Hydropower in the Mekong River Basin

HPPs on the Mekong mainstream have aroused particular environmental concerns.[27][28] The majority of these are based in China's Yunnan Province. Table 1 below indicates the status of each of these HPPs.

Table 1: Hydropower plants on the Mekong mainstream[29]

Notes: COD = Commercial Operating Date; N/A = Not Applicable

Table 2: Commissioned dams in the Mekong River Basin (15 MW installed capacity and above)[29]

Notes: COD = Commercial Operating Date

Table 3: Hydropower dams under construction in the Mekong River Basin (15 MW installed capacity and above)[29]

Notes: COD = Commercial Operating Date

The environmental impacts of Mekong hydropower development are generally well studied and understood. Some the key impacts of Mekong hydropower are as follows:

Hydrological impacts: about 75% of annual flows through the Mekong system occur between late June and early November,[30][31] which drives ecological productivity throughout the system.[32][33] This surge of water is known as the 'flood pulse' and dams (of all kinds) will contribute to its diminution. Wet season flows can be expected to reduce, while dry season flows can be expected to increase.[34] This has significant implications for the Mekong's ecology.

Fisheries impacts: the Mekong's fisheries are threatened in multiple ways, most importantly by dams and excessive fishing pressure.[35] Dams affect fisheries by:[36]

• Acting as barriers to fish migration - either as fish try to migrate upstream to spawn; or for trapping fish fry or eggs as these travel downstream.
• Interrupting natural flood cycles to which fish have evolved and adapted to over thousands of years.
• Riverbed hardening. Dams typically release water in bursts, which removes smaller sediments like silt, sand, and gravel, as well as aquatic plants and animals and debris from vegetation. As a result, the bedrock below the dam becomes exposed and loses its value as a fish habitat.
• Trapping sediment, a significant source of nutrition for fish.[37]
• Altering water temperature. Water released from a dam is typically colder than prevailing temperatures downstream of the dam. This has a direct impact on fish habitats and populations.
• Hydropeaking, which refers to the release of water from HPPs when demand is highest (usually during the day), and much smaller releases when demand is low. This also affects fisheries through the rapid alteration and high and low river flows. Globally, hydropeaking has been found to impact fish biodiversity, and fish community composition.[36][38][39]

The fisheries impact of all existing and planned mainstream dams will be most felt in Cambodia (which will experience three-quarters of the loss), while the balance will be experienced in Vietnam, Lao PDR and Thailand.[39] In terms of tonnages, this will represent a loss of between 580-750,000 Mt per year.[39]

In another study by the Mekong River Commission, fisheries assessments conducted in 2020 suggested that the annual finfish yield from the lower Mekong (i.e. those parts of the basin that fall within Cambodia, Laos, Thailand and Vietnam)) was between 1.51 to 1.71 million tonnes, while the harvest of other aquatic animals (OAAs) was approximately 443,000 tons. This is approximately 25-30% less than yield estimates conducted in 2000 and 2010. The estimated value of the fish catch varies from USD 7.13 billion to USD 8.37 billion annually. In addition, the estimated value of the OAA harvest is approximately USD 1.13 billion.[40]

Sediment impacts: in the Mekong, some 40% of the sediments that reach the Mekong Delta are derived from the Three Parallel Rivers area in Yunnan, while some 52% come from the Central Highlands of Vietnam.[41] The balance comes from those parts of the basin in northern Thailand, and the Tibetan gorges.[41][42] Sediment loads are lowest during the dry season and highest during the first months of the flood season, when loose sediments weathered during the dry season are washed into rivers.[41][43]

Although suspended sediment concentrations in the Mekong have been monitored since 1994, the ‘pre-disturbance’ sediment load is unknown. Nevertheless, studies can demonstrate very significant declines in the Mekong's sediment load since 2001. At Chiang Saen, sediment flows have decreased from about 85 million metric tonnes per year (Mt/yr) to 10.8 million Mt/yr, meaning that the sediment contributions from China to the Mekong mainstream has decreased to about 16% of all sediments in the Lower Mekong as compared to about 55% historically.[44] A similar trend is seen down-stream at Pakse, where average loads have decreased from 147 Mt/yr to 66 Mt/yr between 1994 and 2013.[44]

The declining sediment load has significant implications for the Mekong Delta, recharging sediments otherwise washed away by the sea, consumed by sea-level rise, or in combination with land subsidence. Studies of the possible long-term consequences of system-wide sediment reductions suggest that it is likely that nearly half of the Delta's land surface will be below sea level by 2100, with the remaining areas impacted by saline intrusion from the sea and frequent flooding.[42] Much of the Mekong's sediment decline is attributed to the 'trapping efficiency' of dams.[42][44][45][46]

