Understanding the geologic history of the upper Mekong basin is increasingly important for examining the effects of dam construction, both in terms of seismicity and sediment trapping. The sediment regime of the river has been altered by the construction of dams, which have captured large quantities of sediment. However, the degree to which it has changed the river is uncertain due to the small number of studies done so far. Additionally, agriculture and development have their own effects on the sediment load of the Mekong, which further complicates sediment analysis. More alarmingly, a large magnitude earthquake could cause dam damage or failure, which in turn could cause catastrophic damage downstream. While such an event is unlikely, it is important to properly regulate dam construction as well as encourage the construction of earthquake resistant infrastructure, especially in Yunnan, Northern Thailand, and Laos. The underlying geologic structure of the Mekong River Basin is highly complicated and should be studied in greater detail so that dams are constructed as safely as possible, both to protect downstream communities and to ensure that the sediment load is not being disturbed at the expense of aquatic ecosystems and downstream agricultural communities.
The origin of the Mekong River lies 5,000 meters above sea level, high on the Tibetan plateau. From there the river runs through China’s Qinghai and Yunnan provinces, where it is called the Lancang River. Its name changes to the Mekong as it flows through the five mainland Southeast Asian nations: Myanmar, Lao PDR, Thailand, Cambodia, and finally Vietnam. The River runs a total of 4,350 km before it spreads out over the Mekong delta and into the South China Sea. The Mekong drains an area of 795,000 square kilometers, with an annual discharge of 475 cubic kilometers, making it the longest and largest river by volume in Southeast Asia, and the 12th longest and 8th largest by volume in the world. At 16,000 cubic meters per second, the Mekong has an average discharge comparable to the Mississippi river, despite the Mekong being over 1,000 miles shorter. (Fig 1) Its importance in the region as a source of livelihood and culture cannot be understated; it is the connecting tie between the nations of mainland Southeast Asia. While river ‘capture’, or the seismically induced alteration of river pathways, makes pinpointing the origin of the Mekong River difficult, there is some indication of its modern derivation. According to one study, which took sediment cores from the South China Sea, “The oldest sediments, which are linked to the modern delta body, accumulated in the early mid-Holocene, at about 8000 calibrated years before present preceding the mid-Holocene sea-level highstand in the South China Sea.” (See figure 1, core MD01-2393) Primarily because of sea level rise the Mekong River has changed since then into the basin recognizable today.
The Mekong River Basin is situated off the southeastern edge of the Tibetan Plateau, which as an active converging plate boundary has a strong influence on the tectonics of the Mekong basin. The collision of the Eurasian plate and the Indian Plate are the source of the uplift of the Himalayas and the Tibetan Plateau, and the Mekong River basin lies between this convergent plate boundary and the Sumatran Subduction Trench further south along the southern coast of Sumatra. This intraplate zone is a ‘basin and range’ province, much like the Nevada-Utah basin and range of the United States, and is scattered with faults with different slip-rates, especially in the area in and around northern Thailand. Considering this somewhat unique geologic position which has created different fault zones pulling and pushing in different directions, the basin’s geology is both heterogeneous and, particularly in the northern part, seismically active. To the north of the Mekong River Basin, the Longmenshan fault zone is highly active; responsible for the devastating earthquake in Chengdu in 2008, which claimed the lives of over 68,000 people. The upper Mekong basin is not range of the Longmenshan fault zone, but its basin and range typology is strongly influenced by this fault zone. The most notable fault systems that influence the basin are the “right-lateral, strike-slip Red River and the left-lateral strike-slip Xianshuihe-Xiaojiang fault systems.” These fault systems as a portion of the typical ‘basin and range’ geological province create series of exactly that: similarly trending valleys and mountains that are a direct result of fault blocks falling and rising with respect to each other. This allows different geologic layers to be exposed within relative short distances, meaning that as the Mekong River flows downstream, it quickly gathers different types of sediments.
