- From December 2014 World Rivers Review.
China has built seven hydropower dams on the upper Mekong River (known as the Lancang in China), and plans to build 21 more. The Lancang crosses through Qinghai, Tibet and Yunnan before flowing into Myanmar, Laos, Thailand, Cambodia and Vietnam. There have been many concerns from the Lower Mekong communities on how these dams will impact their lives and livelihoods. People have questioned whether recent sudden changes of water levels and droughts in the Lower Mekong were caused by these Lancang dams. International Rivers prepared a research brief based on a literature review and monitoring data to help understanding the downstream impacts on hydrology, fisheries and sedimentation caused by the Lower Lancang cascade in China.
Because the Lancang River contributes 45% of water to the Mekong basin in the dry season, the flow at Chiang Saen in Thailand, which is located hundreds of kilometers downstream from the Lower Lancang Cascade, can be increased by over 100% in the dry season. An increase in water levels in the dry season will reduce the exposed riverbank areas that are now used for seasonal agriculture. Millions of villagers who live along the Mekong River grow vegetables in riverbank gardens and their livelihoods will be significantly impacted if they lose the gardens. In the wet season, the decrease in flow at Chiang Saen caused by the seven Lancang dams will be about 30%. This will shrink the floodplain area and reduce the flow of nutrients deposited on floodplains.
The Lancang dams have also altered water temperatures. The daily average water temperature at Chiang Saen decreased after Dachaoshan Dam started operation. The annual water temperature range has also increased. Once the Lancang cascade in the middle reaches is completed, the temperature impacts are certain to accumulate and will extend at minimum hundreds of kilometers downstream. The decrease in water temperature and the increase in water temperature fluctuation will change the behaviors of fish species, impacting their reproduction and migration activities.
The dams not only change the river’s flow and temperature, but also block fish migration channels, which are critical for reproduction. The extent of fish migration from the Lower Mekong into the Upper Mekong is unknown. However, the endangered Mekong Giant Catfish has been found to forage and spawn in the Buyuan River (a tributary of Lancang ) between the Jinghong and Mengsong dam sites. Other fish species such as Tor sinensis, Wallago attu, Hemibagrus wychioides may also migrate between the Lower and Upper Mekong.
Several scientific reports have found that half of the sedimentation in the Lower Mekong originates from the Lancang basin. Because of the different measuring methodologies used between Yunnan and downstream Mekong countries, as well as different analysis methodologies, the sedimentation capture rate by the Manwan Dam (completed in 1995) has been estimated to range from 53% to 94%. Some researchers have reported that the sedimentation impact from Manwan Dam extends as far as Vientiane, Laos. The whole cascade of dams will theoretically trap 94% of the suspended sediment load coming from China. The reduction of sedimentation downstream will not only result in riverbank erosion, but will also reduce the nutrients carried in the flow and deposited in floodplain areas, thus undermining the chemical base of the ecosystem. It is also likely to trigger the acceleration of seawater intrusion in the delta. These impacts aren’t easily observed in the short term, but build over decades.
With the two biggest dams of the cascade, Xiaowan and Nuozhadu, put into operation in 2010 and 2012, and the middle Lancang cascade expected to be completed in the next few years, bigger downstream impacts are expected. The changes in hydrology, fisheries and sedimentation brought by the Lancang dams will have extensive and very significant impacts on millions of people who rely directly on the river for their food and livelihoods. Altering the hydrological and sedimentation regimes and blocking fish migration will potentially reduce the quantity and diversity of fish in the downstream Mekong River, and lead to food insecurity and lost livelihoods. Furthermore, the reduction of sedimentation deposit and the seawater intrusion will affect the highly productive agricultural and rice fields in the region, which depend on nutrients transported by the river in its sediment, and therefore create even bigger challenges in food and livelihoods.
It has been demonstrated that China and Chinese dam builders can be more responsive and responsible when planning and operating dams. China has agreed to share more hydrological data with the Mekong River Commission by extending the hydrological data provision by 30 days, starting on June 1 until October 31, every year, as well as increasing the frequency of the data sharing to twice a day. The dam developer of Lancang dams, Hydrolancang, has taken environmental and social concerns into consideration in several cases. Gushui Dam’s height was reduced due to concerns over inundating a protected area in Tibet. Guonian Dam – originally planned between the Gushui and Wunonglong dams – was canceled because of its potential impacts on the Mingyong Glacier. The water level of Wunonglong dam was reduced to avoid some impacts, which therefore led to the reduction of installed capacity. Mengsong Dam, originally planned as the last dam on the Lancang, was canceled due to concerns over its negative impact on fish migration. However, the large impacts from the Lancang dams are not avoidable and the effectiveness of mitigation measures remains to be seen.