Forest impacts: there is a two-way relationship between (reservoir) hydropower and deforestation in the catchments they draw on. In the absence of soil conservation measures, deforestation often contributes to increased erosion, which is then deposited in the reservoir, reducing reservoir capacity. The 1,570 MW Manwan HPP on the Mekong mainstream in China, for example, lost 21.5–22.8% of the total storage capacity to sediments in its first 11 years of operation.[47] But hydropower can also drive deforestation. Reservoirs need to be cleared of vegetation before filling, and given the size of some reservoirs, the area subjected to clearance can be considerable. The deforestation impact of reservoir clearance in the Mekong is unknown. In 2008, an estimated 60% of the timber harvested in Laos was approved under special quotas for hydroelectric and other infra-structure projects.[48] It has been estimated that hydropower development contributes to the loss of 13,100 ha of forest a year in Laos.[49] Over 2006/07, the Lao national timber quota was temporarily raised by 400,000 m³ to allow for logging specifically related to the development of the Nam Theun 2 HPP.[50] It has been argued that logging represents an additional motivation to hydropower development, and is frequently linked to corrupt actors.[51]

River connectivity: 'connectivity' refers to the degree to which matter and organisms can move among spatially defined units in a natural system. ‘River connectivity’ is typically described as lateral (between a river's main channel and its floodplains), longitudinal (between upstream areas in the river channel or catchment, and downstream ones), and vertical (within the water column, between upper water layers and lower ones.[52][53] River connectivity can be conceptualised as a continuum from 'fully connected' to 'disconnected'. River connectivity strongly influences the resistance and resilience of rivers to natural and human-induced disturbances.[53]

Dams interrupt connectivity, and so fish cannot swim upstream to spawn or breed; dams affect water quality in a variety of ways, altering upstream ecosystems so that they contrast starkly with downstream ones. Dam reservoirs are lacustrine (lake-like) environments unlike rapidly flowing waters downstream; upstream, the water is heavy with sediments, while downstream it is not; above the dam, the water is cold, while below it, it is warmer.

A 2014 study explores an HPP build-out of 81 proposed dams across the Mekong Basin.[54] If this were to occur, it would reduce the Mekong's connectivity to just 11% by 2022. This build-out – already well advanced – would make the Mekong one of the most heavily impounded rivers in the world.

Greenhouse gases: hydropower reservoirs do emit greenhouses gases (GHGs), although the volumes and types depend on the reservoir's latitude and age.[55] Young reservoirs tend to release larger amounts of GHGs than older ones due to the availability of decomposing vegetation and other organic matter soon after initial inundation; tropical reservoirs tend to release more than temperate ones due to higher rates of net primary production.[55][56]

In the Mekong, one study looked at 119 reservoirs across the basin, and found that these emit be-tween 0.2–1,994 kg  of CO₂ per MWh over a 100 year lifetime with a median of 26 kg  of CO per MWh.[57] Hydropower reservoirs that also provided irrigation water (22) had generally higher emissions reaching over 22,000 kg  of CO₂ per  MWh.[57] Yearly emissions ranged from 26 to 181,3 000 Mt of CO₂ per year over a 100-year lifetime, with a median of 28,000 Mt of CO₂ per year. Altogether, 82% of hydropower reservoirs (119) and 45% of reservoirs also providing irrigation (22) had emissions comparable to other renewable energy sources (<190 kg  CO₂ per  MWh), while the rest had higher emissions equivalent to fossil fuel power plants (>380 kg  CO₂ per  MWh).[57] These results, the study authors caution, are tentative and they suggest that hydropower in the Mekong Region cannot be considered categorically as low-emission energy. Instead, the GHG emissions of hydropower should be carefully considered case-by-case.[57]

People are affected by hydropower in a diversity of ways. Most immediately are those displaced by an HPP (i.e. resettled due to the presence of the dam itself, its reservoir, and/or ancillary buildings and infrastructure). Then there are those who will be affected by fisheries and sediment losses, and/or losses arising due to hydrological changes. Such populations may be eligible for compensation. Finally, there are broader impacts, related to the ways in which hydropower affects national economies. This section focuses on involuntary displacement and resettlement due to hydropower in the Mekong.

Figures for the number of people displaced by hydropower development are scattered and not generally available. Where data are available, they are as follows:

Table 4: Resettlement of hydropower-displaced people in the Mekong River Basin[29]

In 2011, it was estimated that hydropower development in Laos would result in the resettlement of between 100,000 and 280,000 people.[58] In 2019, the Lao Minister of Energy and Mines estimated that, up until that point, 60,000 people from 12,000 families from more than 200 villages across Laos had been resettled to make way for power projects.[59] Similar estimates are not available for other parts of the Mekong River Basin.

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• Australian Mekong Resource Centre
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• Hobo Maps Laos Hydropower Projects Info
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