Courtesy of PNAS

Dams on the tributaries of the Mekong River could have a greater negative effect on fish biodiversity and food security than those on the main river, researchers say.

Hydropower developments on Mekong tributaries are not subject to the same level of scrutiny as their counterparts on the main river. “Most of the attention has been on proposed dams on the Mekong mainstream, such as the highly controversial Xayaburi dam in Laos,” says lead author Guy Ziv, an environmental scientist now at Stanford University in California. “The impact of tributary dams is little studied.”

The findings, published today in Proceedings of the National Academy of Sciences1, “point to a desperate need to reconsider hydropower development in the entire Mekong River basin”, says Ame Trandem, the Southeast Asia programme director for the environmental group International Rivers in Bangkok.

With a watershed of 800,000 square kilometres, the Mekong River basin supports the world’s largest inland fishery and is home to 65 million people in six countries: China, Myanmar, Laos, Thailand, Vietnam and Cambodia. “Most of the people are poor and get 81% of their protein from subsistence fisheries,” says Ziv.

The steep topography of the region makes the Mekong an attractive place for hydropower development. Driven by increasing demand for electricity and a desire for economic development, 11 dams are being planned on the main river, with 41 on the tributaries expected to be completed within the next 4 years. Another 10–37 tributary dams are likely to be built between 2015 and 2030.

Using a fish migration model, Ziv and his colleagues found that if all of the proposed dams were constructed, they would reduce fish productivity by 51% and endanger 100 migratory fish species.

They then focused on the 27 tributary dams whose fate is yet to be determined, and were surprised to find, says Ziv, that the losses in fish biodiversity and production would be greater than for the proposed dams on the upper reach of the lower Mekong River.

“Individual dams may not make a big difference,” says Ziv. “But if you add all 27 dams together, you may get a catastrophic impact.” This is not only because of the total area that will be blocked for fish migration, but also because some regions are more important fish passages than others, he says.

One area of particular importance, the study shows, is the 3S river system in northeastern Cambodia, southern Laos and central Vietnam that is dominated by three major Mekong tributaries — the Se San, Se Kong and Sre Pok Rivers. Dams in this region would hit fish migration the hardest. The planned Lower Se San 2 Dam in Cambodia, for instance, would cause a 9.3% drop in fish biomass basin-wide. “The impact would be catastrophic,” says Ziv.

“Dams at different locations have different trade-offs between power generation and the loss in fish biodiversity and productivity,” says Ziv. “The Lower Se San 2 Dam will have the highest environmental cost per unit of energy produced.”

The team has created a simple matrix for deciding which dams to build throughout the basin. The tool estimates the loss of fish productivity at different levels of total electricity generation and ranks each dam in terms of its trade-offs. “Dams with better trade-offs can be built first when the energy demand is relatively low,” says Ziv. “And you really should avoid building those with the worst trade-offs, such as the Lower Se San 2 Dam.”

Ziv stresses that the study is just a “starting point” and that other aspects of potential impact, such as effects on sediment, agriculture and the displacement of people and communities, must be incorporated into the scheme for comprehensive trade-off analyses.

According to the 1995 agreement of the Mekong River Commission (MRC), an international body responsible for the sustainable development of the river, each member country is required to consult other nations for any major development projects on the Mekong itself. But there is no such requirement for projects on the tributaries.

To many, the latest findings call for a change in that policy. “It’s really time for the MRC to take a basin-wide approach to assessing the consequences of dams in the region,” says Trandrem.