Even if no dams are built on the mainstream below China, the cascade to which it is committed will ultimately have serious effects on the functioning of the Mekong once the dams are used to control the river’s flow. This will be the case because the cascade will:
• alter the hydrology of the river and so the current ‘flood pulse’, the regular rise and fall of the river on an annual basis which plays an essential part in the timing of spawning and the migration pattern. This will be particularly important in relation to the Tonle Sap in Cambodia, but will have an effect throughout the river’s course;
• block the flow of sediment down the river which plays a vital part both in depositing nutrients on the agricultural regions flooded by the river and also as a trigger for fish migration — at present well over 50% of the river’s sediment comes from China;
• at least initially cause problems by restricting the amount of flooding that takes place most importantly in Cambodia and Vietnam; and
• lead to the erosion of river banks.
So China’s dam-building plans are worrying enough, but the proposed new mainstream dams would pose even more serious concerns. Those built at sites higher upstream would cause the least damage to fish stocks, but if, as currently seems possible, the most likely dams to be built would be at Don Sahong and Sambor the costs to fish stocks could be very serious. This is because unanimous expert opinion judges that there are no ways to mitigate the blocking of fish migration that would occur if these dams are constructed. None of the suggested possible forms of mitigation — fish ladders, fish lifts, and alternative fish-passages — are feasible for the species of fish in the Mekong and the very large biomass that is involved in their migratory pattern. Fish ladders were tried and failed at the Pak Mun dam on one of the Mekong’s tributaries in Thailand in the 1990s.
Why are the governments of Laos and Cambodia contemplating the construction of dams that seem certain to have a devastating effect on their populations’ food security? The answers are complex and include some of the following (a) a lack of knowledge at some levels of government (b) a readiness to disregard available information on the basis that it may be inaccurate (c) a belief or conviction that fishing is ‘old-fashioned’ whereas the production of hydroelectricity is ‘modern’. In Cambodia’s case, and in particular in relation to the proposed dam at Sambor, the fact that a Chinese firm is seeking to construct the dam raises the possibility that Prime Minister Hun Sen is unready to offend the country that has become Cambodia’s largest aid donor and Cambodia’s ‘most trusted friend’. In Laos, the proposal for a dam at Don Sahong is very much linked to the interests of the Siphandone family for whom southern Laos is a virtual fief. Of all the proposed dam sites Don Sahong is the most studied in terms of knowledge of fisheries so that it can be safely said that the planned dam would wreak havoc on a migratory system that involves fish moving through the Hou Sahong channel throughout the year.
In the face of the threats posed by both the Chinese dams and those proposed for the downstream stretches of the river, there is no existing body able to mandate or control what individual countries choose to do on their sections of the Mekong. The agreement establishing the Mekong River Commission (MRC) in 1996 does not include China or Burma, and though the latter’s absence is not important the fact that China is not an MRC member underlines the body’s weakness. In any event, the MRC members’ commitment to maintaining the Mekong’s sustainability has not overcome their basic commitment to national self-interest. A prime example of this is the manner in which the Lao Government has proceeded in relation to the proposed Don Sahong dam. For at least two years while the dam was under consideration there was no consultation with Cambodia. Similarly, so far as can be judged, Cambodia’s consideration of a possible dam at Sambor has taken place without consultation with either the governments of Laos or Vietnam.
At the moment the best hope is that both the Cambodian and Lao Governments will abandon their plans for Sambor and Don Sahong. If they do not, the future of the Mekong as a great source of food, both through fish and agriculture, is in serious jeopardy. This is all the more so as China continues its dam-building program and as the effects of climate change in the region through which the river flows are pointing to a greatly increased precipitation that is likely to cause major increases in flooding in the future, possibly as early as 2030.
Download full paper here https://www.think-asia.org/bitstream/handle/11540/6409/Osborne%2C_The_Mekong_WEB.pdf?sequence=1