) fish. In the 1980s and 1990s, it was quite the opposite, the natural fish production was only one-seventh that of farmed fish, and the indigenous fish were under severe threat (He et al., 2007). During the process of dam impoundment and operation, the elimination of native fish and introduction of exotic fish will make the fish trophic population change.

The natural longitudinal connectivity of a river is a corridor for the flow of energy, material, and fish from upstream and downstream (Santos et al., 2006). Fish population structure along these longitudinal gradients is a good indicator of overall river ecosystem health. The dams, as the obstacles, destroy the longitudinal connectivity of fish assemblages and affect free migration of fish along the rivers (Schiemer, 2000). Studies revealed that the biological integrity index F-IBI of fish in the main stream was 0.41 km⁻¹and that in the tributaries was 0.17 km⁻¹ before the operation of Xiaowan Dam, reflecting the diversity of habitats and fish fauna. However, the values have respectively dropped to 0.01 and 0.09 km⁻¹ afterwards, indicating the habitat and fish fauna homogenisation (Li et al., 2013). After the completion of Manwan dam, the diatom in phytoplankton dominated but the total number of species significantly decreased. The phytoplankton species decreased from 41 to 35, in particular, in the alluvial zone under the dam. With the construction and operation of Xiaowan dam upstream, phytoplankton communities in Manwan Reservoir and downstream areas quickly responded to further changes in the environment. During the construction of Xiaowan dam, in the alluvial zone of Manwan Reservoir, the abundance of phytoplankton decreased from 1.54 × 105 ind./L to 0.05 × 105 ind./L, and in the static water zone, it decreased from 9.34 × 105 ind./L to 0.10 × 105 ind./L (Fan et al., 2015).

Underground water is a crucial resource of the Mekong river. According to estimates, at the time of writing, there are over 1 million wells in the Mekong River Delta region shared by Cambodia and Vietnam for the exploitation of groundwater resources so as to meet local production and living needs (Erban et al., 2013). The global groundwater data from the International Groundwater Resources Assessment Centre (IGRAC) directory indicate that basin groundwater withdrawals are about 550 million m³/year (Ho et al., 2019), which is distinctly higher than that in the1960s. Currently, the dependence on underground water is increasing, especially in Cambodia and Thailand, and this water has become the major source of drinking water supply (Ho et al., 2019). Studies indicate that the excessive exploitation of underground water has made a large part of the Vietnamese side of the Mekong River Delta subside (Erban et al., 2014). The average descending range during 2002–2017 was up to 18 cm (Minderhoud et al., 2017). In some of the delta areas, there are problems, such as the combined effects of rising sea levels due to climate change, depletion of aquifers, deterioration of water quality and intrusion of salt water (Erban et al., 2014; Hamer et al., 2020; Smajgl et al., 2015). Furthermore, the climate change has altered the precipitation and temperature across the basin, the resultant change of underground water supply mode and the downstream flood pulse will affect the underground water system in delta region (Smajgl et al., 2015).

From the perspective of water ecological protection and management in the basin, it is vital to scientifically estimate, safeguard and regulate the ecological environmental water demand. According to the classification coefficient of ecological environmental water demand (GCEWR) and ecological runoff (ER), the minimum, the suitable and optimal environmental flows of the Lancang river channel are respectively 142.53 × 10⁸ m³, 286.46 × 10⁸ m³ and 385.96 × 10⁸ m³, separately accounting for 18.63%, 37.45% and 50.45% of the natural runoff (765 × 10⁸ m³) (Hu et al., 2009). The ecological flow management should also sustain the aquatic habitat. A case study of Chinese knotting fish, the highest priority fish in the middle and lower reaches of the Lancang River, the optimal ecological flow during spawning period and adult period is 2100 m³/s and 1200 m³/s, respectively (Peng et al., 2020).

The building of reservoirs can substantially regulate the environmental flow of the downstream in the basin and effectively improve the guarantee rate of environmental flow during the dry season. There are six dams in the upstream of the main stem of the Lancang River. They can reduce flood peak discharge and reduce sediment flux in China (Fan et al., 2015). Meanwhile, they can effectively regulate the volume of runoff in the basin (Rasanen et al., 2012; Tang et al., 2014a, 2014b; Yun et al., 2020a, 2020b). The construction of these dams has positive impact for the ecohydrological processes in the basin. The operation of upstream reservoirs can release the water stored in the rainy season to mitigate the shortage of water in the dry season, ensuring its environmental flow and promoting the stable development of agricultural and ecological system downstream in the Mekong delta (Kondolf et al., 2018). In face of the severe drought in Southeast Asia at the end of 2015, water was continuously released through the joint operation of reservoirs, which greatly mitigated the drought in the downstream countries (Zeng, 2017). Compared with the long-term average level, the drought had reduced the flow by 16%. Nevertheless, due to the urgent discharge of Chinese dams, the flow in the dry season has increased, Ultimately, it was conducive to alleviating the potential effects of the drought. During the period from March to May, 2016, Jinghong reservoir discharged 12.65 billion cubic meters in total, which was equivalent to 40–89% of the flow in the Mekong River's various reaches. Emergency water replenishment increased the water level or flow along the mainstream of the Mekong River by 0.18 –1.53 m, which was 602–1010 m³/s. Without these emergency discharges, the flow of Jinghong would have been reduced by 47%, Qingsheng by 44%, Langkai by 38% and Shangding by 22%. In addition, the emergency discharge eased the salt water intrusion to the Mekong Delta.

