In 1997, China closed outfall for four days in order to work on a dam that blocked the flow of a large amount of fresh water into the Mekong Delta and make Vietnam lost 100.000 USD per day.

Closure of Mekong Outfalls in China can Create Huge Losses for Vietnam
So far, the assessment for impact of the Mekong River water on Vietnam national security has been performed but it is not sufficient and inconclusive. In fact, in term of different aspects, there are variety of opinions about the impact of Mekong River water on Vietnam national security.
Therefore, in this article, the authors continue to research on effects of Mekong River water security on Vietnam national security on the following aspects:
The security of the Mekong River is being controlled as a “weapon” to dominate and influence international relations. The Mekong River is one of the 10 largest rivers in the world, with a length of 4.880km, a basin area of 795.000 km2, an total annual average flow of approximately 475 billion m3, an average annual flow of 15.000m3/s. 
The upstream of the Mekong River crosses China and Myanmar (accounting for 24% of basin area); downstream areas of Laos, Thailand, Cambodia and Vietnam (accounting for 76% of basin area). The Mekong River has a rich resource and biodiversity, ranking the second in the world (just behind the Mississippi River area), which is very convenient for economic development, especially irrigation, aquaculture and hydropower development. 
However, the current flow of the Mekong River is being regulated and controlled by the construction of hydroelectric dams on the mainstream and stributaries. So far, there are 19 hydropower dams which has been planning to construct on the mainstream of the Mekong River and China is the nation constructing the highest number of large hydropower dams with eight dams completed on the upstream and 4 dams on the plan; Laos and Cambodia make plan to construct 11 dams on the downstream. 
In June 2018, Laos implemented a plan to build the fourth hydropower plant named Pak Lay on the Mekong River (followed by Xayabury, Don Sahong, Pak Beng). Although China is the country which controls directly flow on the upstream of the Mekong River (China called the Lan Thuong River), it refuses to participate in the Mekong Agreement and believes that the water resources of the Mekong River flow through its territory is “internal water”, under its absolute sovereignty, so it has full rights for use and exploitation without consent or cooperation of the countries in the basin. 
In the downstream of the Mekong River in Laos and Cambodia, China also invested in the construction of hydropower dams on the mainstream, which has turned the river on the downstream into a reservoir and transformed the natural essence of the flow. Therefore, there has been a serious impact on the dry season in the Mekong Delta provinces.
Apart from construction of hydropower dams, the countries also adjust flow of the Mekong River to serve the individual interests of each country. Thailand has a strategy of economic development in the northeast of this country. In order to implement this strategy, Thailand has thoroughly exploited the source of the Mekong River for irrigation.
Meanwhile, Cambodia wants to maintain great aquatic resources in the Tonle Sap Lake and thoroughly use the water source of Mekong River for this country. The hydrological measurements show that water source flowing to Vietnam in the Mekong River basin decline sharply in the dry season, leading to severe drought.
Statistics have noted that the water level of the Mekong River is getting lower and lower, and it reaches a new record of low water level in 2016 within the past 90 years, causing seriously severe droughts and saline intrusion. In contrast to the dry season, the countries on the downstream such as Vietnam have to face up with risk of unsafety in the rainy season such as the abnormal flood discharge and broken. 
The reliance on water flow and time of seasonal discharge from upstream water flows has become a major challenge. With the advantage on the upstream of Mekong River, water resources have also been used as a “weapon” in international relations.
In 1997, China closed outfall for four days in order to work on a dam that blocked the flow of a large amount of fresh water into the Mekong Delta and make Vietnam lost 100.000 USD per day. Next, in 2016, the Mekong River Delta provinces have to cope with drought, saline intrusion across 13 provinces. The saline intrusion affects 9/13 provinces with the smallest peak flow in over the past 90 years. 
This has a such great impact that on March 10, 2016, the Vietnam Government has to send a documentary to China to propose charge of upstream dams to improve the drought situation. Security of the Mekong River water threatens Vietnam food security.
The Mekong Delta of Vietnam in the Mekong Basin has contributed to Vietnam 27% of GDP, 90% of rice export, 60% of seafood export turnover. However, the change in water sources has made the surface water of Mekong Delta become scarce, area of arable land become narrowed and less fertile. The decline in food is getting higher and higher over time, which makes the Mekong Delta may no longer be Vietnam’s rice field and seriously affects Vietnam food security.
Currently, the amount of alluvium deposited in hydroelectric dams is no longer sufficient to supply for this area. Hydroelectric dams in China retain 30% of the sediment, and dams built on the mainstream of Laos and Cambodia will retain approximately 5%; At least 50% of farmland in the Mekong Delta is affected by loss of alluvium and nutrition from hydropower projects.
This is one part of the reseach paper named “Security for Water Source of Mekong River and Impacts on Vietnam National Security” written by Le Van Thang, Nguyen Hai Thanh, Nguyen Van Tuan.