New Study Highlights the Impact of Chinese Dams on Lower Mekong

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Dam construction has resulted in widely fluctuating river flow in the Mekong, threatening fish productivity, which is key to the region’s economy.

Large dams on the Mekong River in China’s Yunnan Province have considerable impacts on downstream river flows, new research by myself and colleagues at Aalto University in Finland and published recently in the Journal of Hydropower has shown.

The Mekong originates in China where it is known as the Lancang, before flowing through Thailand, Laos, Cambodia and Vietnam. China has built a cascade of six hydropower dam projects on the upper reaches of the Mekong River.

The first project, known as Manwan, (1,670 MW) became operational in 1993 and the latest, known as Nuozhadu, (5,850 MW) began operations in 2014. The cascade has the capacity to produce 15,000 MW of power and store over 23 square kilometres of water, 28 per cent of the annual river flow at the border of China and Thailand.

Downstream countries are also exploiting their hydropower resources – with Laos recently announcing it will start construction of a third contentious dam on the Mekong.

We found that major changes to the river flow began in 2011 and were the largest in 2014. Hydropower operations caused exceptionally high dry season flows and low wet season flows in northern Thailand.

In 2014 the dry season flows reached record highs of two to three times the long-term average. During the same year, wet season flows reached record low levels and were roughly two thirds of the long-term average.

These changes in flow were observed over 2000 km downstream in Cambodia, where the 2014 dry season flows increased by half.

New Evidence of Dam Impacts

In 2012, we used simulation models to predict how the hydropower cascade would change river flow, and in fact our predictions closely matched the observed fluctuations in flow.

Thus we have a fairly good understanding of the changes taking place. It is now clear that the construction of large dams on the upper reaches of the Mekong have resulted in widely fluctuating river flow, depending on the hydropower operations.

Changes in river flow play an important role in the ecology and economy of the Mekong River, which is rich in biodiversity and the source of livelihoods and food security for millions of people.

Fishing is particularly important to the people as the Mekong River is one of the world’s most productive inland fisheries, with an annual catch of 2.6 million tonnes.

The annual value of the river’s fisheries is estimated to be USD 3.9-7 billion. Over 1,000 fish species have been found in the Mekong, including giant sting ray, freshwater dolphin and giant catfish.

A key driver of the biodiversity and productivity of the Mekong River is the river flow regime driven by monsoon rains, often called the annual flood pulse.

The flood pulse transports large amounts of sediment and nutrients along the river and has created diverse floodplain habitats. As the river flow changes, we have observed an increased variability of this flow regime and a smaller annual pulse. In addition, the dams block nutrient-rich sediment that would otherwise be carried downstream.

Our research predicts that these changes are likely to lead to lowered ecosystem productivity, which may have significant impacts on the ecosystems and communities that depend on the Mekong.

Reports by Chinese scientists have already shown the negative effect of dams on the ecology along the Chinese part of Mekong River and a reduction in number of fish species during the construction period of the six dam cascade.

The effects on the ecology and people in the downstream countries since the completion of the dams are yet to be scientifically documented and published.

By Timo Räsänen

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