In terms of the hydrological and ecological issues in the basin, the countries and organisations along the basin strengthen the monitoring to deal with problems of water quality and underground water. In 1985, in an effort to maintain the proper level of water quality in the entire Mekong River Basin, the Mekong River Commission established water quality monitoring networks in the Lao People's Democratic Republic, Thailand and Vietnam (Cambodia joined in 1993) to detect the water quality change for necessary measures. So far, there have been 132 monitoring points in total, 33 of which are monitoring sites established in the 2000s and 99 old points established before 1995 (Chea et al., 2016). Since 2010, MRC and its member countries have routinely monitored the water quality of the Mekong River and its major tributaries on a monthly basis by measuring 19 water quality parameters at 48 stations, 17 of which are in the Mekong River, 5 stations in the Bassac River, and 26 stations on tributaries of the Mekong River (https://​www.​mrcmekong.​org/​our-work/​functions/​basin-monitoring/​water-quality-monitoring/​). The monitoring has effectively supported water quality management and early warning in the basin.

A series of measures have been taken to mitigate the impact of human activities and climate change on aquatic life in the basin. The critical tributaries have been identified and natural reserves have been established along the river so as to protect the habitat and native fish populations. Ecological reservoirs have been rebuilt and the ecological environment monitoring and evaluation system together with the fishery administration have been improved (Huang, 2013; Yuan, 2012), when planning the number and location of hydroelectric dams, appropriate removal of dams on major tributaries and enhancement of indigenous fish stocks. Furthermore, fish passage facilities have been established in dams along the main stream of the Lancang River, including fish hoist, fish lock and fish ladder (Huang, 2013). Germplasm conservation areas have been established in the main growth and breeding areas of cross-border fish germplasm resources with high economic value and genetic and breeding value, and corresponding management methods have been formulated to strengthen and standardise the management of the conservation areas. Secondly, a regional gene bank centre for the germplasm resources of cross-border migratory fish has been established to strengthen the protection of the genetic germplasm resources of migratory fish, in particular, the rare genetic germplasm resources of migratory fish, strengthening the relevant technical research and advancing the sustainable utilisation of resources (Liu et al., 2008). The ecology scheduling has been implemented in the dams and reservoirs operation management in order to ensure that the discharge of water can sustain the ecological function of the river and protect it from being damaged. Furthermore, the flood peak of artificial flood discharge in the flood season has been adopted based on the time of year when most fish in the basin migrate. Meanwhile, special attention has been paid to the flood process that is basically consistent with the natural conditions to meet the migratory fish demands. When conditions allow, layered discharge can be adopted to increase the drain of surface water, raise the temperature of drainage water, prolong the discharge time, reduce the discharge intensity and reduce the impact downstream (Chen et al., 2007). In a word, the ecology scheduling has been adopted to compensate the environmental demands of river ecosystems on the quantity, quality and temperature of water and keep the flood discharge similar to the natural flow state of rivers, minimising the stress on aquatic organisms caused by adverse environmental effects including low-temperature water and supersaturated gas. At last, fish hatcheries can be built to propagate and set free the native fish (Li et al., 2013). The international aid funds and talents can be utilised to build the endangered fish domestication and breeding base. Key technologies for the domestication and breeding of rare and endangered species can be explored and establish a system for artificial release of endangered fish (Liu et al., 2008).

The flow of water resources in the basin is transboundary. Thus cooperation of water resources management is a necessity to improve and solve the ecohydrological problems in the river basin (Liu et al., 2020a, 2020b). The Lancang-Mekong Cooperation (LM cooperation for short) mechanism established at the end of 2015 is the first regional cooperation initiated by the countries in the basin. Almost all aspects of basin management are included within the Lancang-Mekong cooperation mechanism, but there have been five priority directions: connectivity, capacity of production cooperation, cross-border economic cooperation, water resources cooperation, agriculture and poverty reduction cooperation.

Water resource cooperation, as one of the five priority development directions (connectivity, productive capacity, cross-border economic cooperation, water resources, agriculture and poverty alleviation), dedicates to realising the reasonable distribution, fair utilisation of water resources and sustainable economic development of countries in the basin (Feng et al., 2019). In August, 2020, the third leaders’ meeting of Lancang-Mekong cooperation was successfully held, promoting cooperation in the comprehensive development stage and sending a positive signal of solidarity, cooperation and common development among the six Lancang-Mekong countries. China will start to share annual hydrological information of the Lancang River with Mekong countries from the same year.

At the present stage, over 10 multilateral documents on cooperative river basin flood management have been signed in terms of flood disaster control for countries in the basin, involving identification and responsibility of flood disaster prevention and control, flood monitoring and early warning, and emergency response of flood control. The flood prevention and control work will be promoted comprehensively through multiple methods, such as, high-level government visits, expert exchanges and data and information sharing (Wu et al., 2020). The item 6 of The Five-Year Plan of Action for LM Cooperation (2018–2022) emphasised that the emergency management of flood and drought disasters in the Lancang-Mekong region should be strengthened, and a joint assessment of flood control and drought alleviation in the Mekong river basin should be implemented. China proactively deepened the cooperation on data sharing with MRC member countries, and signed the Steps on Data and Information Exchange and Sharing and the Memorandum of Understanding on the Provision of Daily River Flow and Rainfall Data at Two Monitoring Stations in Yunnan Province during the Rainy Season. In this way, water conservancy construction, flood control, drought relief in basin countries have been significantly advanced. In terms of irrigation, China responds to demands of downstream countries in real time and increases the discharge of the upstream reservoirs to satisfy the irrigation and ecological conservation needs of downstream countries.