The sediment regime in the Mekong is a result of its drainage pattern and the variety of rock types in the river basin. The Mekong River basin itself is atypical of continent draining rivers in its drainage pattern is not dendritic. Rather, the river has a parallel drainage pattern which is much more linear with more direct tributary angles. This pattern is a combined result of the underlying geologic structure and the slope of the topography. The upper basin is particularly narrow which indicates strong, or steep, slope control. Often, underlying structures such as joint systems control the geometry of tributary angles, which are generally narrow. In these steep and narrow gorges, the rapid flowing water of the Mekong quickly erodes the hillsides, making the river a muddy-silt brown. Considering the heterogeneity of the underlying structure, the swift moving water gathers many different minerals, creating a rich sediment regime with lots of chemical elements needed for agriculture and aquatic ecosystems. The upper part of the basin, especially in China, is the primary source for this sediment. Researchers have suggested that “the existing estimate of the mean annual suspended sediment load of the Mekong reported in the literature is ~160 Mt y^-1, and (Roberts) has estimated that about 50% of this load is contributed by the upper part of the basin in China.”
The northern part of the basin “accounts for about 24% of the total area of the basin and about 18% of its total discharge, and sediment yields in these mountainous headwaters, which have steep, unstable slopes, are clearly substantially higher than those from the remainder of the basin.” As it flows the Lancang River quickly becomes a muddy-silt brown, reducing the River’s ability to erode the rock further downstream. Dams allow sediment to settle out, in fact “Kummu and Varis cited estimates that suggest that the Manwan Dam could trap as much as ~50-60 Mt of sediment per year, and this would clearly cause a major reduction in the sediment load of the Lancang River.” What the overall effect this entrapment will be is not yet known. What is known is the exiting water, devoid of sediment, will erode rock more quickly than it did before, possibly replacing the sediment lost but at the cost of downstream slope stability. The increased erosion of stream beds could pose ‘major threats’ to places such as Luang Prabang, Vientiane, and Nongkhai.
Figure 2: Mekong Sediment load, values from 1961 compared with recent years (between 1997 and 2002).
Figure 3: Mekong river discharge, values form 1961 compared with recent years (between 1997 and 2003).
Unfortunately, there have not been a lot of studies done in Southeast Asia on this subject, and research needs to be continued in order to examine how the sediment regime has been and is being altered. So far, research done has shown that variations in sediment discharge are more closely linked with the total water discharge of the Mekong, rather than the construction of new dams. Figures two and three illustrate this problem as there are hardly enough data points, due to a lack of continuous research, to come to a conclusion about the sediment regime and the way dams have affected it. In this way, it is important that these parameters be monitored annually so that a meaningful conclusion can be drawn as to whether or not dams have a negative impact on sediment transport.
Seismicity: Predicting earthquakes in the northern Mekong Basin
Accurately predicting the timing of an earthquake, as seismologists know, is close to impossible. But that doesn’t mean that it’s not worth trying, because properly understanding seismic activity can be effective in protecting human lives. While exceedingly challenging, seismologists use a variety of techniques to predict the likelihood of earthquakes occurring, and what the magnitude of the earthquake might be. These techniques generally involve measuring average slip rates and estimating the likelihood within a given period of time of the fault ‘slipping’ which causes earthquakes. In the Mekong River basin, this is extremely important with regard to the dams that have been built along the river as well as for dams in the planning phases.
Figure 4. Dams along the Mekong River and its tributaries. Courtesy of the WWF
Seismic activity in the Mekong River basin is primarily limited to areas in Yunnan, northern Thailand, and Laos. Some areas in northern Thailand in recent history have been described as seismically inactive, as despite there being several fault zones there are few historical records of destructive earthquakes. There is some mention in different literature that northern Thailand is seismically ‘highly stable’, which happens to be true for recent history, but that does not suggest earthquakes cannot or will not happen. As Fenton says in his 2003 study, “Due to a lack of large, damaging earth-quakes during historical time, Thailand has not been considered to be a seismically active country. Although there are a number of accounts of historical earthquake damage (Nutalaya et al. 1985), the locations and sizes of most of these events are not well constrained.” While earthquakes are generally below 6.5 in magnitude, there are notable exceptions. For example, “[The Red River] fault has produced several earthquakes >M 6.0 including the 4 January 1970, M 7.5 earthquake in Tonghai which killed over 10,000 people.” While this was further north, there are concerns that earthquakes could cause substantial damage to developing infrastructure. One USGS study of a magnitude 6.8 earthquake in the Golden Triangle region of Myanmar in March of 2011 highlights that “Overall, the population in this region resides in structures that are highly vulnerable to earthquake shaking, though some resistant structures exist. The predominant building types are wood and unreinforced brick masonry construction.” This suggests that if a larger magnitude earthquake of were to strike, the damage would be enhanced by the collapse of structures which are not equipped to handle such shaking. These faults are considered capable of generating maximum earthquakes of up to 7.5 in magnitude, which while unlikely on an annual basis, (see figure 6) increase in likelihood over time.
