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, which drives ecological productivity throughout the system. 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. 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. Dams affect fisheries by: • 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. • 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. 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.
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. The balance comes from those parts of the basin in northern
Thailand, and the Tibetan gorges. 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. 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. 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.
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. 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. It has been estimated that hydropower development contributes to the loss of 13,100 ha of forest a year in Laos. 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. It has been argued that logging represents an additional motivation to hydropower development, and is frequently linked to corrupt actors.
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. 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. 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. Hydropower reservoirs that also provided irrigation water (22) had generally higher emissions reaching over 22,000 kg of CO2 per MWh. 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 (2 per MWh), while the rest had higher emissions equivalent to fossil fuel power plants (>380 kg CO2 per MWh). 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. == Mekong hydropower and resettlement of affected communities ==