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. Continue reading “The Mekong river under THREAT”