Figure 6. Faults in Northern Thailand. Note the proximity of faults 3, 11, and 18 to the Mekong and proposed dam site. Note annual probability of fault movement in Fig. 7 Courtesy of the USGS
Figure 7. Annual probability of fault movement among studied active faults in northern Thailand. See fig 6. and key for location of faults. Data courtesy of the USGS
The Xayaburi dam in Laos is controversial for several reasons, but fears of damage from earthquakes are rising. One Thai geologist, Dr Punya of Chulalongkorn University, has estimated there is a “30 per cent chance of a medium-sized earthquake hitting the dam site in the next 30 years, and a 10 per cent chance of a powerful earthquake of up to magnitude 7.” He was reported as saying: “If the fault at the dam site becomes active … there is no chance for seismic engineering to take care of that.” Dr Punya also stated that construction on the dam should “never have started” at such a site without further research into its seismic risk. Dr. Punya’s concerns do not seem unwarranted, as there have been substantial earthquakes in recent years. In 2011, two earthquakes occurred 48 kilometers from the site of the Xayaburi dam, one 5.4 and one 4.6 magnitude. One month later a magnitude 3.9 earthquake occurred 60 kilometers from the dam site. In 2007, a 6.3-magnitude quake hit the Xayaburi area. Further away, in northern Myanmar, a 6.9 magnitude quake on March 24, 2011 killed 151 people.
Apparently, the earthquakes near Xayaburi occurred on what were thought to have been inactive faults, “an unusual development and one that causes additional concern.” It is possible this may be related to dam-induced seismicity, another substantial concern many geologists bring up with regard to dam construction and seismicity. This phenomenon has been documented as far back as 1932, and the Sichuan earthquake in 2008 has been suggested as being a result of this effect. Tectonic movement isn’t a process that changes within the lifetimes of humans, and a trend of increasing seismicity is only likely to continue. In fact, “some studies suggest that due to the high slip rate on this fault, future large earthquakes arehighly possible.” While total dam failure is extremely unlikely, earthquakes will nonetheless be able to cause a lot of damage to dams, costing the dam companies millions. Moving forward, it is imperative that more geologic and seismic studies are done of the northern Mekong basin. This is especially true for dam construction companies as they construct dams; to do so in as safe and secure a way as possible.
Unfortunately, most of the scientific literature on the subjects of seismicity and sediment transport in the Mekong River point to the lack of research done thus far as a limiting factor for their own research. While there has been a fair amount of research done, it is not sufficient to completely assess whether dams are safe to construct or not. Based on preliminary findings, it seems that most earth scientists that have studied this region agree that they feel uneasy about the construction of dams and that more research needs to be done. The construction of dams might ultimately be important for the development of Southeast Asian nations, but proper research needs to be done to ensure they are not irreparably damaging the river. A worst-case scenario would consist of catastrophic dam failure due to an earthquake, which would in turn likely cause downstream dams to fail, and destroying any communities along the river. The economic loss, not to mention the loss of life, would be disastrous. Because of this risk, however small, research and engineering techniques should be paid for ahead of time by dam construction companies rather than afterwards with human lives and livelihoods.
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Fawthrop, T. 2014, November 19. Experts renew quake fears over Xayaburi dam Mekong River in Laos. South China Morning Post. Xayaburi.
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